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In order to study the energy dependence of the cochlear amplifier, transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) were recorded in rats during gradual cooling to 27°C and heating to 40°C. In the range 33–39°C, the TEOAEs and DPOAEs were maximal in amplitude and almost insensitive to temperature. However, they were significantly depressed (reversibly) at higher and lower temperatures. Intensity functions were plotted at 37, 27 and 40°C for both types of oto-acoustic emissions. At 37°C intensity functions were nonlinear, with a notch at mid-intensity regions. At 27°C, the magnitudes were depressed more at the lower intensities and threshold elevations were observed. As a result, the intensity functions were more linear and the notch was no longer seen. This result provides further evidence for a more active, energy-dependent component of the otoacoustic emissions at lower intensities for both TEOAEs and DPOAEs. The cooling probably affects the lower intensity otoacoustic emissions by inducing a depression in the endocochlear potential, by reducing the motility of the outer hair cells and by introducing a small conductive hearing loss.

There have been reports that the developing ear is more sensitive than the adult ear to noise-induced hearing loss. This was investigated by testing auditory function in rats, both electrophysiologically and histologically, following exposure to broad-band noise (12 h/day for 15 days) at different stages of auditory development (neonates and adults), and also in age-matched controls. An exposure of 90 dB SPL broad-band noise caused no long-term change in auditory function in either age group. A higher exposure (102 dB SPL) caused greater long-term changes in hearing in the adult compared to the young noise-exposed rats, although histology showed greater damage to hair cells in the younger animals. Therefore, functionally, the developing ear does not seem more vulnerable than the developed ear to acoustic trauma.

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The effects of cooling rats from 37°C to 27°C and rewarming to 37°C on the conductive mechanism of the middle ear was studied by means of acoustic impedance measurements. Cooling reduced middle ear compliance reversibly, without an effect on external canal volume and middle ear pressure. These results provide evidence for an increase in the stiffness of the tympanic membrane and/or of the ossicular chain and/or a decrease in stapes mobility. Thus a small part of the decrease in the magnitude of otoacoustic emissions during cooling is due to an effect on the conductive mechanism of the middle ear.