Modeling Ear-Canal Acoustics, Incorporating Visco-Thermal Effects and the Influence of the Middle Ear

Abstract

The ear canal, below about 6 kHz, is well described by a uniform cylinder (sound propagates predominantly as plane waves) with the middle ear being a non‐rigid termination. A non‐rigid termination can be viewed as altering, as a function of frequency, the acoustic length and radius of the cylinder. It is generally assumed that sound transmission in the ear canal over this frequency range is lossless. This paper presents a method for calculating the influence of visco‐thermal losses and the middle ear on ear canal acoustics. The acoustic input impedance was derived from sound pressure measurements in the ear canal and then a nonlinear least‐square‐fit to the data with a one‐dimensional model incorporating visco‐thermal losses generated length, radius, and middle ear impedance parameters. It was found that a rigid wall assumption for visco‐thermal calculations was insufficient to account for damping in the ear canal. The properties of the ear canal wall (not being a rigid, low‐friction surface), incorporated into visco‐thermal losses as a scaling factor, provided a better fit to the data. Viscous and thermal losses were both found to affect sound propagation in the ear canal, viscous losses being more significant, altering the acoustic input impedance of the ear primarily in the region of the standing wave frequency. The model data suggests that the middle ear influences ear canal acoustics up to about 3 kHz.