The role of the chinchilla pinna and ear canal in electrophysiological measures of hearing thresholds.

Measurements of the acoustic transfer function (ATF) of the pinnae of 8 chinchillas were compared with the auditory-evoked potential (AEP) thresholds of 16 chinchillas measured in free field and with insert earphones. The ATF was measured in anesthetized chinchillas in a far-field condition in a semi-anechoic room using a logarithmic frequency sweep from 100 Hz to 20 kHz. Probe microphone measurements were collected with the probe opening at the tympanic membrane and in the same approximate position with the chinchilla removed from the sound field. For each animal's acoustic transfer function, the average of five in-the-ear and three free-field measurements were determined. The ATF exhibited a 5-dB passive gain at about 1 kHz and a broad resonance between 2.5 and 6 kHz of about a 10-dB gain. AEP thresholds were obtained from monaural chronically implanted chinchillas at 0.5, 1, 2, 4, and 8 kHz using first free-field and then insert earphone stimuli. The free-field sound pressure was measured with a microphone in the approximate position of the chinchilla's head. The earphone sound pressures were measured with a probe microphone positioned near the tympanic membrane. The free-field AEP hearing thresholds exhibited +10-dB gain at 4 kHz compared to the insert earphone AEP thresholds. The agreement between the ATF and AEP derived transfer function suggested that the threshold differences at 4 kHz between the two testing configurations can be accounted for by the pinna and ear canal gain.