Sound-power collection by the auditory periphery of the Mongolian gerbil Meriones unguiculatus. I: Middle-ear input impedance.
This is the first paper of a series dealing with sound-power collection by the auditory periphery of the gerbil. The purpose of the series is to quantify the physiological action of the gerbil's relatively large tympanic membrane and middle-ear air cavities. To this end the middle-ear input impedance ZT was measured at frequencies between 10 Hz and 18 kHz before and after manipulations of the middle-ear cavity. The frequency dependence of ZT is consistent with that of the middle-ear transfer function computed from extant data. Comparison of the impedance and transfer function suggests a middle-ear transformer ratio of 50 at frequencies below 1 kHz, substantially smaller than the anatomical value of 90 [Lay, J. Morph. 138, 41-120 (1972)]. Below 1 kHz the data suggest a low-frequency acoustic stiffness KT for the middle ear of 970 Pa/mm3 and a stiffness of the middle-ear cavity of 720 Pa/mm3 (middle-ear volume V MEC of 195 mm3); thus the middle-ear air spaces contribute about 70% of the acoustic stiffness of the auditory periphery. Manipulations of a middle-ear model suggest that decreases in V MEC lead to proportionate increases in KT but that further increases in middle-ear cavity volume produce only limited decreases in middle-ear stiffness. The data and the model point out that the real part of the middle-ear impedance at frequencies below 100 Hz is determined primarily by losses within the middle-ear cavity. The measured impedance is comparable in magnitude and frequency dependence to the impedance in several larger mammalian species commonly used in auditory research. A comparison of low-frequency stiffness and anatomical dimensions among several species suggests that the large middle-ear cavities in gerbil act to reduce the middle-ear stiffness at low frequencies. A description of sound-power collection by the gerbil ear requires a description of the function of the external ear.
Ravicz
M E
ME
Eaton-Peabody Laboratory of Auditory Physiology, Massachusetts Eye and Ear Infirmary, Boston 02114.
Rosowski
J J
JJ
Voigt
H F
HF
D000158:Acoustic Impedance Tests; D000818:Animals; D001309:Auditory Threshold / Q000502:physiology; D004432:Ear, Middle / Q000033:anatomy & histology / Q000502:physiology*; D005849:Gerbillinae / Q000033:anatomy & histology / Q000502:physiology*; D006309:Hearing / Q000502:physiology*; D008954:Models, Biological; D010898:Pitch Perception