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US009319794B2
a2) United States Patent (10) Patent No: US 9,319,794 B2
Betlehem et al. (45) Date of Patent: Apr. 19, 2016
(54) SURROUND SOUND SYSTEM FOREIGN PATENT DOCUMENTS
(75) Inventors: Terence Betlehem, Lower Hutt (NZ); wo 03/073791 9/2003
Mark Alistair Poletti, Lower Hutt (NZ) wo 2004/068463 8/2004
(73) Assignee: Industrial Research Limited, Lower wo 2005/013643 2/2005
Hutt (NZ) OTHER PUBLICATIONS
(*) Notice: Subject to any disclaimer, the term of this Terence Betlehem etal, “Theory and design of sound field reproduc-
patent is extended or adjusted under 35 tion in reverberant rooms,” J. Acoust. Soc. Am. vol. 117, No. 4, Apr.
U.S.C. 154(b) by 422 days. 2005, pp. 2100-2111.
. M. Poletti et al., “Sound-field reproduction systems using fixed-
(21) Appl.No.: — 13/817,945 directivity loudspeakers,” J. Acoust. Soc. Am. vol. 127, No. 6, Jun.
(22) PCT Filed: Aug. 22, 2011 2010, pp. 3590-3601.
. Michael Chapman et al., “A Standard for Interchange of Ambisonic
(86) PCT'No.: PCT/NZ2011/000161 Signal Sets Including a file standard with metadata,” Ambisonics
§ 371 (c)(1), Symposium, Jun. 25-27, 2009 pp. 1-6.
(2), (4) Date: May 16, 2013 Mark Poletti, “Unified Description of Ambisonics Using Real and
(87) PCT Pub.No.: W02012/023864 Complex Spherical Harmonics”, Ambisonics Symposium, Jun.
PCT Pub. Date: Feb. 23, 2012 25-27, 2009, pp. 1-10, ;
(Continued)
(65) Prior Publication Data
US 2013/0223658 Al Aug. 29, 2013 Primary Examiner — Sonia Gay
. on vos (74) Attorney, Agent, or Firm —Dann, Dorfman, Herrell
(30) Foreign Application Priority Data and Skillman, PC.
Aug. 20,2010 (NZ) .... vwone S87A83— (57) ABSTRACT
(51) Int. Cl. A surround sound system for reproducing a spatial sound
HOAS 3/00 (2006.01) field in a sound control region within a room having at least
HOR 5/02 (2006.01) one sound reflective surface. The system uses multiple steer-
GIOL 19/008 (2013.01) able loudspeakers located about the sound control region,
(52) U.S.CL. each loudspeaker having a plurality of different individual
CPC HOAR 5/02 (2013.01); HO4S 3/002 (2013.01); directional response channels being controlled by respective
, GIOL 19/008 (2013.0 1) speaker input signals to generate sound waves emanating
(58) Field of Classification Search from the loudspeaker with a desired overall directional
CPC H04S 3/002: HO4R 5/02 response. A control unit connected drives each of the loud-
USPC "381307 Speakers and has pre-configured filters based on measured
See application file for complete search history. . acoustic transfer functions for the room for filtering the input
. spatial audio signals to generate the speaker input signals for
(56) References Cited all the loudspeakers to generate sound waves with coordi-
U.S. PATENT DOCUMENTS
5,142,586 A * 8/1992 Berkhout .. 381/63
5,199,075 A 3/1993 Fosgate
(Continued)
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reproduce the spatial sound field.
33 Claims, 11 Drawing Sheets
L Congurable
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12Page 2
US 9,319,794 B2 Page 2 (56) 5,809,150 5,870,484 7,092,541 7,133,530 7,515,719 7,577,260 8,594,350 2006/0072773 2006/0165247 2006/0222191 2007/004 1599 2007/0263889 2008/0101631 2009/0060236, 2009/0103753 2009/0214046 2009/027 1005 References Cited U.S. PATENT DOCUMENTS A 9/1998 Eberbach A 2/1999 Greenberger Bl 8/2006 Eberbach B2 11/2006 Poletti B2 4/2009 Hooley et al. BL* 8/2009 Hooley et al. B2* 11/2013 Hooley etal. AL 4/2006 Hughes, II et al. Al 7/2006 Mansfield et al. Al 10/2006 Hung et al. AL 2/2007 Gauthier et al. Al 11/2007 Melanson Al 5/2008 Jung et al. AL* 3/2009 Johnston et al. AL 4/2009 Hsu AL* 8/2009 Suzuki et al. AL* 10/2009 Christensen OTHER PUBLICATIONS Marinus M. Boone et al., “Design of a Loudspeaker System with a Low-Frequency Cardioidlike Radiation Pattern,” J. Audio Eng. Soc., vol. 45, No. 9, Sep. 1997, pp. 702-707. Laura Fuster et al., “Room Compensation using Multichannel Inverse Filters for Wave Field Synthesis Systems,” AES Convention Paper 6401, 118th Convention, May 28-31, 2005, Barcelona, Spain, pp. 1-9. M.A. Poletti, “Three-Dimensional Surround Systems Based on Spherical Harmonics.” J. Audio Eng. Soc., vol. $3, No. L1, Nov. 2005, pp. 1004-1025. Alexander Mattioli Pasqual et al., “Application of Acoustic Radiation Modes in the Directivity Control by a Spherical Loudspeaker Array” Acta Acoustics United with Acustica, vol. 96, 2010, pp. 32-42. Peter Kassakian et al., “Characterization of Spherical Loudspeaker Arrays,” AES Convention Paper 6283, 117th Convention, Oct. 28-31, 2004, San Francisco, CA, pp. 1-16 * cited by examiner
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US 9,319,794 B2
U.S. Patent Apr. 19,2016 Sheet 1 of 11
7
12
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16 Loudspeaker Units
1 44 2M +1
! Surround | ] Listener in
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FIGURE 2Page 4
U.S. Patent Apr. 19,2016 Sheet 2 of 11 US 9,319,794 B2 12 L Congurable Loudspeaker Units 22 Microphone 14 \ Test y Signals Array in -"> ~~, Sound Control Central 43 rec Tua Region Control ' Unit OM #1 tA +20 \ . N44 24 : 12 22 ~o~ f 22 Mode Weighting Matrix Fourier Matrix function coefficients Surround Sound Processor FIGURE 4
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US 9,319,794 B2 Sheet 3 of 11 Apr. 19, 2016 U.S. Patent Bs zi soia gq ft e4 ry ay! SAS] & ali KR porn ncn c nner ncn cnc nnn TE SEES pS SSS ge sssisssigs) eee --------------------------, \ Y = go: =e! _ a e2@ifogfq |S = S 9 Sala ¥ ays x as z H + Bee Wal cz] sS [hax] Sh “S| 2 SI SE PEN ep [Sp TENER TR SN SR TEE a a: ll fe lend =| |S as zl | = N Oso! = Sy} [e& % OB] Po x: ! ! a} © © Sy} any 1 ag ee ee Senne eee See ee eee ee oe = a 2% a coy & ae) FIGURE 5
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U.S. Patent Apr. 19,2016 Sheet 4 of 11 US 9,319,794 B2 i] — 1 r— Loudspeaker ! ! ! ' 1 ' 1 2M+1 1 D H 13< Sound | Beamspace Drivers} Inputs : Matrix 25 __] B i i I i) 1 i) 1 ! I 1 1 be ee ee eee ee ee ee ee ee eee ' Volume 2? Control 25 25 25. 25 25 FIGURE 6B
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US 9,319,794 B2 Sheet 5 of 11 Apr. 19, 2016 U.S. Patent 11 2a 1 15 B, 156 12 < ~ ee = é) a FIGURE 7B 1S) | <3 =4 =) e) — <= B, ati
Page 8
U.S. Patent Apr. 19,2016 Sheet 6 of 11 US 9,319,794 B2 FIGURE 8A FIGURE 8B H Order aa : FIGURE 8C Si oe
Page 9
U.S. Patent Apr. 19,2016 Sheet 7 of 11 US 9,319,794 B2 0°%o Image 12-4 Directional Loudspeaker ; 12 1st Order FIGURE 10
Page 10
U.S. Patent Mean Square Error Apr. 19, 2016 Sheet 8 of 11 US 9,319,794 B2 40 42 46 10°>-—+ _\ “ay ~ _\ iD —No Walls + = =North Wall only isee 4 Walls, 7=.9 or (rn ae Se Od CCCI 4 Walls, 7=.5 10 —=UCA, 4 Walls, ¥=.9 10° 10° — 0 10 20 30 40 50 60 70 80 90 Loudspecker Weight Energy(dB) Panning Angle (°) FIGURE 11A 40 42 ; \ ty) 46 direct 1st order ““O 10 20 30 40 50 60 70 80 90 Panning Angle (°) FIGURE 11B
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U.S. Patent Apr. 19,2016 Sheet 9 of 11 US 9,319,794 B2 40” 20 Panning Angle (°) 0 2 10 15 DRR (dB) FIGURE 12A FIGURE 12B
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U.S. Patent Apr. 19,2016 Sheet 10 of 11 US 9,319,794 B2 FIGURE 13
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U.S. Patent Apr. 19,2016 Sheet 11 of 11 US 9,319,794 B2 12b 12a 12d FIGURE 14
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US 9,319,794 B2 1 SURROUND SOUND SYSTEM. FIELD OF THE INVENTION The present invention relates to a surround sound system. for reproducing a spatial sound field within a room. BACKGROUND TO THE INVENTION S Inhome theatre, typical surround sound is performed using, 5 or 7 loudspeakers plus a subwoofer, such as in the Dolby surround format. Such surround sound systems are able to create direct fields from various directions and ambient (dif- fuse) fields, but they cannot perform a full ambisonies repro- duction that is required to recreate a sound over a spatial area or volume. The more high-end and complex ambisonics surround sound systems typically require a large circular or spherical arrangement of loudspeaker drivers surrounding the sound control region to reproduce a spatial sound field. However, the 2 requirement for such large arrays of loudspeakers is not com- patible with the demands for compact surround sound sys- tems in home theatre and entertainment systems. A fundamental challenge to sound field control is the pres- ence of room reverberation. Many current surround sound systems simply ignore the presence of room reverberation, although there are some possibilities for avoiding reverbera- tion or cancelling reverberation outside the sound control region [4-8,22] In this specification where reference has been made to 3 patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the inven- tion. Unless specifically stated otherwise, reference to such external documents is not to be construedas an admission that 3 such documents, or such sources of information, in any juris- diction, are prior art, or form part of the common general knowledge in the art. It is an object of the present invention to provide an 8 8 improved compact surround sound system that is capable of 40 reproducing spatial sound fields with a reduced number loud- speakers, or to at least provide the public with a useful choice. SUMMARY OF THE INVENTION & Ina first aspect, the present invention broadly consists in a surround sound system for reproducing a spatial sound field in a sound control region within a room having at least one sound reflective surface, comprising: multiple steerable loud- speakers located about the sound control region, each loud- 5: speaker having a plurality of different individual directional response channels being controlled by respective speaker input signals to generate sound waves emanating from the loudspeaker with a desired overall directional response cre- ated by a combination of the individual directional responses; 5. and a control unit connected to each of the loudspeakers and which receives input spatial audio signals representing the spatial sound field for reproduction in the sound control region, the control unit having pre-configured filters for fil- tering the input spatial audio signals to generate the speaker 6 input signals for all the loudspeakers to generate sound waves with co-ordinated overall directional responses that combine together at the sound control region in the form of either direct sound or reflected sound from the reflective surface(s) of the room to reproduce the spatial sound field, the filters being pre-configured based on acoustic transfer function data rep- resenting the acoustic transfer functions measured in the s 2 sound control region from the individual directional responses of each of the loudspeakers at their respective loca- tions in the room. Preferably, the input spatial audio signals may be in an ambisonics-encoded surround format that is received and directly filtered by the filters in the control unit to generate the speaker input signals for the loudspeakers. Alternatively, the input spatial audio signals may be in a non-ambisonics sur- round format and the control unit further comprises a con- verter that is configured to convert the non-ambisonics input signals into an ambisonics surround format for subsequent filtering by the filters in the control unit to generate the speaker input signals for the loudspeakers. Preferably, the control unit may be switchable between a configuration mode in which the control unit configures the filters for the room and a playback mode in which the control unit processes the input spatial audio signals for reproduction of the spatial sound field using the loudspeakers. Preferably, the control unit may comprise a configuration. module that is arranged to automatically configure the filters in the configuration mode based on input acoustic transfer function data for the room that is measured by a sound field recording system. Preferably, the input acoustic transfer function data for the room may be measured by a sound field recording system comprising a microphone array located in the sound control region and the acoustic transfer function data represents the acoustic transfer functions measured by the microphone array in response to test signals generated by each of the loudspeak- ers for each of their directional responses. More preferably, the configuration module may receive raw measured acoustic transfer function data from the sound field recording system and converts it into an ambisonics representation of the acoustic transfer function data which is used to configure the filters of the control unit. Preferably, the filters of the control unit may be ambisonics loudspeaker filters. In one form, the surround sound system may be configured. to provide a 2-D spatial sound field reproduction in a 2-D sound control region. Preferably, the sound control region may be circular and has a predetermined diameter. More preferably, the sound control region may be located in a horizontal plane and the loudspeakers are at least partially co-planar with the sound control region. Preferably, each loudspeaker may be located within a respective loudspeaker location region, the room being radi- ally and equally segmented into loudspeaker location regions about the origin of the sound control region based on the number of loudspeakers, and wherein each loudspeaker region is defined to extend between a pair of radii boundary lines extending outwardly from the origin of the sound con- trol region. Preferably, the angular distance between each pair of radii boundary lines may correspond to 360°/L, where L is the number of loudspeakers. Preferably, each loudspeaker may be spaced apart from every other loudspeaker by at least half of a wavelength of the lowest frequency of the operating frequency range of the surround sound system. This condition will ensure de-corre- lated room excitations above the Schroeder frequency. Preferably, each loudspeaker may be spaced apart from any reflective surface(s) in the room by at least quarter of a wave- length of the lowest frequency of the operating frequency range of the surround sound system. Preferably, each loudspeaker may be spaced at least 0.5 m from the perimeter of the sound control region. More prefer- ably, each loudspeaker may be spaced at least 1 m from the perimeter of the sound control region.
