Head-Related Transfer Function (HRTF)

A head-related transfer function (HRTF) describes how the anatomy of the head, pinnae and torso filters incoming sound before it reaches the eardrum, as a function of the direction of the sound source. In the source dossier on Holophonics, HRTF synthesis is named as one of the mechanisms that adequately explains spatial-audio reproduction.

Role in the source

The dossier states that Holophonics, like Binaural Recording, reproduces "the interaural differences (arrival time and amplitude between the ears), as well as rudimentary head-related transfer functions (HRTF). These create the illusion that sounds produced in the membrane of a speaker emanate from specific directions."

It further states that "there is abundant literature proving that properly presented spatial cues via HRTF synthesis (mimicking binaural heads) or binaural recording is adequate to reproduce realistic spatial recordings comparable to real listening, and comparable to the Holophonics demonstrations."

The cited counter-evidence base is Gilkey & Anderson, Binaural and Spatial Hearing in Real and Virtual Environments.

Relation to the head-shaped microphone

The head-shaped acoustic monitoring device described in US4680856 — Process for Forming an Acoustic Monitoring Device and EP0050100 — Device for the Spatial Codification of Sounds is, in effect, a physical realization of the HRTF: it reproduces the auricles, auditory meatus, Eustachian tubes and oral cavity, and places microphones at the eardrum position and orientation. The EP0050100 wig provides asymmetry for front/back discrimination.

Relation to the disputed theory

HRTF synthesis is the alternative explanation the dossier offers in place of the Auditory Interferometer Hypothesis. The source's position is that HRTF cues are sufficient to account for the Holophonics demonstrations, so the interferometer mechanism is unnecessary as well as unsupported.