Modeling individual differences in ferret external ear transfer functions.

Individual variations in head and outer ear size, as well as growth of these structures during development, can markedly alter the values of the binaural and monaural cues which form the basis for auditory localization. This study investigated individual differences in the directional component of the head-related transfer function of both adult and juvenile ferrets. In line with previous studies in humans and cats, intersubject spectral differences were found to be reduced by scaling one of the directional transfer functions on a log-frequency axis. The optimal scale factor correlated most highly with pinna cavity height. Optimal frequency scaling reduced interear spectral difference equally well for adult-juvenile comparisons as for comparisons between pairs of adult ears. This illustrates that the developmental changes in localization cue values should be at least partly predictable on the basis of the expected growth rate of the outer ear structures. Predictions of interaural time differences (ITDs) were also derived from the physical dimensions of the head. ITDs were found to be poorly fitted by the spherical head model, while much better predictions could be derived from a model based on von Mises spherical basis functions. Together, these findings show how more accurate estimates of spatial cue values can be made from knowledge of the dimensions of the head and outer ears, and may facilitate the generation of virtual acoustic space stimuli in the absence of acoustical measurements from individual subjects.