Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation

Hearing Protector Attenuation and Noise Exposure Among Metal Manufacturing Workers

OBJECTIVES

This study utilized personal noise measurements and fit-testing to evaluate the association between noise exposures and personal attenuation rating (PAR) values among participating workers, and second, to compare the attenuated exposure levels received by the workers and the British Standards Institute's recommended noise exposure range of 70 to 80 dBA.

DESIGN

We measured hearing protection device (HPD) attenuation among a sample of 91 workers at 2 US metal manufacturing facilities, through performance of personal noise dosimetry measurements and HPD fit-testing over multiple work shifts. We compared this testing with participant questionnaires and annual audiometric hearing threshold results.

RESULTS

The average 8-hr time-weighted average noise exposures for study participants was 79.8 dBA (SD = 7.0 dBA), and the average PAR from fit-testing was 20.1 dB (±6.7 dB). While differences existed between sites, 84% of the 251 PAR measurements resulted in effective protection levels below the recommended 70 dBA (indicating overprotection), while workers were underprotected (i.e., effective exposures >80 dBA) during <1% of monitored shifts. Our results also demonstrated a significant positive relationship between measured noise exposure and PAR among non-custom-molded plug users (p = 0.04). Non-custom-molded plug wearers also showed a significant increase in PAR by sequential fit-test interaction (p = 0.01), where on average, subsequent fit-testing resulted in increasingly higher HPD attenuation. Workers at site 1 showed higher PARs. PARs were significantly related to race, even when adjusting for site location. While age, hearing threshold level, task, and self-reported tinnitus showed no significant effect on individual PAR in an unadjusted model, site, race, and sand- or water-blasting activities were significant predictors in adjusted models. Within-worker variability in time-weighted averages and PARs across repeated measurements was substantially lower than variability between workers.

CONCLUSIONS

Careful selection of HPDs is necessary to minimize instances of overprotection to workers in low and moderate occupational noise environments. The use of fit-testing in hearing conservation programs to evaluate PAR is recommended to avoid overprotection from noise exposure while also minimizing instances of under-attenuation.

Using auditory steady-state responses for measuring hearing protector occlusion effect

INTRODUCTION

The currently available methods for measuring the occlusion effect (OE) of hearing protection devices (HPDs) have limitations. Objective microphonic measurements do not assess bone-conducted sounds directly transmitted to the cochlea. Psychophysical measurements at threshold are biased due to the low-frequency masking effects from test participants' physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is used as a technique that might overcome these limitations.

PARTICIPANTS AND METHODS

Pure-tone stimuli (250 and 500 Hz), with amplitude modulated at 40 Hz, were presented to twelve adults with normal hearing through a bone vibrator at three levels in 10-dB steps. The following two conditions were assessed: the unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level were linearized using least-square regressions. The ASSR-based "physiological" OE was then calculated as the average difference between the two measurements.

RESULTS

A significant statistical difference was found between the average threshold-based psychophysical OE and the average ASSR-based OE.

CONCLUSION

This study successfully ascertained that it is possible to objectively measure the OE of HPD using ASSRs collected on the same participant both with and without protectors.