Although noise may be innocuous in many vocational environments, there is a growing concern in industry that it can reach hazardous levels when amplified by hearing aids. This study examined the daily noise exposures associated with hearing aid use in industry. This was done by both laboratory and site measurements in which hearing aids were coupled to the microphone of an integrating sound level meter or dosimeter. The former method involved the use of recorded railroad and manufacturing noise and a Bruel and Kjaer 4128 Head and Torso simulator. In the latter procedure, a worker wore one of three hearing aids coupled to a dosimeter during 8-hour shifts in a manufacturing plant. Both methods demonstrated that even when amplified by mild-gain hearing aids, noise exposures rose from time-weighted averages near 80 dBA to well above the OSHA maximum of 90 dBA. The OSHA maximum was also exceeded when moderate and high gain instruments were worn in non-occupational listening environments. The results suggest that current OSHA regulations that limit noise exposure in sound field are inappropriate for hearing aid users.
We examined the effects of 1-min tonal exposures on the amplitude of the 2f1-f2 distortion product in the ear canals of cats. These effects were compared with the changes in the whole-nerve action potential (AP) responses to tone bursts following similar exposures. Both the distortion product and AP amplitudes were reduced following exposure. The time courses of AP and acoustic emission (AE) recoveries were to a certain extent similar, and the post-exposure effects depended in similar ways upon the frequency and intensity of the exposure tone (adapter). Assuming that the 2f1-f2 AE is generated by nonlinearities in the motion of the cochlear partition, the reduction in its amplitude suggests that 1-min sound exposures can alter cochlear mechanics. The analogous aspects of the AP and AE post-exposure effects further suggests that components of long-term adaptation as measured by the AP are associated with changes in the mechanics of transduction. Finally, alterations of the middle ear cavities of the cat by opening the bulla or bony septum can result in marked changes in the effectiveness of an adapter in reducing AE amplitude.
Frequency effects in auditory backward masking were examined by psychophysically determining the thresholds of 0.5, 1, 3, 5, and 7 kHz tone burst probes followed at different delay times by a noise burst masker. It was found that for each probe frequency condition, the amount of backward masking (defined as the shift in probe threshold due to the presence of the masker) as a function of the time interval between the probe and masker (Δt) could be approximated reasonably well by a simple decaying exponential function. The rate of decay of the masking effect as a function of Δt was found to increase with increasing probe frequency. This latter result qualitatively supports Duifhuis's [J. Acoust. Soc. Am. 54, 1471–1488 (1973)] theory which attributes short term backward masking to the temporal overlap of cochlear responses to the probe and masker.
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