After guinea pigs were exposed to octave‐band noise, which in chinchillas produces behavioral threshold shifts of more than 50 dB, as well as significant shifts of cochlear microphonic potentials, only minimal changes in guinea pig cochlear microphonic responses were found. Differences in sensitivity to noise exposure between the two species could be accounted for by four factors. (1) At lower frequencies, the attenuation caused by the closed bulla is important up to frequencies which are higher for guinea pig than for chinchilla. (2) The resonance associated with sound‐pressure transformation from tragus to tympanic membrane is observed at frequencies about an octave higher in the guinea pig than in the chinchilla. (3) Cochlear microphonics with sound pressure measured at the drum and the bulla opened are less sensitive for guinea pig than for chinchilla. (4) Departure from linearity and/or onset of distortion of cochlear microphonics occurs at sound‐pressure levels which are different for guinea pig than for chinchilla. For frequencies less than about 5 kHz, to produce a criterion change in cochlear sensitivity the level of octave‐band noise in the field must be 15 to 25 dB greater for intact guinea pig than for chinchilla.

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