Use of auditory steady-state responses in measuring the attenuation of hearing protection devices.

It is often difficult, for technological or economic reasons, to reduce noise at its source. A common solution is to protect workers from noise exposure by using hearing protection devices (HPD). Unfortunately, considerable non-compliance occurs in the use of HPD by workers. One reason for non-compliance is the difficulty in providing workers with an attenuation level that is appropriate for a given work environment, since it is difficult to measure the effective attenuation of HPD for each individual. For example, objective measurements such as field- microphone in real ear ( F-MIRE) do not assess bone conducted sound and psychophysical measurements, such as real ear attenuation at threshold (REAT), are biased due to the low-frequency masking effect  from test-subjects’ physiological noise, in addition to be affected by the variability inherent in subjective measures. We investigated using auditory steady state responses (ASSR) as a technique which might overcome limitations of these other methods. ASSRs were recorded in ten normal hearing adults, using both “normal” and “occluded” conditions. Pure tone stimuli (500 and 1000 Hz) were amplitude modulated at 40 Hz and presented through headphones at 45, 55, and 65 dB SPL. “Physiological-attenuation” was calculated as the average difference between the normal and occluded conditions using linear least-square regression of ASSR amplitude and phase data. Physiological-attenuation estimates were compared to F-MIRE (objective) and REAT (subjective) measurements. Physiological attenuation estimates were expected to be different from REAT-based attenuation results, because we used supra-threshold stimulation levels to eliminate the low-frequency masking effect. However, Wilcoxon tests failed to show any statistical difference. This suggests that the effect of low-frequency masking may not be as large an influence as previously assumed. Further research, using an extended frequency range, should be done to validate this hypothesis.