Daniele Pralong
John R. Phillips
Simon Carlile
Univ. Lab. of Physiol., Parks Rd., Oxford OX1 3PT, UK
An inherent problem in accurately measuring HRTFs is the use of recording systems placed in the ear canal that may interfere with the sound field. A simple earprint and electroplating method is described for making customized, ultra-thin (0.2-mm) holders, which can be used to position a small (1.25-mm-diam) polyethylene probe coupled to a miniature microphase deep within the auditory canal. Acoustic recordings using the system on a model ear equipped with a microphone in place of the eardrum show that (1) spectral notches created by standing waves within the canal can be used to precisely place the probe relative to the eardrum; (2) the system does not significantly affect the sound spectrum as it is transformed from the free-field to the eardrum ((plus or minus)1 dB from 0.2 to 12 kHz, (plus or minus)3 dB from 12 to 14 kHz); (3) it does not introduce direction-dependent spectral components; and (4) successive insertions of the set produce highly repeatable recordings. Finally, the system was used to test the reproducibility of headphones placement, a crucial element in the high-fidelity simulation of auditory space. [Work supported by the Swiss National Science Foundation, the Foundation for the 450th Birthday of the University of Lausanne, the McDonnell--Pew Centre for Cognitive Neurosciences, the Beit Foundation, and the Wellcome Trust.]