On the minimum-phase approximation of head-related transfer functions

The head-related transfer function embodies the amplitude and phase transformations accompanying a sound source from a fixed position in space to the eardrum. These transformations, which primarily result from the interaction of the acoustic wave with the complex geometry of the head and pinna, are known to be the principal determinants of source location. It is especially known that the interaural time differences (ITDs) are encoded by the phase spectra of the HRTFs at the two ears. In this study we explore a model of the HRTF which approximates the HRTF as a minimum phase function. The model is tested both theoretically and through psychophysical experiments. Results suggest that a minimum phase representation of the phase spectra, augmented by a position-dependent, frequency-independent ITD, is an adequate description of the HRTF phase. Both the scientific and the practical consequences of this result shall be discussed.