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US 9,319,794 B2 3 Preferably, each loudspeaker may be configured to gener- ate overall directional responses having up to M” order direc- tivity patterns, where M is at least 1. More preferably, each loudspeaker may be configured to generate overall directional responses having up to M“ order directivity patterns, wherein Mis equal to 4. Typically, the value 2M+1 corresponds to the number of individual directional response channels available for each loudspeaker. Preferably, each loudspeaker comprises at least an indi- vidual directional response channel corresponding to a first order directional response. In one form, each loudspeaker may comprise at least indi- vidual directional response channels corresponding to 2M+1 phase mode directional responses. In a preferred form, each loudspeaker may comprise at least individual directional response channels corresponding to an omni-directional response, and cos(mo) and sin(mp) for m=1,2,...,M, and where @ is equal to the desired angular direction of the loudspeaker overall directional response rela- 2 tive to the origin of the loudspeaker. Preferably, the overall directional response of each loud- speaker may be steerable in 360° relative to the origin of the loudspeaker. Preferably, each loudspeaker may comprise multiple driv- ers configured in a geometric arrangement within a single housing, each driver being driven by a driver signal to gener- ate sound waves, and wherein each loudspeaker further com- prises a beamformer module that may be configured to receive and process the speaker input signals corresponding to the individual directional response channels of the loud- speaker and which generates driver signals for driving the loudspeaker drivers to create an overall sound wave having the desired overall directional response. Preferably, each loudspeaker may comprise a housing within which a uniform circular array of monopole drivers of apredetermined radius are mounted, and wherein the number of drivers and radius may be selected based on the desired maximum order of directivity pattern required for the loud- speaker. More preferably, the monopole drivers may be spaced apart from each other by no more than half a wave- length of the maximum frequency of the operating frequency range of the surround sound system. Preferably, the surround sound system may comprise at least four steerable loudspeakers. Preferably, the control unit may be configured to automati- cally step-up the order of the directivity patterns of the overall directional responses of the loudspeakers as the frequency of the spatial sound field represented by input spatial audio 5 signals increases to thereby maintain a substantially constant size of sound control region. Preferably, the contro] unit may be configured to automati- cally step-up the order of the directivity pattern of the overall directional responses of the loudspeakers at predetermined frequency thresholds in the operating frequency range of the surround sound system, the thresholds being determined based on the number of loudspeakers and the desired size of sound control region. Preferably, the loudspeakers may be equi-spaced relative to each other about the sound control region. More preferably, the loudspeakers may be sparsely located about the sound control region. Preferably, each loudspeaker may be located near a reflective surface, such as a wall in the room or in the vicinity of a corner of the room. Preferably, the spatial sound field may be represented in the sound control region by direct sound in combination with first a 35 40 4 65 4 order, second order, and/or higher order reflections from sound waves reflected off one or more reflective surfaces of the room. Preferably, the surround sound system may be config- urable to reproduce higher order ambisonics spatial sound fields. Preferably, the diameter of the sound control region may be at least 0.175 m. Typically, the diameter of the sound control region may be in the range of about 0.175 m to about 1 m. In another form, the surround sound system may be con- figured to provide a 3-D spatial sound field reproduction in a 3-D sound control region. More preferably, the 3-D sound control region may be spherical in shape. It will be appreciated that other shapes of 2-D and 3-D sound control regions could alternatively be used, but typi- cally using a sound control region that is a circular (spherical) shape in 2-D (3-D) is most efficient due to the physics regard- ing sound field reproduction. Ina secondaspect, the present invention broadly consists in an audio device for driving multiple steerable loudspeakers to reproduce a spatial sound field ina sound control region, each loudspeaker having a plurality of different individual direc- tional response channels being controlled by respective speaker input signals to generate sound waves emanating from the loudspeaker with a desired overall directional response created by a combination of the individual direc- tional responses, and where the loudspeakers are located about a sound control region in a room having at least one sound reflective surface, the device comprising: an input interface for receiving input spatial audio signals represent- ing a spatial sound field for reproduction in the sound control region; a filter module comprising filters that are configurable based on acoustic transfer function data representing the acoustic transfer functions measured in the sound control region from the individual directional responses of each of the loudspeakers at their respective locations in the room, and which filter the input spatial audio signals to generate speaker input signals for all the loudspeakers to generate sound waves with co-ordinated overall directional responses that combine together at the sound control region in the form of either direct sound or reflected sound from the reflective surface(s) of the room to reproduce the spatial sound field; and an output interface for connecting to all the loudspeakers and for send- ing the speaker input signals to the loudspeakers. In one form, the input interface may be configured to receive input spatial audio signals in an ambisonics-encoded surround format for direct filtering by the filters of the filter module to generate the speaker input signals for the loud- speakers. In another form, the input interface may be configured to receive input spatial audio signals in a non-ambisonics sur- round format and which further comprises a converter that is configured to convert the non-ambisonics input signals into an ambisonics surround format for subsequent filtering by the filters of the filter module to generate the speaker input signals for the loudspeakers. Preferably, the device may be switchable between a con- figuration mode in which the device configures the filters of the filter module for the room and a playback mode in which the device processes the input spatial audio signals for repro- duction of the spatial sound field using the loudspeakers. Preferably, the device may further comprise a configura- tion module that is arranged to automatically configure the filters of the filter module in the configuration mode based on input acoustic transfer function data for the room that is measured by a sound field recording system.
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US 9,319,794 B2 5 Preferably, the input acoustic transfer function data for the room may be measured by a sound field recording system comprising a microphone array located in the sound control region and the acoustic transfer function data represents the acoustic transfer functions measured by the microphone array in response to test signals generated by each of the loudspeak- ers for each of their directional responses. Preferably, the configuration module may receive raw mea- sured acoustic transfer function data from the sound field recording system and converts it into an ambisonics repre- sentation of the acoustic transfer function data which is used to configure the filters of the filter module. Preferably, the filters of the filter module may be ambison- ics loudspeaker filters. The second aspect of the invention may have any one or more of the features mentioned in respect of the first aspect of the invention. The phrase “direct sound” in this specification and claims is intended to mean sound waves propagating directly from 2 the loudspeaker into the sound control region without reflec- tion of any reflective surfaces. The phrase “reflected sound” in this specification and claims is intended to mean sound waves propagating indi- rectly from the loudspeaker into the sound control region after being reflected off one or more reflective surfaces, whether 1° order reflections, 2” order reflections, or higher order reflec- tions, such that the sound waves appear to be arriving from virtual sound sources not corresponding to the loudspeakers. The term “comprising” as used in this specification and claims means “consisting at least in part of”. When interpret- ing each statement in this specification and claims that includes the term “comprising”, features other than that or those prefaced by the term may also be present. Related terms such as “comprise” and “comprises” are to be interpreted in the same manner. As used herein the term “and/or” means “and” or “or”, or both. As used herein “(s)” following a noun means the plural and/or singular forms of the noun. The invention consists in the foregoing and also envisages constructions of which the following gives examples only. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will be described by way of example only and with reference to the drawings, in which: FIG. 1 is a schematic diagram of the surround sound sys- tem in accordance with an embodiment of the invention, in 5 playback mode; FIG. 2 isa schematic diagram ofa central control unit of the surround sound system in accordance with an embodiment of the invention; FIG. 3 is a schematic diagram of the surround sound sys- tem in accordance with an embodiment of the invention, ina configuration mode using a microphone array sound field recording system; FIG. 4 is a schematic diagram ofa microphone array sound field recording system for measuring acoustic transfer func- tion data for the surround sound system in its configuration mode in accordance with an embodiment of the invention; FIG. 5 is a schematic diagram of the configurable loud- speaker filters in the central control unit in accordance with an embodiment of the invention; FIG. 6A is a schematic diagram of a steerable loudspeaker in accordance with an embodiment of the invention; 8 w 65 6 FIG. 6B is aschematic diagram of the driver array configu- ration for a steerable loudspeaker in accordance with an embodiment of the invention; FIG. 7A is a schematic diagram of another possible geo- metric arrangement of four loudspeakers of the surround sound system in the form ofa corner-like configuration about a sound control region in a room in accordance with an embodiment of the invention; FIG. 7B is a schematic diagram of a possible geometric arrangement of four loudspeakers of the surround sound sys- tem in the form ofa diamond-like configuration about a sound control region in a room in accordance with an embodiment of the invention; FIG. 7C is a schematic diagram of a possible geometric arrangement of five loudspeakers of the surround sound sys- tem in the form ofa Dolby-surround-like configuration about a sound control region in a room in accordance with an embodiment of the invention, FIGS. 8A-8C are schematic diagrams depicting the first and second order image-sources for the respective loud- speaker arrangements of FIGS. 7A-7C; FIG. 9 is a schematic diagram of another geometric arrangement of loudspeakers of the surround sound system about a sound control region in a room in the form ofa corner array in accordance with an embodiment of the invention; FIG. 10 is a schematic diagram of the corner array surround, sound system of FIG. 9 and various possible direct sound and reflected sound waves from the steerable loudspeakers; FIGS. 11A and 11B show graphical representations of mean square error and loudspeaker weight energy respec- tively against panning angle for a performance comparison between a conventional uniform circular array of loudspeak- ers and a corner array surround sound system in accordance with an embodiment of the invention; FIGS. 12A and 12B show graphical representations of mean square error against phantom panning angle and direct- to-reverberant ratio (DRR) for performance comparison between a convention uniform circular array of loudspeakers and a corner array of the surround sound system in accor- dance with an embodiment of the invention respectively; FIG. 13 shows a schematic diagram of the beampatterns required from the loudspeakers in a corner array geometric configuration of the surround sound system to place a phan- tom source in-line with a direct ray D and in-line with a reflected ray R; and FIG. 14 shows screen shots of wave propagation generated by a corner array surround sound system for generating a sound wave propagating into the sound control region from an angle of 45° in the plane. DETAILED DESCRIPTION OF PREFERRED. EMBODIMENTS 1. Overview The present invention relates to a surround sound system for reproducing a spatial sound field in a room, typically for domestic home entertainment systems. The surround sound system is scalable to suit rooms of varying size and shape. Typically the room is substantially enclosed by a floor and ceiling, and comprises at least one but preferably multiple sound reflective or reverberant surfaces, typically provided by a wall(s) defining the room or other vertical surface adjoin- ing the floor and ceiling. The levels of reverberation are measured by the critical reverberation distance which repre- sents the distance from a source at which the reverberant and direct sound energies are equal. In an average living room or
