On the role of head rotation in solving the HRTF inverse problem

Abstract

The sound arriving at the eardrum is a complex composition of the sound source spectrum, the acoustic properties of the listener’s environment, and the directionally dependent filtering of the listener’s head-related transfer function (HRTF). Listeners only ever have access to this mixture, and it remains unknown how they are nonetheless capable of disentangling features intrinsic to the sound source from those created by the source’s position in the environment. We hypothesized that head movements, by changing only the HRTF-related components of an arriving sound, inform a listener’s judgement of source spectrum. We tested this with virtual acoustics, presenting listeners with a speech sound rendered at an elevated frontal location and asked them to judge which of two subsequent copies of the same sound rendered at the horizon had spectral noise added to them. The test sounds were either head-locked, world-locked, or moved inverted to the listener’s movements, and were rendered with either individual or non-individual HRTFs. Error patterns across HRTF and motion conditions appear to confirm the hypothesis that head rotation plays a role in the estimation of sound source spectrum, and further indicate that listeners have expectations about how the signal at the ear should change as the head moves.