Acoustic impedance measurements in the gerbil ear

Abstract

The external‐ear radiation impedance ZE and the middle‐ear input impedance ZM were measured in the Mongolian gerbil from 10 Hz to 18 kHz. The frequency range of measurement was spanned by two acoustic sources of different characteristics. Here, ZE was measured looking out from the tympanic ring through the external ear; ZM was calculated at the tympanic membrane from measurements at the lateral surface of the skull. The magnitude of both ZE and ZM is higher in gerbil than in chinchilla, cat, or human, consistent with the smaller ear dimensions in gerbil. Also, ZM is stiffness dominated below 1 kHz and has an equivalent volume of 140 μ/, approximately two‐thirds of the volume of the middle‐ear air spaces. This result and measurements of ZM with the middle‐ear air spaces opened suggest that these spaces dominate the impedance in this animal at low frequencies. The external ear can be modeled roughly by a rigid‐walled tube of 2.5‐mm diameter and 10.6‐mm length. Comparisons of ZE and ZM suggest that roughly half of the sound power available at the tympanic membrane at frequencies above 2 kHz enters the middle ear and that a much smaller proportion is transmitted at lower frequencies.