Audibility of Direct Switching Between Head-Related Transfer Functions

In binaural synthesis, signals are filtered with head-related transfer functions (HRTFs). In dynamic conditions HRTFs must be constantly updated, and thereby some switching between HRTFs must take place. For a smooth transition it is important that HRTFs are close enough so that differences between the filtered signals are inaudible. However, switching between HRTFs does not only change the apparent location of the sound but also generates artifacts that might be audible, e. g. clicks. Thresholds for the audibility of artifacts are defined as the smallest angular separation between switched HRTFs for which the artifacts are just audible. These thresholds were measured for temporal and spectral characteristics of HRTFs separately, and were defined as the minimum audible time switching (MATS), and the minimum audible spectral switching (MASS) respectively. MATS thresholds were in the range of 5-9.4 mu s, and MASS thresholds were in the range of 4.1-48.2 degrees being more dependent on the direction of sound than MATSs. Generally, results show that for dynamic binaural synthesis time switching imposes higher demands on spatial resolution than those imposed by spectral switching.