A dipole model for estimating frequency versus elevation-angle trajectory of the first spectral notch of head-related transfer functions in the median plane

• Published in: The Journal of the Acoustical Society of America Vol. 140; no. 4; p. 2957
• Main Authors: Takemoto, Hironori, Mokhtari, Parham, Kato, Hiroaki, Nishimura, Ryouichi, Iida, Kazuhiro
• Format: Journal Article
• Language: English; Japanese
• Published: 01.10.2016
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.4969130

The first (lowest) spectral notch (N1) of head-related transfer functions is known as a cue for sound localization in the median plane. This may be due to the fact that N1 frequency gradually increases as the sound source approaches the direction above the head. The mechanism of this phenomenon, however, is still unclear. To clarify the mechanism, using finite-difference time-domain simulation, the normalized pressure amplitude patterns were visualized around the head for 12 subjects when the analysis field was excited at various frequencies from a point just outside the blocked meatus. As a result, strong radiations with different phases occurred in the upper and lower directions, while in the intermediate narrow zone, the pressure amplitude became minimum. This zone corresponded to N1 and bent upward with increasing excitation frequency. These facts implied that the two radiated sound waves canceled each other in the zone, where the path difference amounted to a phase difference of half a wavelength. Thus, the shape of the intermediate zone could be modeled by a hyperboloid whose foci were two hypothetical points on the pinna. The model could roughly estimate, for each individual, the frequency versus elevation-angle trajectory of N1 in the median plane.