Dependence of distortion on cochlear transducer characteristics.

Salt AN, Gill RM.

Abstract

The cochlea generates distortion primarily due to the known nonlinear transduction characteristics of the hair cells. An understanding of how distortion varies with stimulation conditions requires detailed knowledge of the transducer characteristics under different stimulation conditions. In the present study, cochlear microphonics (CM) recorded from the round window were analyzed by fitting Boltzmann functions using methods established by Kirk et al. (Hear Res 112, 69, 1997). Derived parameters include transducer slope (z), saturation voltage (Psat)and operating point (Po; representing the position on the transducer curve at zero-crossings of the stimulus). Derived transducer parameters were compared as stimulus frequency and level were varied systematically. For a 500 Hz stimulus, z was little changed as level was varied from 75 to 95 dB SPL, while Psat increased by more than a factor of 2. Po was typically negative for low stimulus levels but became positive for levels of 100 dB SPL and higher. At frequencies where Po became close to zero, very low levels of second harmonic distortion were observed. With higher frequencies, z decreased progressively while Psat remained unchanged. These changes account for the decreases in second and third harmonic distortions as frequency increases. Analysis of CM responses to two tones presented simultaneously showed similar slope and operating point characteristics, but differences in Psat were observed at both high and low frequencies. We conclude that for a given stimulus condition, the amount of distortion generated depends on the both the transducer curve properties and on the region of the transducer curve over which the stimulus operates. Since cochlear transducer properties appear to be dynamic and vary with the applied stimulus frequency and level, an understanding of these relationships will aid in the interpretation of distortion data.
 

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

---

Back to Publications page