Spatial Distribution of Sound Pressure in the Ear Canal

Abstract

For many experiments in physiological and psychological acoustics results are reported using, as a reference level, the sound pressure measured at the eardrum. However at higher frequencies there can be rather dramatic variations of sound pressure level within an ear canal and across the tympanic membrane. Different locations of a reference microphone can lead to quite different results, with the introduction of artifacts that relate only to peculiarities of the sound field. As a first step toward understanding this problem, measurements have been made of the spatial variation of sound pressure in scaled replicas of human ear canals and in the ear canals of live cats. The measured pressure distributions can be described reasonably well using a theoretical model that has been developed. This model is an extension of Webster’s horn equation, taking into account the curvature and variable cross section of the ear canal and the absorption of acoustic energy at the eardrum. From both theory and experiment it is clear that variations in sound pressure level of over 20 dB can occur over the surface of the tympanic membrane.