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US 9,319,794 B2 7 bedroom, this distance is typically 50 cm to 1 meter. Any further than the critical reverberation distance, sound energy is dominated by the reverberation. In brief, the surround sound system is configured to gen- erate spatial or surround sound by creating the impression that sound is coming from one or more intended directions. Referring to FIG. 1, the system comprises a small array of configurable loudspeaker units 12 that surround or are located in a spaced-apart geometric arrangement, random or orga- nized, about a sound control region 11 in the room within which the listener or listeners 15 are located. In this embodi- ment, all the loudspeakers are located relative to the sound control region such that they at least have a direct sound path to the sound control region. The loudspeakers 12 are each configurable or steerable in that they have variable directional responses that can be controlled by the speaker input signals 13 which control them. The system further comprises a con- trol system or unit 14 that generates the speaker input signals for driving all the loudspeakers 12 in a co-ordinated manner to generate sound waves with particular directional responses that combine together in the sound control region 11 to repro- duce a spatial sound field in that region based on an input audio spatial signal 16 representing the spatial sound field to be reproduced. The central contro] unit is configured to use all loudspeakers in reproducing the spatial sound field by utilis- ing direct sound waves directed into the sound control region from one or more of the loudspeakers in combination with reflected or reverberant sound waves directed into the sound control region 11. The reflected sound waves are generated by the loudspeakers directing sound waves at reflective or rever- berant surfaces, such as walls in the room. The reflected sound may have undergone one, two or multiple reflections before propagating into the sound control region. The pur- pose of the reflected sound waves is to exploit the room’s natural reverberation to create additional acoustic impres- sions or acoustic sound directions from what appear to be virtual sound sources thereby enabling a full spatial sound field reproduction without requiring a large array of speakers surrounding the listener from all directions. The surround sound system could be implemented with a 2-D spatial sound field reproduction or a more complex 3-D sound field reproduction. The example embodiments of the surround sound system to be described focus on the 2-D implementation with the sound control region located in a substantially horizontal plane in space within the room envi- ronment and with the array of loudspeakers located in sub- stantially the same plane in space, but the design modifica- tions required for providing a 3-D implementation will also be discussed, which may involve a spherical sound control region and employing loudspeakers in locations on the ceiling 5 and floors. More specifically, in this specification unless the context suggests otherwise, 2-D spatial sound field reproduction is intended to relate to reproduction of the spatial sound ina 2-D sound control region, typically circular, which may have a desired predefined height or thickness vertically, and in which the surround sound system may typically comprises a circular array of loudspeakers surrounding the 2-D sound control region and which are arranged to propagate sound waves horizontally into the sound control region. The thickness of the 2-D sound control region may be determined by the loud- speaker vertical dimensions, or whether the loudspeakers are vertical line arrays or electrostatic loudspeakers that are capable of propogating sound waves horizontally toward the sound control region over a vertical range corresponding to the thickness of the 2-D sound control region. In this speci- fication, unless the context suggests otherwise, 3-D spatial 0 a w 8 w 40 45 8 sound field reproduction is intended to relate to the spatial sound in a 3-D sound control region, typically a spherical region, and in which the surround sound system may com- prise a spherical array of loudspeakers surrounding the 3-D sound control region and which are oriented or configured to propagate sound waves into the 3-D sound control region at any desired elevation angle, whether horizontal, vertical or any other angle. In this embodiment, the control unit 14 has two modes of operation, a configuration mode and a playback mode. The configuration mode must be operated at least once before the playback mode can operate effectively. During set-up of the surround sound system, the configuration mode is initiated once all the loudspeakers are positioned about the sound control region in the room. The configuration mode custom- ises the performance of the system to the loudspeaker layout and reverberance properties of the room so as to configures the responses of the loudspeakers to exploit the natural reverabaration in the room, and to use both the direct sound path and available reverberant reflections to reproduce the spatial sound field represented by an input spatial audio signal when in playback mode. Once configured, the system can be switched into playback mode for sound field reproduction. The system typically remains in playback mode until the loudspeaker positions are altered or the room reverberation properties changed in any way, in which case the configura- tion mode is typically re-initiated to re-calibrate the system for the new set-up or environment. FIG. 1 shows the system in the playback mode. The system receives input spatial audio signals 16 representing the spatial sound field for reproduction and processes that input signal to generate and deliver 2M+1 speaker input signals 13 over wiring or wirelessly to each of a number L of “smart” con- figurable loudspeaker units 12 represented by the pentagonal boxes, which then play out directional sound for reconstruct- ing the spatial sound filed in the sound control region. The input spatial audio signal may be in any format, including by way of example ambisonics or Dolby surround or any other spatial format. The number M represents the order of the directional responses achievable by each loudspeaker 12 and this may be altered to suit system requirements as desired. By way of example only, the system is capable of repro- ducing a full ambisonics sound field, but also emulating or reproducing other spatial sound signal formats, including Dolby surround and others. The surround sound system may be a stand-alone system that receives the input spatial audio signals 16 from another audio playback device, Personal Computer, or home theatre or entertainment system, or may be integrated as a component or functionality of such systems or devices. The various components and mode operations of the sur- round sound system will now be individually described in more detail. 2. Control Unit Referring to FIG. 2, the control unit 14 will be described in more detail. During playback mode, the control unit 14 receives the input spatial audio signals 16 and comprises pre-configured filters 17 that are arranged to filter the input signals 16 into speaker input signals 13 for driving each of the loudspeakers 12 to generate sound waves with a desired directional response for recreating the spatial sound field in the sound control region. In this embodiment, the control unit is configured to work in an ambisonics sound format and comprises ambisonics loudspeaker filters.
Page 18
US 9,319,794 B2 9 In this embodiment, the input spatial audio signals 16 containing the spatial audio information is delivered to the control unit 14 as several input sound channels. By way of example, it may be composed of (i) ambisonically-encoded sound information, (ii) spatial information on the phantom source location(s) from which each sound channel will be played, or (iii) one of a variety of surround-formatted signals. By way of example only, the surround multi-format signals could include: stereo, Dolby Digital™, DTS Digital Sur- round™, THX Surround EX, DTS-ES and others. In this embodiment, the control unit 14 is configured to receive either an ambisonically-encoded input signals 16a or one or more other formats of surround-encoded input signals 166. The ambisoncially-encoded 1 6a input signals are filtered directly by the filters 17, while other format signals 16d are first processed by an ambisonics converter 18 and converted into an ambisonics format for subsequent processing by the filters 17. It will be appreciated that other embodiments of the control unit need not necessarily provide this multi-format input capability and may provide only one format of input 2 signal if desired. In operation, the central control unit 14 processes and delivers each of the excitation input signals to the directional response components of each smart loud- speaker unit 12 for playback of and reproduction of the spatial sound field. As previously discussed, the pre-configured filters 17 are configured or customised for the arrangement of loudspeak- ers 12 and room reverberation characteristics in the configu- ration mode. This is achieved by measuring acoustic transfer functions for each of the loudspeaker directional responses in the sound control region, which will be explained in further detail later. The signal processing performed by the central control unit 14 and the storage of acoustic transfer functions in the ambisonically-encoded spatial sound format will be described in further detail below. As shown, the control unit 14 also comprises a configura- tion module in the form ofa surround sound processor 19 that is configured to measure the acoustic transfer functions to the sound control region at a number of frequencies in the con- figuration mode of the system and then configure the filters 17 based on those measured acoustic transfer functions. As shown in FIG. 3, the acoustic transfer functions of each loud- speaker channel are best obtained using a microphone array 20 located in the sound control region. The configuration mode involves generation of test signals and playing through each channel of each smart loudspeaker and converting the resulting microphone array signals into an ambisonic repre- sentation of the acoustic transfer functions. As mentioned above, the acoustic transfer functions are then used to con- figure each of the ambisonic loudspeaker filters 17. The ambisonics input signal 16, surround sound processor 19, ambisonics converter 18, and ambisoncics loudspeaker filters 17 will each be described in further detail below. 2.1 Ambisonics Input Signal The central control unit 14 requires information regarding the spatial placement of the sound. Ambisonics pertains to the representation of a spatial sound field. Ambisonics has both 2-D and 3-D versions. The B-format recording is one of the earliest realizations of ambisonics, which records the sound pressure and 3 components of velocity at a point in space, then reproduces the sound field using an array of loudspeakers [9]. For 2-D reproduction, only two components of velocity are measured. The ambisonics B-format thus consists of 3 signals in 2-D (pressure plus two components of velocity) and 4 signals in 3-D (pressure plus three velocity components). This sound field is reproduced accurately over a large area only at low frequencies. Since the area of accurate reproduc- w 35 40 4 5C B 10 tion reduces with frequency, this spatial sound reproduction is inadequate over much of the audible frequency range. For a disc-shaped (2-D) or spherical control region (3-D) the radius for accurate reproduction is only R=s,/21f=55 mm at 1 kHz where s, is the speed of sound. For sound field reconstruction over a larger area, one may use Higher Order Ambisonics (HOA), which is adopted in the surround sound system of the invention. In HOA, the sound field at each point (r,p) over a circular region at frequency f can be written in terms of the ambisonics expansion about the origin: uv a Pr BLD = YY Balf Mather iN where J,,(-) is the Bessel function of order n, B,,(f) is the 2-D ambisonics coefficient at frequency f, k=2zf/s, is the wave number and N is the order of the ambisonics field related to the radius of the circular region by R=Ns,/2zf (Fora B-format recording, N=1). We record the sound field by measuring the coefficients over a finite range n=-N, . . . , N producing the Nth order ambisonics signal set. One requires at least 2N+1 drivers to reproduce the Nth order HOA in 2-D. The sound field at each point (r,0,) over a 3D spherical region can be written in terms of the ambisonics expansion about the origin: cae Q) Por, OID = YY) BE MalkrDYP (0, d) 1 era where j,(-) is the spherical Bessel function of order n, Y”(-) is the spherical harmonic function and B,?(f) is the 3-D ambisonics coefficient. One requires at least (N+1)° drivers to reproduce the Nth order HOA in 3-D. There are equivalent ambisonic representations to the com- plex angular functions e””” (2-D) or Y,’(0.) (3-D) which are real. Either the real or the complex functions could be used in the surround sound system of the invention. Real representa- tions have implementation advantages but are easy to obtain from the complex functions [11]. Alternatively to ambisonics, the input audio signal spatial information delivered to the central contro] unit 14 could consist of a number of sound channels, each for several phan- tom source, each channel additionally having the following specified: (i) a polar orientation angle 9 for a 2-D system, (ii) an orientation angle pair consisting of an azimuth angle and elevation angle @ for a 3-D system, and (iii) an optional phantom source range r. There are standard equations for converting such spatial sound information into an ambisonics format. Such equations shall be used to reconstruct the sound fields up to Nth order ambisonics for the loudspeaker location of a Dolby Surround, DTS or other commercial surround system. 2.2 Surround Sound Processor and Configuration Mode As mentioned above, the surround sound processor 19 of the control unit 14 is operable to receive and process acoustic transfer function data 21 representing the acoustic transfer functions measured during the configuration mode by the microphone array 20. At a general level, to determine the acoustic transfer functions in the room, a number of test
Page 19
US 9,319,794 B2
at
signals are played out of each smart loudspeaker, and the
response recorded by the central control unit 14 using a
microphone array.
For determining each of the acoustic transfer functions, a
test signal 22 is generated and directed to each channel of
each smart loudspeaker. Each channel of the loudspeaker
generates a different directional response. The impulse
response to each microphone in the microphone array is then
measured. The test signal used may be a pulse signal, but
more practically a wideband chirp or Maximum Length
Sequence signal may be used. The filters 17 can then be
configured in the frequency domain, using just the positive
frequencies, so it is possible to measure the complex
ambisonics coefficients of the acoustic transfer functions.
Ambisonics is an efficient means of storing the acoustic trans-
fer function for each channel of each smart loudspeaker at a
number of frequencies. This contro] unit 14 stores the acous-
tic transfer function data in the form of the ambisonic loud-
speaker filters 17 after signal processing to be detailed below.
In brief, the surround sound processor 19 takes the measured
acoustic transfer function data, applies FFT and mode
weighting matrices, then does a matrix inversion before it
stores the data into the ambisonic loudspeaker filters 17.
More particularly, the surround sound processor 19 is con-
figured to receive and convert the raw microphone array
acoustic transfer function data at each frequency into the
(ambisonic) modal decomposition of the acoustic transfer
functions in equations (3) and (5) below, in 2-D by using a
FFT matrix 23 followed by a phase mode weighting matrix 24
dependent on the array radius and type of housing [4] or in
3-D by using a spherical harmonic transform matrix followed
by a 3-D mode weighting matrix [14]. The surround sound
processor is then arranged to configure the ambisonic loud-
speaker filters 17 based on the measured and processed acous-
tic transfer function coefficients, and which is explained in
further detail below.
The use of a microphone array for sound field recording is
known by those skilled in the art. Any suitable microphone
array design may be used that is capable of measuring the
acoustic transfer functions from each loudspeaker to any
point in the sound control region [1-4]. 2-D implementation
may use a uniform circular array geometry 20 as shown in
FIG. 4. A 3-D implementation may use a spherical array. At
least Q=2N+1 elements for 2-D and Q=(N+1)? elements in
3-D are required where N=kr, arranged at radius comparable
to the desired size of the sound control region 11. In a 2-D
embodiment, there may be advantage in using directional
microphones that are pointed horizontally along the plane of
the control region, so that reverberation due to lateral reflec- 5
tion could be reduced.
