Current part of a triple study [Riederer, Proc. 18th Intern. Congr. Acoust. ICA 2004] concentrates on microphone placement variation in blocked head-related transfer function (HRTF) measurements, employing two full-grown males measured two times carefully by the author with Sennheiser KE4-211-2 capsules connected to a microphone-tailored Unides Design UD-MPA10 preamplifier. The entrances of the ear canals were puttied with mouldable silicone polymer; 252 sound incidents including azimuths at 10° intervals and elevations ±30°, ±15°, 0°, 60° and 90° were investigated.
When the microphone capsule position is moved 2-3 mm within the cavum conchae, from the tragus edge proximity to its midpoint, three distinguishable elevation and azimuth-dependent resonance structure transforms shifting smoothly in frequency are noted in the point-to-point dB-magnitude 2-D difference plots of the 1/10-octave-smoothed system-compensated HRTFs. A minor change (+0..5 dB) occurs in the 6-9 kHz band and two major ones (-10..-15 dB) in the 9-16 and 16-20 kHz bands; all variations vanish at 90° elevation. Also, peaks below 3 kHz are attenuated 2-3 dB. These major trends are indicated in both the subjects’ highly individual, unchanged overall, HRTF characteristics. Therefore, the general underlying errors in HRTF repetition measurements, e.g., subject positioning and system artefacts, can be considered undetectable below 16-20 kHz. Moreover, extensive HRTF measurements show high general repeatability and consequences of different measurement methods and conditions [Riederer, J. Audio Eng. Society (Abstracts), 46, p. 1036 (1998 Nov.), preprint 4846; Riederer, PhD Thesis to be published].
The findings give further empirical verification to the natural resonances of the pinna found by Shaw and Teranishi in 1968 [J. Acoust. Soc. Amer. 64, pp. 689-691] and suggest that even small positional changes of the microphones at the blocked cavum conchae cause variation to the eigenfrequencies creating notable changes in HRTFs.
[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