Transducer Operating Point Shifts Account for Post-Onset Distortion Changes with Low Frequency Tones.

Colin S. Rodger and Alec N. Salt

Abstract

Prior studies have shown that acoustic emissions show systematic changes with time as tonal stimuli are applied continuously. We have observed similar time-varying changes of second harmonic distortion in the cochlear microphonic response to a single tone. Averaged cochlear microphonic waveforms obtained from guinea pigs were analyzed spectrally and the transducer operating point was estimated using methods established by Kirk and Patuzzi, 1997 (Hear. Res. 112, 49). Measurements were performed repeatedly at 15 sec intervals during continuous tone delivery for 3 min and afterwards, with stimuli then only applied briefly as needed for data collection. A considerable variety of distortion changes with time was observed, with some animals showing sharp minima. The time courses were largely explained by operating point movements of the transducer. Operating point shifts were consistent across animals but their effects on distortion depended on the absolute value, which varied in different individuals. The relationship was studied as the 500 Hz tone level was systematically varied from 70 to 120 dB SPL. Below 80 dB operating point could not be reliably determined, although distortion changes comparable to those with higher levels were observed. With high level stimulation, larger operating point changes with more rapid time course were observed. The inclusion of a second tone for the measurement of acoustic emissions substantially changed the dependence of operating point and second harmonic on stimulus duration. Understanding the dynamic changes in transduction during tonal stimulation is essential for the interpretation of acoustic emission data. This is especially important for situations where operating point is disturbed, such as with endolymph volume disturbances.
 

This study was supported by the National Institutes of Health through the National Institute on Deafness and other Communication Disorders, Grant number DC01368

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