As mentioned, the computation and configuration of the
ambisonic loudspeaker filters 17 for sound reproduction is
implemented within the Surround Sound Processor 19. This
process for the 2-D implementation is first explained, fol-
Jowed by the 3-D implementation. It is desired to reproduce a
number of ambisonic sound fields using a set of L smart
loudspeakers.
For the 2-D implementation, consider a sound field with
expansion about an origin given by ambisonics expansion in
equation (1). The ambisonics coefficients of the desired
sound field are ,,(f) expressed in the frequency domain. The
control unit 14 requires a set of acoustic transfer functions for
each loudspeaker. The acoustic transfer functions are effi-
ciently stored as a set of ambisonically-encoded modal coef-
ficients a,,(l.mlf) defined in terms of the sound field created
by the mth directional response of each loudspeaker 1:
a
8
0
40
B
12
N @)
Hyults & f= YY atl ml trie”.
_N
The coefficients «,,(Imlf) are measured in the configuration
mode of operation at the intended listening position with aid
of the microphone array 20. A total of (2M+1)L sets of 2N+1
coefficients are produced.
As mentioned, the surround sound processor 19 of the
central control unit 14 determines the loudspeaker filters to be
applied to the spatial audio signals based on the measured
acoustic transfer functions. In a preferred embodiment, the
loudspeaker filters are designed to reconstruct the nth spatial
sound mode J, (kr)e”®. We determine the loudspeaker filters
G,,(mlf) to recreate each nth spatial mode as follows: The
sound pressure resulting in the room from the loudspeaker
weights for creating the nth mode {G,(mlf): m=l,...,
2M+1, I=1... L} is:
LM
Inllone”® = YY) Gall m| fein 8 1S).
FL pel
Substituting in equation (3), we determine an equation for
determining each loudspeaker filter:
LM
DD) Galle Aaretls mL Ayre" *,
1ST ml
In kre"? =
=n
which by orthogonality of complex exponentials is satisfied if
the following set of equations are satisfied:
nan
Laue L,
DD aul ml Galt min={
ak 0, otherwise,
for n'=-N, ..., N. This set of 2N+1 equations can be written
in matrix-vector form:
ANSD-Eny
where [AOI aver Real j2a+1)4m 70 (Lmlf),
[g,Pl@iyeae1y-G, (mf) and e, is an 2N+1-long vector
where element n+N+1 is one and all other elements are zero.
Here [M],, denotes the element in the ith row an jth column in
matrix M whilst [v], denotes the ith element of vector v. Vector
g,(f) contains the L(2M+1) loudspeaker filter weights at fre-
quency fto apply to the configurable loudspeaker channels to
create the spatial mode corresponding to the nth ambisonic
coefficient. As a result, a matrix G(f) [g_{f). gevai(f...-.
2\(f)], whose 2N+1 columns are the loudspeaker weight vec-
tors for creating the ambisonic spatial sounds at frequency f
up to order N, can be determined by taking the regularized
pseudo-inverse of A(f) through the Tikhonov-regularized
least squares. The matrix A(f) is long, since a robust solution
would entail using more drivers, L(2M+1), than the 2N+1
reproducible ambisonic channels. As a result the solution is:
GHANIAN ANT
where A is a single regularization parameter. The parameter
may either be tuneable or have a fixed value selected in the
device.
(4)Page 20
US 9,319,794 B2
13
The required filters to create the 2-D ambisonics spatial
sound field are shown to be related to the 2M+1 acoustic
transfer function coefficients for each of the L configurable
loudspeakers. There are L(2M+1) acoustic transfer functions
for each mode. The Surround Sound Processor 19 hence
determines the ambisonics loudspeaker filters directly from
the measured acoustic transfer function coefficients.
The approach presented here represents a frequency-do-
main approach, where the output is a collection of loud-
speaker weights at a number of frequencies. This approach
culminates in a time-domain approach, where the output is a
collection of time-domain filters. The solutions may be cal-
culated at each frequency, and the inverse FFT used to pro-
duce the required digital filter for filtering the nth ambisonics
signal for the mth mode of the Ith loudspeaker.
Ina3-D implementation, the desired spatial sound field can
be written as equation (2) where (1) is now an ambisonics
coefficient of the desired sound field. The acoustic transfer
functions are efficiently stored as a set of ambisonically-
encoded modal coefficients o.,”(I.mlf) defined in terms of the
sound field created by the mth directional response of each
loudspeaker I:
ny ¢
Hylts 0. 63.9) = 2 Dell ma] Prin Kkra¥G(@, 6)
omg
In a preferred embodiment, the loudspeaker filters are
designed to reconstruct the (p.q)th ambisonic spatial sound
mode j (kr)Y ,?(p.0). We determine the loudspeaker weights
G,?(Lmif) to recreate each spatial mode (p,q) at frequency fas
follows. The sound pressure resulting in the room from loud-
speaker weights is:
L (M+
JkOVPO, 6)= YY) GEL m| Hint, 881):
©
Substituting in equation (5), we obtain an equations for
determining the (p,q)th loudspeaker filter
aes
pg
mn?
, wy
2, Gt | fra’ (l,m fig Yh (0, Oe
wa
N
Ul kNYE(0, O = Y
7
which by orthogonality of spherical harmonics is satisfied if
the following set of equations are true:
P
ara? , lL. pepa'eq
GPL, m| fia? (Lm | f) {
14 MID IG MID =D nccwise
Me
zy
for {(p'.q’): q’=0, 1,....N, p'=-q', ..., q'}. The set of (N+1)”
equations for each (p.q) can be written in matrix-vector form
as:
Ag Ome?
where [A 2vgepstc- Deer ?en Oy j (mip, — [g,?
(Olenaren=G/ (unify and e,? is an (N41)? -long vector
where element P P+geptl is one and the other elements are
zero. As a result, a matrix G(f)=[go°(). 2,7), .--. gy]
S
w
8
w
3
&
5
3:
2
s
14
whose (N+1)? columns are the loudspeaker weight vectors for
creating the ambisonic spatial sounds at each frequency up to
order N can be determined by taking the regularized pseudo-
inverse of A(f) through the Tikhonov-regularized least
squares. The matrix A(f) is again long, since a robust solution
would entail using more drivers L(M+1)* than the (N+1)?
reproducible spatial modes. The solution is again given by
equation (4).
The required filters to create the (p,q)th 3-D ambisonics
spatial sound field are again related to the (M+1)’ acoustic
transfer function coefficients for each of the L smart loud-
speakers corresponding to the same mode (p,q). There are
L(M+1) acoustic transfer functions for each mode.
2.3 Ambisonics Loudspeaker Filters
As mentioned above, the ambisonics loudspeaker filters 17
of the control unit 14 are configured for the room during the
configuration mode prior to switching to the playback mode
of the surround sound system. The filters may be digital
filters, such as Finite Impulse Response (FIR) filters for
example. The ambisonics loudspeaker filters 17 apply the
appropriate filtering to construct the appropriate spatial sound
field from each ambisonics input signal channel in playback
mode shown in FIG. 1.
In the 2-D embodiment of the system, the sound field
represented by coefficients {B,,(f): n=-N ... N} is reproduced
using several smart loudspeakers 12, each of which is capable
of generating 2M+1 polar responses, M being the order of the
directional response. In this embodiment, each configurable
loudspeaker may contain from M=1 to 4, although higher
order directional responses, e.g. up to 20 order orhigher still
may be required for higher operating frequencies. As shown.
in FIG. 5, performing this ambisonics reproduction requires a
set of loudspeaker filters for each ambisonics coefficient
B,,(f). For example, the Ambisonics Loudspeaker Filters 17
process ambisonic signals of the spatial sound field by the set
of configurable filters {G,(Lm;f): n=-N...N,F1...L,
. . 2M+1} to yield the output signals S(1,m;f) for each
channel m of each configurable loudspeaker ]. The number of
smart loudspeakers in FIG. 5 is L, numbers of configurable
channels on each loudspeaker is 2M+1 and numbers of
ambisonic coefficients is 2N+1 (where N is the order of the
ambisonics reproduction), making a total of L(2N+1)(2M+1)
loudspeaker filters required in the Ambisonics Loudspeaker
Filters box 17 of the Central Control Unit 14. As previously
5 discussed, the filters are set during the configuration mode by
the Surround Sound Processor 19. In a 3-D embodiment of
the system, the sound field is represented by coefficient {B,,”
(f): m=-n...n,n=0...N}. This is completely analogous to
the 2-D case but for Mth order, each smart loudspeaker must
be capable to generate (M+1)° 3-D directional responses, and
requires a total of L(N+1)°(M+1)? loudspeaker filters
required for the Ambisonics Loudspeaker Filters box 17.
By way of example only, to reconstruct sounds at 1 kHz (2
kHz) in a disc of diameter 60 cm (30 cm) sound control
region, at least an ambisonics order of N=6 is required. The
numbers of temporal loudspeaker filters for any conceivable
6” order 2-D ambisonics reproduction system are: 156sL
(2N+1) (2M+1)<936 for L=4 to 8 configurable loudspeakers,
and where M=1 to 4 in this embodiment, although it will be
appreciated that the limits will alter ifhigher order loudspeak-
ers are employed. More loudspeaker filters are required if the
desire is to increase the size of the reproduction region
beyond what is mentioned here.
2.4 Ambisonics Converter
In the embodiment shown in FIGS. 1 and 2, the central
control unit 14 is capable of processing a multi-format sur-
round signal 166 for reproduction with the surround soundPage 21
US 9,319,794 B2
15
system. The central contro] unit 14 comprises an ambisonics
converter module 18 that is configured to process a multi-
format surround signal into an ambisonics signal format for
processing by the filters 17 for playback over the loudspeak-
ers 12, as is the case with the direct ambisonic input signal
16a.
In one embodiment, the Ambisonics Converter 18 is used
for converting Dolby 5.1 surround signals 166 into ambison-
ics coefficients 18a to generate phantom sources positioned in
the standard five loudspeaker ITU geometry used in Dolby :
Digital and DTS Digital Surround. In an alternative embodi-
ment, the Ambisonics Converter 18 could also support stereo
sound or the seven loudspeaker layouts of THX Surround EX
and DTS-ES where the loudspeaker locations are different.
The converter 18 makes the surround sound system down-
ward compatibility with currently-available technologies.
By way of example, we show one possible method of
converting these surround sound formats into an ambisonic
format given the desired loudspeaker locations. For an acous- >,
tic monopole in 3-D, the sound pressure at point x=(r,0,)
truncated to Nth order ambisonics is:
exp{-ékllx = yl}
a
He)
ORI aT TD Marg, 6 lbGO. 8)
0 pg
where y=(r,.0,.9,) is the position of the monopole source and
S(f) is the transmitted sound signal. For an acoustic monopole
in 2-D, the sound pressure at point x=(r.p) for a monopole
source located at y=(r,,,) the Nth order ambisonic recon-
struction of the sound pressure is:
y
HP Cklbe= YI) =D Hk de* J lkrde™*,
where H,,°(-) is the Hankel function of the second kind of
order n. The ambisonics coefficients of an acoustic monopole
are hence B ”({)=ikh, (kr, LY,’ (0,.,)|*(-D embodiment)
and (=H, (kr, yerints (2+ D embodiment) multiplied by
the spectrum of the audio signal for playback. Whatever the
surround sound format, the ambisonics signals can be deter-
mined from a list of the format’s standard loudspeaker posi-
tions, the audio playback signals and depending upon the
format, perhaps the required loudspeaker directivity patterns.
3. Configurable Loudspeaker Design and Room
Arrangement
3.1 Design of Loudspeaker
Fach loudspeaker 12 is capable of creating a number of
configurable directional responses over a number of frequen-
cies, and may preferably have the capability of steerability of
the beam pattern in 360° in the 2-D implementation. Each
smart loudspeaker 12 is driven by several speaker input sig-
nals 13, each signal line drives a separate loudspeaker direc-
tional response. The loudspeakers 12 may provide onboard
amplification to each driving signal, or alternatively the
amplification may be provided in the central control unit or
other amplifier module(s), whether integrated with the central
control unit or each loudspeaker or provided as a separate
component.
FIGS. 6A and 6B shows a possible design of a loudspeaker
12 in an embodiment of the surround sound system. FIG. 6A.
a
35
40
4
35
6
16
shows a block diagram of a loudspeaker processing 2M+1
speaker input signals 13 to feed D drivers 25 through a master
volume control 26 and FIG. 6B shows a possible physical
construction of a smart loudspeaker with an outwardly ori-
ented symmetrical circular arrangement. While preferred, the
loudspeaker arrangements need not necessarily be circular,
spherical or cylindrical. An alternative geometry could in
theory be used, as long as it performs well. A frequency
domain embodiment of the unit is shown by virtue of using a
beamspace matrix 27 which processes and mixes the speaker
input signals 13 to generate the overall desired directional
response from the individual directional response channels.
