Audibility Curve of the Chinchilla

The audibility curve of the chinchilla was measured by behavioral audiometry. The animals were trained to respond to tones by the method of instrumental avoidance conditioning. Threshold tests were conducted in a sound-treated quiet room, with the loudspeaker located over the chinchilla's head. Thirty-six chinchillas were made monaural by surgical destruction of the left cochleas. Their auditory sensitivity was measured for tones spaced at equal logarithmic intervals over the range from 0.09 to 22.8 kHz. In addition, a few animals were tested at 32.0 kHz. For tones between 0.62 and 6.0 kHz, the chinchilla's threshold is about 2.4 dB sound-pressure level (SPL). For low frequencies, sensitivity declines about 5 dB/oct from 0.62 to 0.19 kHz, and about 17 dB/oct from 0.19 to 0.09 kHz. For high frequencies, sensitivity declines about 7 dB/oct from 6.0 to 16.0 kHz, and about 22 dB/oct from 16.0 to 22.8 kHz. Similar measurements were made for 10 binaural chinchillas. On the average, the binaural animals are 2.0–8.0 dB more sensitive than monaural animals. Two old chinchillas, 12 and 14 years of age, were found to have nearly normal thresholds, but abnormal inner ears.