Acoustic pressure field alongside a manikin’s head with a view towards i n s i t u hearing-aid tests

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Abstract
The frequency responses of hearing aids measured in a free field differ from those measured on the head of a person or manikin due to the scattering of the sound by the head and torso. To compare and interpret the response of hearing aids located on the head at various frequencies, it is necessary to know precisely the spatial pressure distribution. The amplitude and phase of the acoustic pressure were measured alongside a manikin''s head in increments ranging from 2-5 mm with frontal sound incidence. The acoustic driver was located in front of the manikin at distances of 1.0 and 3.5 m from the ear-canal axis. The test frequencies were the octave band center frequencies from 0.5-4.0 kHz and the 3rd octave band center frequencies from 4.0-8.0 kHz. The sound pressure level varied smoothly, as a function of position, alongside the head for frequencies .ltoreq. 2.0 kHz. At frequencies .gtoreq. 4.0 kHz, the pressure level changed rapidly with position. Particularly severe pressure minima were found around the pinna at 6.3 and 8.0 kHz. The smoothing effect of test signals using pink noise of 6 and 29% bandwidth on the acoustic pressure variation alongside the head and behind the pinna was also shown.

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