As shown in FIGS. 6A and 6B, each smart loudspeaker 12
has a directivity response determined by beamformer drivers
(loudspeaker elements) and configured by the speaker input
signals 13. In this embodiment, the beamformer consists of a
loudspeaker beamspace matrix 27, which is embodied as
either:
1. A frequency domain implementation where a set of F
beamspace matrices operates on the input signals 13,
over F frequency subbands. Each beamspace matrix cre-
ates 2M+1 beam patterns intended for D drivers over the
frequency subband.
2. A time domain implementation where a matrix of time
domain filters creates 2M+1xF beam patterns over the
entire frequency band for the D drivers.
As mentioned, a series of D amplifiers 26 may be provided
for magnifying the signals to volume levels appropriate for
playback. The amplified signals are each delivered to a loud-
speaker (driver) co-located in common housing. In this
embodiment, the housing is compact and the driver 25 geom-
etry in each loudspeaker 12 is chosen to generate directional
patterns over a range of directions. A circular driver geometry
is shown in FIG. 6B for 2-D reproduction but for 3-D field
reproduction a spherical or cylindrical geometry would be
better suited.
The number of drivers and input channels 13 for the loud-
speakers 12 may vary depending on the surround sound sys-
tem playback requirements. For the surround sound system to
exploit room reflections, it is generally required for each
configurable loudspeaker to be able to create at least a M=1*
order directivity pattern, and preferably up to 4” order.
The loudspeakers 12 create directional responses up to Mth
order using a small number D of drivers (Dz2M+1 in 2-D and
Dz(M+1)° in 3-D). The 2-D implementation of the smart
loudspeaker might include (i) constructing the 2M+1 phase
mode directional responses {e”"*: m=-M, . . . , M}, (ii)
constructing an omni-directional response, as well as each of
the directional responses cos(m) and sin(m) for m=1,
.,M. Fora 3-D implementation, the smart loudspeaker
could construct an omni-directional response, as well as the
real parts {Re[Y,,"(0,)]: m=O ...n, n=1 ... M} and imagi-
nary parts {Im[Y,,”(0,9)]: m=1 ... n, n=l. . . M} of the
spherical harmonic functions. The Loudspeaker Beamspace
Matrix 27 and the geometric arrangement of the drivers
within the housing of the configurable loudspeaker unit 12 are
selected to create such directional responses over a wide
range of frequencies. These design aspects are further
described below.
The physical layout of the drivers within the loudspeaker
12 will now be described. The far-field directivity pattern
D,(gl£) of loudspeaker | at frequency f can be written as the
phase mode expansion:Page 22
US 9,319,794 B2
17
Me
DubLF)= Yi aml Hem
—M
where a,,(IIf) are the weighting coefficients for the nth order
phase mode. Each directional loudspeaker is realized by
arranging a number D of monopoles drivers into a uniform
circular array of radius r. To ensure loudspeaker responses up
to Nth order are obtainable, one designs each monopole array
choosing rand D as follows:
Choose r=M/k to excite a necessary number of spatial
modes, up to order M [16].
Choose Dz2M+1 to ensure adequate that number of
degrees of freedom are available to create the loud-
speaker responses.
This scheme ensures monopoles are spaced 4/2 or less
apart to avoid spatial aliasing at frequency f, corresponding to
the lowest frequency in the operating frequency of the sur-
round sound system. The array design may be constructed by
housing the D drivers inside a cylindrical loudspeaker box.
The driver weights are then chosen according to regularized
least squares to suit the sound field reproduction problem.
Typically, the audio operating frequency range of the sur-
round sound system is preferably in the range of 60 Hz-12
kHz, more preferably 30 Hz-20 kHz.
As discussed, the beamformer module of each loudspeaker
12 may be in the form of a beamspace matrix. Each loud-
speaker is designed to generate the 2M+1 directional
responses (2-D implementation) or (M+1)° responses (3-D
implementation) up to order M, using D drivers. By way of
example, the following illustrates the design for acoustic
monopole drivers in free-space in one embodiment of the
loudspeaker design. In alternative 2-D embodiments, the
drivers are mounted onto the equator of a hard cylinder or
sphere. Suppose each monopole d of a directional loud-
speaker at frequency fis excited by loudspeaker weight b,,,,{f)
where m=-,..., M and d=l, 2,..., D. To choose the
loudspeaker weights to construct the nth phase mode in the
far-field, it is necessary to match the directivity pattern e”"”
across the continuous angular range (e[0,27]:
>
Di Pnei etna’ = ele
a1
where 4,-[cos 9, sin 9,”, 0,, is the orientation angle of
monopole m and $=[cos 4, sin >|”. If the loudspeaker vector
for the D element array to construct the mth order phase mode °
is b,—[Dnis Dos» +s Ban)’ then b,, can be designed by
matching the directivity pattern at Q angles {1,2,---5 da}:
Ebne=Prm
where [p,.],-€”* is the vector of phase mode p, [E] ne "°*"*
is the matrix of beam steering vectors to each direction 6,,=
[cos 6,,, sin 8,,]”, @,=[c08 ,, sin p,]” and we choose @,=27
(q-1)/Q. Define the matrix of phase mode weights B=[b_,,
b_ya}+++- byl’, for which we obtain through the least squares
solution:
BrE'P
where P=[p_,p .. - . p,,] and E*=(E7E)"'E* is the pseudo-
inverse of E. The matrix B for each loudspeaker transforms
the 2M+1 phase mode weights into D driver weights.
The preferred directional responses for the channels of the
loudspeakers are an omnidirectional pattern, cos m6 patterns
S
a
w
8
w
40
60
18
and sin m@ patterns, (for m up to order M) are preferred.
However, also acceptable are the phase mode responses e’”"°
(for m equalling -M up to M).
3.2 Physical Arrangement of Loudspeakers in Room
FIGS. 7A-7C depicts various possible example plan view
configurations of loudspeakers 12 in an enclosed rectangular
room 5 in terms of the dimension distance of a loudspeaker
from a wall |,,,, distance of loudspeakers from each other
1,4, and distance of a loudspeaker from center of the sound
control region 1_,,,,45;- Shown are example four and five loud-
speaker geometries where the loudspeakers are adequately
spaced and roughly surrounding the sound control region.
The geometric arrangement may be varied depending on the
shape and configuration of the room, the number of loud-
speakers 12 provided in the surround sound system, and the
position and orientation of the sound control region 11. Gen-
erally, the geometric arrangement of the smart loudspeaker
array in the room may vary provided that is appropriate for
creating the spatial sound effects in a robust manner. Typi-
cally, the physical layout consists of several loudspeakers 12
positioned at several positions in the room around the sound
control region 11. To create the sensation of spatial sounds
robustly, one requires the smart loudspeakers 12 to be posi-
tioned to surround the sound control region.
Typically, the surround sound system will function with
L=4 to 8 configurable loudspeakers 12, although additional
loudspeakers may increase performance of the system in
certain environments.
Inpreferred embodiments, the room 5 is equally divided or
segmented radially about the origin 6 at the center of the
sound control region into loudspeaker location regions L,,
L,, . .. L,, where L=the number of loudspeakers in the
surround sound system. A loudspeaker is located at any loca-
tion within its respective loudspeaker location region, such
that there is one loudspeaker per loudspeaker location region.
Each loudspeaker location region is defined to extend
between a pair of dotted radii boundary lines B,, B>, . . . Bz
that extend outwardly from the origin of the sound control
region. The angular distance 0, between each pair of radii
boundary lines is equal and corresponds to 360°/L, where L is
the number of loudspeakers. In these preferred embodiments,
additionally the loudspeakers are located at spaced-apart
minimum distances from each other, adjacent walls, and the
perimeter of the sound control region by the conditions 1,,,,,,
lyase ANd |.,,4o7% Which are further discussed below.
In FIG. 7A, a corner-like array configuration is provided.
with four loudspeakers 12a-12d. As shown, each loudspeaker
12a-12d is located in its respective loudspeaker location
region L,-L,. As shown, the dotted boundary lines B,-B,
defining the loudspeaker location regions are spaced apart
equally by 6,;=90°. This configuration comprises left 12a and
right 126 loudspeakers in front of the listener 15 and two left
12c and right 12d loudspeaker behind the listener. In a pos-
sible modification of the configuration shown, each of the
loudspeakers 12a-12d may be located closer toward a respec-
tive corner of the room in a true corner array.
In FIG. 7B, a diamond-like array configuration of four
loudspeakers 12a-12d is shown. The configuration comprises
center front 12¢ and rear 126 loudspeakers, and also left 12¢
and right 12d loudspeakers are located on respective sides of
the listener 15. The loudspeaker location regions L,-L, are
similar to those shown in FIG. 7A, except the boundary lines
B,-B, are rotated by about 45°.
