The sound field in life‐size replicas of human ear canals occluded by a hearing aid

Abstract

In addition to the longitudinal sound pressure distributions that form in the human ear canal, large transverse variations can arise in the vicinity of an occluding hearing aid. These effects are being studied, numerically and experimentally, making use of life‐size ear canal replicas. Using digital representations of real ear canal geometries [Stinson and Lawton, J. Acoust. Soc. Am. 85, 2492‐2503 (1989)], a polyjet fabrication process forms replicas with a spatial accuracy of better than 0.1 mm. A hearing aid test fixture, with vent, receiver, and an inner microphone, occludes the replica canal and provides the acoustical input. The sound field inside the canal is measured using a 0.2 mm o.d. probe microphone. In parallel, the interior sound field is calculated using a boundary element method, using the same ear canal geometry as the replicas and accounting for the acoustical boundary conditions presented by the eardrum and the hearing aid. In the current series of ear canal replicas, the eardrum is rigid. Measurements and calculations are in good agreement. Large transverse variations of sound pressure level, as much as 20 dB at 8 kHz, are observed across the inner face of the hearing aid, particularly near the receiver and vent ports.