Sound-power collection by the auditory periphery of the mongolian gerbil Meriones unguiculatus. II. External-ear radiation impedance and power collection.

Acoustic power flow into the external and middle ear of the gerbil is computed from acoustic measurements and models of the external ear and used to predict the behavioral auditory threshold. The external-ear radiation impedance from the tympanic ring ZE measured in six gerbil ears with a calibrated acoustic source at frequencies from 10 Hz to 18 kHz is mass dominated below about 8 kHz, with a mean mass of 2720 kg/m4. ZE shows resonant behavior near 8 and 14 kHz. The frequency dependence of ZE is similar to that measured in cat, chinchilla, and models of the human external ear, but the mass and the resonant frequencies are higher. The power utilization ratio (PUR) computed from ZE and measurements of the middle-ear input impedance ZT presented previously [Ravicz et al., J. Acoust. Soc. Am. 92, 157-177 (1992)] suggests that appreciable power is transmitted to the middle ear only above 1.5 kHz. Mathematical external-ear models, consisting of tube segments and conical horns that include viscous and thermal losses, were developed from anatomical dimensions to match ZE over the entire frequency range of measurement. The radiation efficiency eta R computed from the models is near unity only above 12 kHz and falls to 10(-5) as frequency decreases to 10 Hz. Predictions of the mean pressure gain from an external diffuse sound field to the tympanic membrane resemble measurements in another gerbilline species. The effective area of the ear in a diffuse sound field at the tympanic membrane EATMDF computed from PUR and eta R approaches the anatomical area of the pinna opening, 71 mm2, above 1.5 kHz and the geometric limit of lambda 2 /4 pi determined by the wavelength lambda above 12 kHz but decreases sharply below 1.5 kHz to 0.002 mm2 at 10 Hz. The diffuse-field sound pressure required to deliver 5 x 10(-17) W to the middle ear between 10 Hz and 18 kHz resembles the behavioral auditory threshold [Ryan, J. Acoust. Soc. Am. 54, 1222-1226 (1976)]. This result supports the idea that the cochlea acts as a power detector at the auditory threshold.