In FIG. 7C, an array configuration of five loudspeakers
12a-12e in the form ofa more conventional Dolby-surround-
like configuration is shown. With five loudspeakers, five loud-
speaker location regions L,-L, are defined by five boundaryPage 23
US 9,319,794 B2 19 lines B,-B; that are equally spaced by angular distance 6,-72°. This configuration provides loudspeakers in the fol- lowing locations: center front 12a, left front 124, right front 12c, left rear 12d, and right rear 12e. As shown in FIGS. 7A-7C, the loudspeakers are position- able in various locations and configurations within their respective loudspeaker location regions and the configuration of the loudspeakers need not necessarily be symmetrical. It will be appreciated that the number of front, rear, and/or side loudspeakers may be increased depending on requirements. As shown, each loudspeaker 12 is located outside the sound control region 11 in each configuration and located or posi- tioned near the walls and/or corners of the room 5 to exploit any reverberation for sound reflections. One metric for suitability of a particular loudspeaker array configuration is the range of directions in which the image- sources are positioned. By way of example, FIGS. 8A-8C depicts the first and second order image-sources for the respective configurations of FIGS. 7A-7C. Comparing the range of directions for the four-speaker configurations in FIGS. 8A and 8B shows that obtaining a diverse range of directions is relatively independent of the specific loud- speaker geometry used. However, FIG. 8C shows that increasing the number of loudspeakers to five creates phan- tom sources in a greater number of directions relative to the four-speaker configurations and is therefore capable of higher performance. By higher performance is meant either (i) cre- ating spatial sound fields in the control region more accu- rately, or (ii) increasing the size of the sound field we can control. Statistical room acoustics, where the reverberant sound field is modelled as diffuse, would dictate that for the acoustic transfer functions at different loudspeaker locations to be uncorrelated and hence sufficiently different from each other, the loudspeakers must be located at least half a wavelength 2/2 apart. However at low frequencies, the surround system will tend to control individual room modes. The boundary between the statistical and modal descriptions of room acous- tics is given by the Schroeder frequency, which is given by £5-2000V T¢o/V where T ¢, is the standard room reverberation time and V is the room volume. Below the Schroeder fre- quency, the acoustic transfer functions become completely correlated. Hence 1,,,,=).y/2 and 1,,,.,=A.,/4 are chosen using A -s,/f,to ensure the loudspeaker acoustic transfer functions are uncorrelated and hence sufficiently different down to as Jow a frequency as possible. By way of example, in a living room of dimensions 5 mx4 mx2.5 m with a typical room reverberation time of 500 msec, the Schroeder frequency is 200 Hz. Using the above criteria, the loudspeakers should be 5 spaced at least I,,4,: walls. A reasonable distance of loudspeakers from the centre of the sound control region 1.,,,,:07 18 required to help ensure that the direct sound is not large in comparison to the sound of a reverberant reflection. This condition helps ensure exploiting areflection for surround sound is robust. The actual distance will depend on both the directivity of the array which is related to loudspeaker order M, and to a lesser extent the strength of wall reflections. Considerations for choosing Aeonnoz are elaborated on below. In other embodiments, the geometrical arrangement of the loudspeakers may correspond to the ITU-R BS 775 5.1 Dolby Surround geometry if there are five loudspeakers employed, with a center speaker at 0° in front of the listener in the sound control region, left and right front surround speakers located at +/-22.5-30° and left and right rear surround speakers =86 cm apart and 1,,,,,-43 cm away from 45 20 located at +/-90-110°. Additionally, if seven loudspeakers are employed, the Dolby Surround 7.1 geometry may be employed. 3.3 Number of Loudspeakers and Loudspeaker Order The requirements on the number loudspeakers L and the directional loudspeaker order M are a function of the radius of the sound control region R and the acoustic frequency f and can be approximately determined from the rule of thumb: AnfR fk LOM +1) = Sy To determine the directional loudspeaker order M as a function of R, f and L, this equation can be rearranged to obtain: where [x] is the integer ceiling function of x. To create a control region ofa constant size with frequency, the directional loudspeaker order must be stepped up progres- sively at pre-determined frequency thresholds. By way of example, for a sound control region of radius R=0.2 m, the frequency thresholds for typical choices of the numbers of loudspeakers 12 are shown in Table 1. This table shows that the requirements on loudspeaker order can be reduced by increasing the numbers of loudspeakers 12. TABLE | ‘Threshold frequencies (Hz) to transition to a higher order M of loudspeaker directivity pattem, for different numbers of loudspeakers L for 2-D reproduction in a circular region of radius R of 0.2 m. Speaker No. of Loudspeakers L. Order M 4 5 7 1 408 544 816 2 1497 1905 2722 3 2585 3266 4627 4 3674 4627 6532 5 4163 5987 §437 6 5851 B48 10342 7 6940 8709 12247 8 8029 10070 14152 In preferred embodiments, the control unit of the surround sound system is configured to automatically step-up the order of the directivity patterns of the overall directional responses of the loudspeakers as the frequency of the spatial sound field represented by the input spatial audio signals increases to thereby maintain a substantially constant size of sound con- trol region. As shown by the above example, the control unit is preferably configured to step-up the order of the directivity pattern at predetermined frequency thresholds that are prede- termined and calculated based on the number of loudspeakers and the desired size of the sound control region. 3.4 Preferable Sound Control Region Size The diameter 2R of the sound control region cannot be any smaller than the size of the listener’s head, and would pref- erably include both the head and shoulders. On average, the diameter of a human head is accepted to be 0.175 m. Due to the heavy requirements on number of drivers required to perform sound reproduction at high frequencies, the sound control region diameter would typically be no larger than 1 m
Page 24
US 9,319,794 B2 21 in most commercial applications, although larger control regions could be provided for as will be appreciated. 3.5 Preferable Room Conditions The preferable room conditions of the surround sound system are a function of the strength of wall reflections, and 5 the relative lengths of the paths of direct propagation and the reflected propagation path, from loudspeakers 12 to the sound control region. To exploita reflection, due to the longer propa- gation distances and the energy absorbed by each wall reflec- tion, the sound directed toward the wall will have to be boosted by the loudspeaker 12 over the levels required for direct sound propagation. Strong boosting of the sound directed toward the wall reflection however is ill-advised, as such boosting increases the average sound energy levels outside the sound control region [5]. These sound levels may be perceived as unpleasant toa listener standing outside. The external sound levels can be reduced to acceptable levels by appropriate choice of Tikhonov regularization parameter. For good system perfor- 2 mance, room conditions must hence be able to ensure the sound energy levels outside are not required to be made significantly larger than those inside the sound control region. By way of example consider an room with identical reflect- ing walls of sound energy absorption coefficient a. Define 1. cntrot 28 the distance of loudspeakers from the sound control region and I,,,=4V/S as the mean free path where V is room volume and S is total room surface area. For an nth order reflection, the propagation distance to the control region is approximately n I,,¢,. For 2-D line sources, the loudspeakers energy will have attenuated down to 100g) o(leoneo70 Ing) of the direct sound field energy due to the propagation distance losses, and 10n log, (1-c) due to wall energy absorption. Reflections must hence be boosted by the loudspeaker to counteract this level of attenuation: tan 1 Boost for nth order reflection (dB) = roo 7 ua }. lon bogu( => } -@ control This equation assumes specular reflection only and does not include air absorption losses which are assumed small. For loudspeakers |.,,,,,.<=1 m away from the sound control region in the 5 mx4 mx2.5 m room (so that |,,,,=2.4 m) with walls having 50% sound absorption, to exploit 1°", 2”@ and 3’@ order reflections, these reflections must be boosted by 6.7 dB, 13 dB and 18 dB respectively, with the more significant contributor of the attenuation being the greater distance of the higher order reflections from the sound control region. The © control unit is configured to boost or amplify the signals relating to the reflected sound to account for wall attenuation. We note that approximate line sources can be built using vertical line arrays or electrostatic loudspeakers. Similar analyses can be applied for 3-D sources, where the depen- dence of propagation loss on distance | is proportional to 20 Jog,, | instead. Typically, the system preferably exploits 1%, 2”? and 3” order reflections in rooms with a wall energy absorption coefficient no greater than 75%, and preferably less than 50% to ensure higher order reflections do not require excessive boosting. Due to the distance and wall reflection attenuation aspects, the surround sound system would typically not be configured to exploit reflections beyond 3 order. Due to the difference in lengths of the propagation paths between the direct sound and higher order reflections, loud- 8 w 8 s 22 speakers should typically be spaced at least l.,,,,,.7-1 m away from the center of the sound control region, and preferably more than 1.5 m 4. Applications Embodiments of the surround sound system may have the following applications: Improved home theatre surround sound, High quality surround sound in the home in the form ofe.g. higher order ambisonics fields, and High end holographic sound systems with a large number of high directivity loudspeakers are appropriate for use in auditoriums. The system provides these benefits through a surround sound system that employs the use of multiple configurable directional loudspeakers to exploit reverberant reflection in the performing of surround sound. The system employs a sparse array geometry of loudspeakers, with loudspeakers located near the edges or corners of the room, for exploiting the reverberant reflection. The system employs a smaller number of loudspeakers than would be required by a tradi- tional higher order ambisonics system. Further, the surround sound system creates the impression of sound originating from a wall reflection utilising to some extent all loudspeak- ers, and to not only create the spatial sound impression but also utilise the loudspeakers to cancel at least some of the unwanted reverberation caused by other sound reflections, as the system performs sound field reproduction by means of reverberant compensation. 5. Experimental Example 1 A first experimental example of the surround sound system will be described by way of example and is not intended to be limiting. Like reference numbers in the drawing refer to the same or similar components. In this experimental example of the surround sound system it is shown that using a small number of directionally-controlled loudspeakers, a sound field may be accurately reproduced in a reverberant room. The goal of surround sound is to reproduce a sound field within a control region. Using constructive and destructive interference from the waves emitted from a set of directional loudspeakers, sound field reproduction can be used to create an arbitrary sound field in the control region. A common objective in surround sound is to place one or more phantom sources around the listener. To place a phan- tom source at any intended orientation, one would ideally distribute adequate loudspeakers evenly around the listener, with sufficient numbers to avoid spatial aliasing. One such geometry is the uniform circular array (UCA). To meet alias- ing requirements in 2-D, at least 2kR+1 loudspeakers are required [19]. However, neither this loudspeaker geometry nor the large numbers of loudspeaker are practical, as both aspects demand a large amount of physical space in the room which carries a low spouse-acceptance-factor. The surround sound system of the invention reduces the heavy requirements on numbers and arrangement of loud- speakers by using a loudspeaker configuration which exploits room reverberation. Referring to FIG. 9, in this experimental example, it is shown that reverberant reflections can be exploited to enhance the application of surround sound in home theatre. Instead of surrounding the listening area with a UCA of a large number of elements, a sparse set of steerable directional loudspeakers 12 located near the corners of a room 5 could be used (herein a “corner array”). This configuration operates to
Page 25
US 9,319,794 B2
23
exploit wall reflections in a typical room which generate the
reverberation to produce a large number of virtual loudspeak-
ers locations for creating a phantom source or sources 6. FIG.
9 shows the creation of a virtual sound source 6 from a first
order reflection. FIG. 10 shows, by way of example only, a
few possible virtual sound source directions available from
utilizing direct source (30), the first order reflections (32) and
second order reflections (34).
Through exploring the performance of the corner array
shown in FIG. 9, it is shown that the surround sound system.
has a reproduction accuracy and robustness than can be com-
parable to that of the UCA. An array of four loudspeakers 12,
each with a configurable directivity pattern, is used in the
experiment. Performance is quantified with the mean square
error in the reproduced sound field to indicate accuracy and
measure to quantify robustness to perturbation of system
parameters.
In this experimental example, we consider reproducing the
sound field over a volume of space with a small number L of
steerable directional loudspeakers 12. Each configurable
directional loudspeaker is realized using an identical array of
2-D monopole elements, so that reverberation can be easily
simulated using the image-source method [13]. Here the
loudspeakers synthesise directional responses up to approxi-
mately M=3” order. In this experiment, we restrict attention
to 2-D reproduction in a room using vertical line sources. The
purpose of the steerable loudspeaker approach is to generate
additional phantom image directions by creating beams
which bounce off reflective walls. Quantitative features of the
reverberant sound field are accurately modelled by the image-
source method for the case of specular reflection. By exploit-
ing specular reflections, we can improve performance in
reverberant environments.
We first overview the pressure matching approach to sound
field reproduction. We then describe the approach to model-
ling the directional loudspeaker.
5.1 Pressure Matching
In the pressure matching approach, one reproduces a
desired sound field by matching the pressure at a finite num-
ber of points within the sound control region. We shall refer to
these points as the matching points. The control region is a
circular 2-D region of radius R. To reproduce the desired
pressure field P,(x;f) over the control region using the L
directional loudspeakers of D 2-D monopole elements, one
needs to satisfy the equation at every point x in the sound
control region:
D
Dy Gust HU | yues F) = Pal | Ps
ml dl
Me
+
where H(xlv,,.f) is the acoustic transfer function between a
monopole driver at y,; and a point x. Pressure matching is
performed over a dense grid of Q' matching points {x,,...,
Xg} located within the control region. The set of equations
required to be satisfied can be manipulated into the matrix-
vector form
Hg=pa
where [H],<o.)-H&,ly;af) is a matrix of acoustic transfer
functions, [2]p1.4-G,(f) is a vector of loudspeaker weights
and [p,],-P.Ax,|f) is a vector of desired pressures at the
matching points. The loudspeaker weights g required to
achieve a small mean square error robustly can be calculated
through the regularized least squares solution:
go [HtHeMy Hp, Gy)
S
a
w
8
w
8
40
45
5C
60
24
where A is the Tikhonov regularization parameter. A class of
desired pressure fields that shall be reproduced here is the 2-D
phantom monopole source:
PAxPoHoM-RODs
where R, is phantom source radius, ,=[cos ,,8in |’, , is
the orientation angle of the phantom source and P, is a pres-
sure amplitude constant.
For accurate sound field reproduction over a circular 2-D
region of radius R, the number of monopoles required at
wavenumber k [15] is:
L=2kR+1 a)
This number corresponds to the number of spatial modes.
active within the control region.
5.2 Directional Loudspeaker Design
A directional loudspeaker can be modelled with an Mth
order directivity pattern. The far-field directivity pattern
D,(glf) at frequency f can be written as the phase mode expan-
sion:
at
Dbl f=
ey ert
41
where «,,,,(f) are the weighting coefficients for the mth order
phase mode. Each directional loudspeaker is realized by
arranging a number D of monopoles drivers into a uniform
circular array of radius r. To ensure loudspeaker responses up
to Mth order are obtainable, one designs each monopole array
choosing M/k and Dz2M+1 as described above. Here we
ensure the directional loudspeakers are designed to achieve
second order directivity responses. The monopole weights
are then chosen according to regularized least squares to suit
the sound field reproduction problem.
The near-field directivity pattern D,glf) of each config-
urable directional loudspeaker | that results from the above
pressure matching design is:
D
Dip. 61) = Y) Gul NG lrgs — ped
«a
where p is the distance from the centre of the uniform circular
array of the loudspeakers, » the angle made with the x-axis,
=[cos 6, sin o]’, 6,=[cos $,, sin ,]’ and 9, is the orientation
angle of each loudspeaker m.
5.3 Pressure Matching with a Uniform Circular Array
For comparison in this experiment, we shall also reproduce
the sound field with L'=LD acoustic monopoles arranged into
auniform circular array. Matching the pressure over Q' points
inside the sound control region, the loudspeaker weights are
again obtained through the regularized least squares solution
in equation (6) where instead [H],,,-H(xly,,f) is now the
acoustic transfer function between a monopole at located at y,
in the UCA and a point sensor at x.
