Impedance of the Ear—A Variable Quantity

The impedance of the ear at any given frequency in the auditory range is by no means a fixed or stable quantity, but may vary moment by moment over a range of magnitude as great as a factor of 10. This paper reports the results and interpretations of impedance measurements performed on guinea pigs that have led to the following conclusions. Impedance changes which occur in a short period of time appear to be associated with the conductive mechanism of the middle ear. These short-term impedance changes apparently reflect a readjustment of the conductive mechanism. They are produced by any factor that may shift the “resting position” of the tympanic membrane, such as unequal pressure between middle and outer ear (due to barometric pressure changes, swallowing, temperature changes in anesthetized subjects, etc.) or action of the tympanic muscles. At low frequencies, below 2000 cps, the tympanic membrane becomes stiffer until its impedance is about equal to that of the air in the middle-ear space. Above 2000 cps, the impedance of the total ear decreases, indicating a change in the transformation ratio between tympanic membrane and cochlea; the cochlea is apparently “decoupled” from the conductive mechanism. This is confirmed by measurement of cochlear microphonics. These facts lead to the hypothesis that tensor tympani protects the cochlea and conductive system at low frequencies by increasing the impedance of the tympanic membrane, and at frequencies above 2000 cps the cochlea is protected by the action of the stapedius muscle in disconnecting it from the chain.