2021 年 42 巻 4 号 p. 181-192
The interaural time difference (ITD) plays an important role in spatial hearing, particularly in azimuthal localization of sound images. Although the ITD is essentially determined by the geodesic distance between two ears, researchers have reported that the ITD is greater for lower frequencies. However, the origin of this frequency-dependence has not been revealed. This study investigates how the ITD is physically characterized to have a frequency-dependent nature by conducting measurements and numerical simulations. Dummy head measurements show that the ITD varies with frequency because the apparent propagation time to the ipsilateral ear decreases for low frequency. Dummy head simulations confirmed this phenomenon and revealed that the apparent propagation time decreases because of a sound pressure phase shift due to reflections from the head. Circular plate simulations revealed that the circular profile including its lateral surface and edge produces reflections that are relevant to the phase shift, yielding the frequency-dependence of the apparent propagation time. Furthermore, rigid sphere simulations showed that such reflections are produced even by smooth convex surfaces without clear-cut edges. These results strongly suggest that a major factor in the production of the frequency-dependence of ITDs is backscatter diffractions from convex surfaces of the head and the pinna.