5.4 On Robust Design
We briefly discuss aspects which contribute to the robust-
ness of a surround sound system. The way the robustness is
quantified is through the loudspeaker weight energy ||g|. The
white noise gain [17, p. 69], quantifies the ability of a loud-
speaker array to suppress spatially uncorrelated noise in the
source signal. The major errors such as those in the amplitude
and phase of the acoustic transfer functions and loudspeaker
position errors are nearly uncorrelated and affect the signalPage 26
US 9,319,794 B2 25 processing in a manner similar to spatially white noise [18]. As the loudspeaker weight energy is inversely proportional to the white noise gain, it provides a relative measure of the reaction to such errors. We examine the factors affecting robustness with aid of the singular value decomposition (SVD). In the case L'sM, the SVD of the acoustic transfer function matrix H can be written: H= Soul where u,, are the orthonormal output vectors of the sound fields reconstructible by H, v,, are the orthonormal input vec- tors of loudspeaker weights and o,, are the singular values of matrix H describing the strength of the sound field created by each loudspeaker weight v,,. We shall assume singular values are ordered o,>0,>...>0,,.After substituting the SVD of H_ 5 into equation (6), the loudspeaker weights can be shown to be: where c,=u,,"p, is the projection of p, on the subspace of sound fields reconstructable by H. A straight-forward way of improving robustness is to increase the Tikhonov regularization parameter ». The loud- speaker weight energy can be shown to be: v it? = 9( 45) ba. = w 8 3 which is inversely related to A. It is largest if we choose a 40 vector as the sound field g=u,. with the smallest singular value, where loudspeaker weight energy is equal to 0, 7(0,7+ 2. Increasing » however reduces the size of the loudspeaker weight energy at the expense of performance. In contrast, manipulating the acoustic environment’s geometry so that the desired sound field p, projects onto only the reconstructable sound fields u,, having large singular val- ues G,, would also improve robustness. Robustness can be improved by: choosing a loudspeaker array geometry which couple 5 strongly the principal components of the acoustic trans- fer function matrix to the desired set of sound fields. One way to do this is to place a loudspeaker in-line with the desired phantom source; changing the acoustic sound environment to achieve the same ends. One way is to introduce reverberation to create an image-source in-line with the desired phantom source. As illustrated by the arrows 32 and 34 in FIG. 10, first and second order reflections greatly increase the range of direc- tions a phantom can be placed. There appears good scope for improving performance by exploiting these reflections. In the case of the array of directional loudspeakers, the loudspeaker weight energy includes a component attributable to the ease of realizing the directional patterns with the D monopole drivers. The measure hence relies on the direc- tional loudspeaker being properly designed, which will be the 4 2 s 26 case if the number and geometry of the monopoles are chosen correctly for the design frequencies. 5.5 Results and Discussion In this experiment, we demonstrate typical performance of a surround sound system with L=4 smart loudspeakers and 8 drivers in each configurable loudspeaker simulating perfor- mance at 500 Hz. The loudspeakers 12 were arranged in a corner array in a room 5 as shown in FIG. 9. We compared performance of the corner array with a uni- form circular array (UCA) in a 6.4x5 m room under different reverberant conditions (cases): 1. anechoic chamber, 2. a single (north) wall only with reflection coefficient 09, 3. all wall reflection coefficients set to y=0.9 and 4. the same room with coefficients y=[0.4, 0.8, 0.2, 0.6] The array geometries being compared are summarized as: A comer array consisting of L=4 smart configurable loud- speakers, each composed of D=8 drivers (monopole sources) arranged into a uniform circular array of radius 10.2 m, which can robustly generate accurate second order loudspeaker responses (and allow creation of up to 3.5” order directivity patterns). Each of the smart loud- speakers was placed in a corner of the room at 1.5 m from both walls. An uniform circular array (UCA) consisting of LD=32 drivers were arranged into an uniform circular array at R,=2 m from the centre of the sound control region. The sound control region 11 was located at the centre of the room 5 with a radius of R=0.5 m. We positioned the loud- speakers of the corner array away from the walls to increase the range of directions that can be attained from low order reverberant reflections. Room reverberation was simulated using a 2-D implemen- tation of the image-source method [13], with acoustic transfer functions computed using: Hos yl f= YG HG bsg — 9D. at where &, denote the accumulated reflection coefficient for the ith image-source and y,"” the position of the ith image-source of monopole 1, truncating the impulse responses to the T35 reverberation time. The T;, reverberation times are 530 msec and 100 msec for reverberant rooms 3 and 4 respectively. Sound field reproduction was carried out using the regular- ized pressure matching in with Tikhonov regularisation parameter A=0.1 to create a 2-D monopole phantom source at 2 m from the centre of the control region. Due to the symme- try in the room geometry, it was sufficient to pan the phantom source angle over a 90° angular range. We compare the performance of the corner array with that ofan UCA of 32 loudspeakers in reverberant room case 3. For a0.5 m control region radius, only 11 monopoles are required by (7) at 500 Hz, so there are a number of additional degrees of freedom with which to perform the reproduction. These degrees of freedom are not wasted, as adding loudspeakers above the Nyquist sampling requirements improves the robustness. FIGS. 11A and 11B show a performance comparison between the corner array and UCA as a function of panning angle for a virtual source at 2 m. The MSE is shown in FIG. 11A and the loudspeaker weight energy is shown in FIG. 11B. Directions to the loudspeaker and first and second order image-sources are as marked. The plots clearly show that one
Page 27
US 9,319,794 B2 27 or more wall reflections improves the reproduction perfor- mance of the corner array by up to two orders of magnitude above anechoic room conditions. Marked with vertical lines are the direct sound direction 40 and the most dominant reflection 42. The MSE reproduction performance of the corner array in several acoustic environments is shown in FIG. 11A, where we study the effect of adding one or more reflective walls to the room. In the anechoic environment, the corner array per- forms poorly when panning angles away from the directional loudspeakers as shown by curve 44. One or more strong reflections however improves the sound field reproduction performance of the corner array configuration, by up to two orders of magnitude. The corner array compares favourably with the uniform circular array. Both configurations perform with an error in the range 10-* to 107°, except in the cases of sound propagating from either the north or east walls. Re- creating a phantom sound propagating from the north wall (9,=90°) is the most difficult, as the loudspeaker image- sources are furthest away from this phantom source direction. Marked on FIGS. 11A and 11B also are angles of the direct source and most significant first order image. The MSE in the direction of the first order image at 67° is good; it almost matches the performance of placing the phantom source in- line with a directional loudspeaker at 30°. The loudspeaker array here is clearly exploiting the reverberant reflection to improve MSE. The first order image of the bottom-right directional loudspeaker beyond the bottom wall produces the most impact here, pulling down the MSE by two orders of magnitudes below the anechoic case at 67°. Higher order images also contribute to improving MSE performance. In FIG. 11A the MSE is lower in the four wall cases than for the single wall and anechoic case. First order reflections are the easiest to exploit. Higher order images however, being further away from the control region, produce reflections that are diminished in amplitude. These reflections would be more difficult to exploit robustly than first order reflections, and neither is their impact on the MSE perfor- mance as dramatic. The level of performance is dependent upon the strength of reverberant reflections. Reducing the strength of reverberant reflections decreases performance. The dotted curve 46 in FIG. 11A, where the average reflection coefficient is reduced from 0.9 to 0.5, shows a performance that is slightly degraded. There appears to be an optimal choice of wall reflection coefficient. If wall reflection coefficients are too weak, then exciting a wall reflection becomes difficult. How- ever, if they are too strong, then exciting a first order reflection is not possible without also exciting much higher order reflec- tions. Higher order reflections are more susceptible to pertur- 5 bation. FIGS. 12A and 12B show the mean square error (MSE) performance of (a) a 32 element uniform circular array and (b) the four element corner array of directional loudspeakers in reproducing a phantom source at 500 Hz. MSE is plotted against both phantom panning angle and direct-to-reverber- ant-ratio (DRR). -20 dB of white Gaussian noise has been added to each element of the matrix of acoustic transfer functions. FIGS. 12A and 12B show how the level of the performance varies with direct-to-reverberant energy ratio as wall reflec- tion coefficient varies from 0.1 to 0.9. These plots corroborate the hypothesis that there is an optimal reverberation level. Here we introduced -20 dB of noise into the acoustic transfer function matrix H to emulate imperfect acoustic transfer function measurement. Both the circular array and the corner array perform very similar at -6 dB reverberation. The raised S a w 8 w 30 40 28 curves for the circular array in FIG. 12A at 0° and 90° are remnants of the degeneracy of the symmetrical room geom- etry. In regard to beampatterns, the directional loudspeaker cor- ner array performance is best when the phantom source is in-line with either a loudspeaker or a low order reflection. By way of example, phantom sources are placed in directions of D and R illustrated in FIG. 13 in room 5 case 3. More par- ticularly, FIG. 13 illustrates the beampatterns required of all four corner loudspeakers to place a phantom source in-line with direct ray D at 6,(D)=-30.5° (dotted beampatterns) and in line with reflected ray R of the top-right loudspeaker 0,(R)=-74.2° (solid beampatterns) at a radius of 2m. The beampatterns for the four steerable loudspeakers 12 are shown at the four comers of the room. For both cases, the beampatterns exhibits a non-trivial structure but possess the properties: (i) a large main lobe in the phantom source direc- tion for the loudspeaker whose image is in-line with the phantom source, and (ii) several other lobes used to cancel the reverberation created from other reflections. The main lobe may be obscured by the reverberation-cancelling lobes if the reproduction is not sufficiently regularized. Here we used a larger regularization parameter A=0.5 to ensure the main lobe is visible. 5.6 Summary This experiment tested an approach to surround sound for exact sound field reproduction in a reverberant room by uti- lizing steerable loudspeakers with configurable directional responses. An array of four configurable steerable loudspeak- ers with roughly second order directivity was shown to pos- sess a reproduction performance comparable with a much larger circular array of loudspeakers, by exploiting the wall reflections in a reverberant room. The level of performance was seen to be dependent on the strength of specular reflec- tions. For optimal performance the room was seen to require strong wall reflections. The pressure matching method in practise relies upon mea- surement of the acoustic transfer functions from each loud- speaker to a number of points in the sound control region. The approach must be made robust to error in these measurements and can be made robust through regularization. A preliminary study of performance was presented using a corner array geometry for the smart loudspeakers. Other geometries also show potential, including a diamond and pentagon, and others. Although some geometries perform 5 better than others for generating certain sound fields, the geometry studied here demonstrates the key features of using, multiple steerable directional loudspeakers to exploit rever- beration. 6. Experimental Example 2 In this experimental example, a simulation of the surround. sound system employing a 4 smart loudspeaker 12 corner array can generate a | kHz acoustic pulse propagating into the sound control region from an angle of 45 degrees. FIG. 14 demonstrates how a small number of smart loud- speakers 12 can control the sound field in the sound control region 11 within a reverberant room 5. It shows how we can create a 1 kHz acoustic pulse inside the control region 11 without reverberation from reflections. In this simulation, a surround sound system of a corner array of four smart loud- speakers 4 (each comprising eight drivers or elements) has been set the task of creating the acoustic pulse to propagate into the sound control region at 45°. To create the spatial sound pulse, the array first excites the bottom-left “smart” loudspeaker 12a at 0 msec which then bounces off the bottom wall at 4-8 msec. The bottom-right
