Investigation of the Effect of Impedance on Azimuth Cues Derived from Spherical Head Models

Recent implementations of virtual sound using head related transfer functions often include an analytical low frequency diffraction model to reproduce stable azimuth cues which are independent of the quality and accuracy of high frequency pinnae data. This paper investigates the previously neglected effects of surface acoustic impedance on the salient azimuth cues produced by a single sphere diffraction model. Results indicate an increase in interaural level difference with both a decreasing impedance magnitude and an increasingly negative impedance phase component. The interaural time difference appears to be less sensitive to impedance changes although an increase is evident with a decreasing impedance magnitude, particularly for sources near the interaural axis. These changes produce offset interaural cues which correspond to a shift in source location in the virtual environment. The effect of incorporating a frequency dependent impedance based on the approximate acoustic properties of hair is also discussed.