Reducing individual differences in the external-ear transfer functions of the Mongolian gerbil

This study examines individual differences in the directional transfer functions (DTFs), the directional components of head-related transfer functions of gerbils, and seeks a method for reducing these differences. The difference between the DTFs of a given animal pair was quantified by the intersubject spectral difference (ISSD), which is the variance in the difference spectra of DTFs for frequencies between 5 and 45 kHz and for 361 source directions. An attempt was made to reduce the ISSD by scaling the DTFs of one animal in frequency and/or rotating the DTFs along the source coordinate sphere. The ISSD was reduced by a median of 12% after optimal frequency scaling alone, by a median of 19% after optimal spatial rotation alone, and by a median of 36% after simultaneous frequency scaling and spatial rotation. The optimal scaling factor (OSF) and the optimal coordinate rotation (OCR) correlated strongly with differences in head width and pinna angles (i.e., pinna inclination around the vertical and front-back axes), respectively. Thus, linear equations were derived to estimate the OSF and OCR from these anatomical measurements. The ISSD could be reduced by a median of 22% based on the estimated OSF and OCR.