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US 9,319,794 B2 29 loudspeaker 12d adds some to the initial sound energy as it propagates past at 12 msec, before switching to the top-right loudspeaker 12c to contribute more energy to the wavefront at 16 msec. The wavefront then bounces off the right and top walls at 26 msec to again propagate past the top-right loud- speaker 12c which contributes more sound energy at 26-30 msec. After constructing the 45 degree wavefront in the sound control region at 34 msec, the four smart loudspeakers then antiphase the propagating sound to reduce its intensity and so ensure that no further reverberation reaches the control region. The foregoing description of the invention includes pre- ferred forms thereof. Modifications may be made thereto without departing from the scope of the invention as defined in the accompanying claims. 7. References The following disclosure in the following documents is herein incorporated by reference. [1] Mark A. Poletti, “Effect of noise and transducer variability on the performance of circular microphone arrays,” Jour- nal of the Audio Engineering Society, Vol. 53, No. 5, pp. 371-384, May 2005. [2] Mark Poletti, Microphone Arrays for High Resolution Sound Field Recording, U.S. Pat. No. 2,373,128, January 2004. [3] Paul D. Teal and Mark A. Poletti, “Adaptive phase cali- bration of a microphone array for acoustic holography.” J. Acoustic Soc. Am., Vol. 127, No. 4, pp 2368-2376, May 2010. [4] Terence Betlehem and Thushara D. Abhayapala, “Theory and Design of Sound Field Reproduction in Reverberant Rooms.” J. Acoustic Soc. Am, Vol. 117, No. 4, pp. 2100- 2111, April 2005. [5] M. Poletti, F. Fazi and P. A. Nelson, “Surround sound systems using directional loudspeakers,” J. Acoust. Soc. Am., Vol. 127, No. 3590, 2010. [7] Sacha Spors, Herbert Buchner and Rudolf Rabenstein, “Efficient active listening room compensation for wave field synthesis,” Proceedings of the 116" Audio Engineer- ing Convention, Berlin, May 8-11, 2004. [8] Philippe-Aubert Gauthier, Alain Berry, “Adaptive wave field synthesis for sound field reproduction: theory, experi- ments and future perspectives,” Proceedings of the 123”¢ Audio Engineering Convention, Oct. 5-8, 2007. [9] Gerzon, Michael A., “Ambisonics in Multichannel Broad- casting and Video,” Journal of the Audio Engineering Soci- ety, Vol. 33, No. 11, pp. 859-871, 1985 [10] Chapman, Michael, et. al, A Standard for Interchange of Ambisonic Signal Sets, Ambisonics Symposium 2009, Graz, Jun. 25-37, 2009. [11] M. Poletti, “Unified Description of Ambisonics using Real and Complex Spherical Harmonics,” Proceedings of the Ambisonics Symposium 2009, Graz, Jun. 25-37, 2009 [13] Allen, J. and D. Berkley, “Image method for efficiently simulating small-room acoustics,” Journal of the Acousti- cal Society of America, vol 65, no. 4, pp. 943-950, 1979. [14] Terence Betlehem and Mark Poletti, “Sound field repro- duction around a scatterer in reverberation,” Proceedings of the International Conference on Acoustics Speech and Signal Processing, pp. 89-92, 2009. [15] Poletti, M. A., “A Unified Theory of Horizontal Holo- graphic Sound Systems,” Journal of the Audio Eng. Soc., Vol. 48, No. 12, 2000. 30 [16] Ward, D. B. and T. D. Abhayapala, “Reproduction of a plane-wave sound field using an array of loudspeakers”, IEEE Trans. Speech and Audio Processing, Vol. 9, No. 6, pp. 697-707, 2001. 5 [17] Van Trees, H. L., Detection, Estimation, and Modulation Theory: Optimum Array Processing, New York: John Wiley and Sons, 2002. [18] Cox, H., R. M. Zeskind, and T. Kooij “Practical Super- gain,” IEEE Transactions on Acoustics, Speech, and Signal 10 Processing, Vol. ASSP-34, No. 3, 393-398, 1986. [19] Ward, D. B. and T. D. Abhayapala, “Reproduction of a plane-wave sound field using an array of loudspeakers”, IEEE Trans. Speech and Audio Processing, Vol. 9, Issue 6, pp. 697-707, 2001. 15 [20] Boon, M. M and O. Ouweltjes, “Design of a Loud- speaker System with a Low-Frequency Cardioid Radiation Pattern,” Journal of the Audio Eng. Soc., Vol. 45, No. 9, 1997, [21] Fuster, L. et al. (2005). “Room compensation using 20 multichannel inverse filters for wave field synthesis sys- tems”. Proc. 118th Convention of the AES, preprint 6401. [22] Spors, S. et al. (2007). “Active listening room compen- sation for massive multichannel sound reproduction sys- tems using wave-domain adaptive filtering,” Journal of the Acoustical Society of America, Vol 122, No. 1, pp. 354-369. [23] M. A. Poletti, “Three-dimensional surround sound sys- tems based on spherical harmonics,” Journal of the Audio Eng. Soc., Vol. 53., No. 11, pp. 1004-1025, 2005. [24] Gauthier, P-A. and A. Berry, “Adaptive wave field syn- 30 thesis for sound field reproduction: theory, experiments and future perspectives,” J. Audio Engin. Soc., Vol. 55, No. 12, pp. 1107-1124, 2007. The invention claimed is: 1. Asurround sound system configured to produce a spatial 35 sound field in a sound control region within a room having at least one sound reflective surface, comprising: multiple steerable loudspeakers located about the sound control region, each loudspeaker configured to receive a plurality of speaker input signals, each speaker input 40 signal controlling one of a plurality of different indi- vidual directional beam response patterns which may be generated by the loudspeaker, and wherein the overall directional response of the sound waves emanating from the loudspeaker is that created by a combination of the 45 individual directional beam response patterns as dic- tated by the speaker input signals; and a control unit connected to each of the loudspeakers and which in a playback mode receives input spatial audio signals representing a spatial sound field for production 50 in the sound control region, the control unit having pre- configured filters for filtering the input spatial audio signals to generate the speaker input signals for driving the loudspeakers to generate sound waves with respec- tive overall directional responses that are co-ordinated to 55 combine together at the sound control region to produce the spatial sound field in the form of direct sound ema- nating into the sound control region directly from one or more loudspeakers and reflected sound emanating into the sound control region from the reflective surface(s) of 60 the room, the filters of the control unit being pre-config- ured in a configuration mode prior to operating in play- back mode based on acoustic transfer function data mea- sured by a sound field recording system comprising a microphone array located in the sound control region 65 and where the acoustic transfer function data represents the acoustic transfer functions measured by the micro- phone array in response to test signals generated by each w
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US 9,319,794 B2 31 of the loudspeakers for each of their individual direc- tional beam response patterns at their respective loca- tions in the room. 2. A surround sound system according to claim 1 wherein the input spatial audio signals are in an ambisonics-encoded surround format that is received and directly filtered by the filters in the control unit to generate the speaker input signals for the loudspeakers. 3. A surround sound system according to claim 1 wherein the input spatial audio signals are in a non-ambisonics sur- round format and the control unit further comprises a con- verter that is configured to convert the non-ambisonics input signals into an ambisonics surround format for subsequent filtering by the filters in the control unit to generate the speaker input signals for the loudspeakers. 4. A surround sound system according to claim 1 wherein the control unit is switchable between the configuration mode in which the control unit configures the filters for the room and the playback mode in which the control unit processes the input spatial audio signals for production of the spatial sound field using the loudspeakers, and wherein the control unit comprises a configuration module that is arranged to auto- matically configure the filters in the configuration mode based on input acoustic transfer function data for the room that is measured by the sound field recording system. 5. A surround sound system according to claim 4 wherein the configuration module receives raw measured acoustic transfer function data from the sound field recording system and converts it into an ambisonics representation of the acoustic transfer function data which is used to configure the filters of the control unit. 6. A surround sound system according to claim 1 wherein the filters of the control unit are ambisonics loudspeaker filters. 7. A surround sound system according to claim 1 wherein the surround sound system is configured to provide a 2-D spatial sound field production in a 2-D sound control region, and wherein the sound control region is circular and has a predetermined diameter. 8. A surround sound system according to claim 7 wherein the sound control region is located in a horizontal plane and the loudspeakers are at least partially co-planar with the sound control region. 9. A surround sound system according to claim 1 wherein each loudspeaker is located within a respective loudspeaker location region, the room being radially and equally seg- mented into loudspeaker location regions about the origin of the sound control region based on the number of loudspeak- ers, and wherein each loudspeaker region is defined to extend between a pair of radii boundary lines extending outwardly 5 from the origin of the sound control region, and wherein the angular distance between each pair of radii boundary lines corresponds to 360°/L, where L is the number of loudspeak- ers. 10. A surround sound system according to claim 1 wherein. each loudspeaker is spaced apart from every other loud- speaker by at least half of a wavelength of the Schroeder frequency of the room within which the surround sound sys- tem operates. 11. A surround sound system according to claim 1 wherein each loudspeaker is spaced apart from any reflective surface(s) in the room by at least quarter of a wavelength of the Schroeder frequency of the room within which the sur- round sound system operates. 12. A surround sound system according to claim 1 wherein each loudspeaker is spaced at least 1 m from the center of the sound control region. S a w 8 w 8 40 32 13. A surround sound system according to claim 12 wherein each loudspeaker is spaced at least 1.5 m from the center of the sound control region. 14. A surround sound system according to claim 1 wherein each loudspeaker is configured to generate overall directional responses having up to M“ order directivity patterns, where Misat least 1, and wherein the value of 2M+1 corresponds to the number of individual directional beam response patterns available for each loudspeaker. 15. A surround sound system according to claim 14 wherein each loudspeaker is configured to generate overall directional responses having upto M” order directivity pat- terns, wherein M is equal to 4. 16. A surround sound system according to claim 14 wherein each loudspeaker comprises at least an individual directional beam response patterns corresponding to a first order directional response. 17. A surround sound system according to claim 14 wherein each loudspeaker comprises at least individual direc- tional beam response patterns corresponding to 2M+1 phase mode directional responses. 18. A surround sound system according to claim 14 wherein each loudspeaker comprises at least individual direc- tional beam response patterns corresponding to an omni- directional response, and cos(m@) and sin(m) for m=1, 2,...,M,and where 9 is equal to the desired angular direction of the loudspeaker overall directional response relative to the origin of the loudspeaker. 19. A surround sound system according to claim 1 wherein the overall directional response of each loudspeaker is steer- able in 360° relative to the origin of the loudspeaker. 20. A surround sound system according to claim 1 wherein each loudspeaker comprises multiple drivers configured in a geometric arrangement with in a single housing, each driver being driven by a driver signal to generate sound waves, and wherein each loudspeaker further comprises a beamformer module that is configured to receive and process the speaker input signals corresponding to the individual directional beam response patterns of the loudspeaker and which gener- ates driver signals for driving the loudspeaker drivers to create an overall sound wave having the desired overall directional response. 21.A surround sound system according to claim 1 wherein each loudspeaker comprises a housing within which a uni- 5 form circular array of monopole drivers of a predetermined radius are mounted, and wherein the number of drivers and. radius is selected based on the desired maximum order of directivity pattern required for the loudspeaker, and wherein the monopole drivers are spaced apart from each other by no more than halfa wavelength of the maximum frequency ofthe operating frequency range of the surround sound system. 22. A surround sound system according to claim 1 com- prising at least four steerable loudspeakers. 23. A surround sound system according to claim 1 wherein the loudspeakers are equi-spaced relative to each other about the sound control region. 24. A surround sound system according to claim 1 wherein the spatial sound field is represented in the sound control region by direct sound in combination with first order, second order, and/or higher order reflections from sound waves reflected off one or more reflective surfaces of the room. 25. A surround sound system according to claim 1 wherein the surround sound system is configurable to produce higher order ambisonics spatial sound fields. 26. A surround sound system according to claim 1 wherein the diameter of the sound control region is in the range of about 0.175 m to about 1] m.
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US 9,319,794 B2 33 27. A surround sound system according to claim 1 wherein the surround sound system is configured to provide a 3-D spatial sound field production in a 3-D sound control region, and wherein the 3-D sound control region is spherical in shape. 28. An audio device for driving multiple steerable loud- speakers to produce a spatial sound field in a sound control region, each loudspeaker having a plurality of different indi- vidual directional beam response patterns being controlled by respective speaker input signals to generate sound waves emanating from the loudspeaker with a desired overall direc- tional response created by a combination of the individual directional beam response patterns as dictated by the speaker input signals, and where the loudspeakers are located about a sound control region in a room having at least one sound reflective surface, the device comprising: an input interface for receiving input spatial audio signals representing a spatial sound field for production in the sound control region; a filter module comprising filters that are configurable based on acoustic transfer function data representing the acoustic transfer functions measured by a sound field recording system comprising a microphone array located in the sound control region and where the acous- tic transfer function data represents the acoustic transfer functions measured by the microphone array in response to test signals generated by each of the loudspeakers for each of their individual directional beam response pat- terns at their respective locations in the room, and wherein the filters filter the input spatial audio signals to generate speaker input signals for driving the loudspeak- ers to generate sound waves with respective overall directional responses that are co-ordinated to combine together at the sound control region to produce the spa- tial sound field in the form of direct sound emanating into the sound control region directly from one or more w 4 of the loudspeakers and reflected sound emanating into the sound control region from the reflective surface(s) of the room; and an output interface for connecting to all the loudspeakers and for sending the speaker input signals to the loud- speakers. 29. An audio device according to claim 28 comprising wherein the input interface is configured to receive input spatial audio signals in an ambisonics-encoded surround for- iat for direct filtering by the filters of the filter module to generate the speaker input signals for the loudspeakers. 30. An audio device according to claim 28 wherein the input interface is configured to receive input spatial audio signals in a non-ambisonics surround format and which fur- ther comprises a converter that is configured to convert the non-ambisonics input signals into an ambisonics surround format for subsequent filtering by the filters of the filter mod- ule to generate the speaker input signals for the loudspeakers. 31. An audio device according to claim 28 wherein the device is switchable between a configuration mode in which the device configures the filters of the filter module for the room and a playback mode in which the device processes the input spatial audio signals for production of the spatial sound field using the loudspeakers, and wherein the device further comprises a configuration module that is arranged to auto- matically configure the filters of the filter module in the con- figuration mode based on input acoustic transfer function data for the room that is measured by the sound field recording system. 32. An audio device according to claim 31 wherein the configuration module receives raw measured acoustic trans- fer function data from the sound field recording system and converts it into an ambisonics representation of the acoustic transfer function data which is used to configure the filters of the filter module. 33. An audio device according to claim 28 wherein the filters of the filter module are ambisonics loudspeaker filters. ee OR kk
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