Sound pressure distribution in the human ear canal

Abstract

The geometry of the human ear canal differs considerably from its conventional description as a uniform cylinder with a plane perpendicular end. The differences can be expected to produce significant effects especially at frequencies greater than 10 kHz. Measurements of sound pressure distribution within an accurately scaled (×2.5) replica of a real ear canal confirm the effects of the tapered end observed previously in simple models [J. Acoust. Soc. Am. Suppl. 1 71, S88 (1982)] and show that the standing wave patterns in the main body of the canal are appreciably disturbed. In agreement with the earlier work, the variations in SPL across the human eardrum must be expected to exceed 15 dB at 15 kHz. An approximate theory has been developed to describe the sound field within ear canals of varying cross section and direction. The theory is an extension of Webster's horn equation, quantifying the canal geometry using effective cross-sectional areas defined along an appropriate curved axis. Agreement between theoretical and experimental pressure distributions is good.