Ear simulators are used in many applications. Some are used to simulate the acoustic load to device such as telephones, hearing aids, headphones, earphones, inserts, or ear buds. Meantime, some are installed in dummy heads to simulate the sound pressure signals received at human eardrums in sound fields. Unfortunately, current standard ear simulators cannot simulate human ears above 10 kHz due to difficulties in constructing accurate acoustic components. Moreover, they cannot simulate individual differences in external ears. Those ear simulators are not satisfactory when it is desired to simulate realistic sound pressure responses over the full range of audible frequencies at human eardrums.
Without constructing physical ear simulators, a novel method of constructing digital filters to simulate transfer functions of various external ears has been presented by DENG Huiqun and YANG Jun from the Institute of Acoustics, Chinese Academy of Sciences. The method can help improve binaural sound signal processing and earphone or hearing aid design for more faithful sound transmission to the eardrum.
Given a sound field, let P0 be the sound pressure at the entrance to the blocked ear canal, P1 be the sound pressure at the entrance to the open ear canal, Ped be the sound pressure at the eardrum, Ued be the total volume velocity at the eardrum, Zed be the eardrum acoustic impedance, and Z0 be the radiation impedance looking from the entrance of the ear canal into the space. Thevenin equivalent circuit is used to derive the transfer function from P0 to Ped as shown in Fig. 1. In the discrete-time domain, an ear canal transmission line, which has variant cross sectional area along its length, is modeled using an M-sectional tube (see Fig. 2).
Fig. 1 The acoustic model for deriving the transfer function of external ears (Image by DENG).
Fig. 2 The acoustic model of an ear canal (Image by DENG).
Let the reflection coefficient at the mth boundary be rm, the eardrum reflection coefficient be red, and the reflection coefficient from the opening of the ear canal be r0, the radiation impedance of the opening be Z0, and eardrum impedance be Zed. Then, the Z domain transfer function from P0 to Ped is:
The proposed filter model has been validated. The calculated transfer function given the ear canal parameters of the BK HATS 4128-C dummy head agrees well with the measured one on the dummy head over the specified frequency range.
This research was supported by Strategic Priority Research Program of the Chinese Academy of Sciences under Grants XDA06040501.
References:
DENG Huiqun,YANG Jun. Simulating External Ear Transfer Function. Proceedings of the 21st International Congress on Sound and Vibration, ISSN2329-3675/ISBN 978-83-62652-66-2, International Institute of Acoustics and Vibration (IIAV), R41-212, 2014.
Contact:
DENG Huiqun
Key Laboratory of Noise and Vibration Research, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
Email: denghuiqun@mail.ioa.ac.cn