Second part of the present triple study [Riederer, Proc. 18th Intern. Congr. Acoust. ICA 2004] employs the same two male subjects and focuses to the effect of varying cavum conchae blockage, relating to head-related transfer function (HRTF) repeatability.
When the cavum conchae is fully puttied with mouldable silicone the recorded HRTFs change notably compared to the inadequate blockage. 1/10-octave-smoothed system-compensated data shows that the idiosyncratic peaks around 4 kHz increase 0-3 dB and widen ~1/3 octave upwards in frequency (generally from 7 to 9 kHz). This causes the remaining resonance structures (valleys) above ~6 kHz to be compressed and somewhat deeper (0-3 dB). The end product is most lucidly demonstrated in point-to-point dB-magnitude 2-D difference plots from 90° elevation, showing a uniform performance at all azimuth angles: a significant 10 dB pressure increase around 7-9 kHz, a minor 3-5 dB boost below 11 kHz and a small 3-5 dB drop around 11-16 kHz, up to which frequency both subjects demonstrate well similar behaviour. The major 10 dB pressure increase is still visible at lower elevations, but significantly modulated by the azimuth and elevation-dependent distinctive HRTF characteristics and therefore non-removable by simple calculus. The improper cavum conchae blockage causes a complex high-frequency scaling effect that can be well estimated from the 90° elevation responses, e.g., it is distinguishable from microphone displacements that do not show a strong pressure difference below 10 kHz.
The presented studies give supplementary illumination of the HRTF measurement complexity and evidence the previous outcomes that ear plug type (material) causes variation to the HRTF characteristics by achieving different kind of ear canal blockage and predominant microphone capsule position [Riederer, J. Audio Eng. Society (Abstracts), 46, p. 1036 (1998 Nov.), preprint 4846; Riederer, PhD Thesis to be published].
[Work supported by Graduate School of Electronics, Telecommunication and Automation.]
Keywords: head-related transfer function (HRTF), three-dimensional (3-D) sound, azimuth, elevation, binaural cue, monaural spectral cue, acoustic measurement, repeatability, quality, cavum conchae, pinna, pinna cues, eigenfrequency, natural resonance, idiosyncratic response, dummy head, real head