Measurement and validation of human HRTFs for use in hearing research

This article offers a brief review of the theory underlying production of virtual auditory stimuli and a discussion of various issues a researcher might encounter in applying the theory. The focus of the discussion is on head-related transfer functions (HRTFs) which describe how a sound wave is transformed as it propagates from a point in space to a listener's eardrums. The discussion covers both technical and psychophysical issues. Most of the technical issues will be familiar to anyone who produces virtual auditory stimuli, focusing on how and where HRTF measurements are made and how the data are processed. Specific issues include: 1) measurement at the eardrum or at the entrance of a blocked meatus; 2) the meaning of HRTF phase, and the consequences of representing HRTFs as minimum-phase systems; 3) individual differences in HRTFs. A few important technical issues will be discussed that have received relatively little attention. Chief among these are problems involved with compensating for the on-ear frequency response of the headphones used to present virtual auditory stimuli. Since the final test of the adequacy of any virtual synthesis must rest with the human listener, psychophysical issues are at least as important as technical issues. Among the psychophysical issues that will be reviewed are front-back confusions and various difficulties with response modalities that restrict either stimulus or response alternatives.