An Interim LAeq8 Criterion for Impulse Noise Injury

We present a method to account for the effects of the hearing protection devices (HPDs) for use with the 8 hours equivalent A-weighted energy (LAeq8) criterion. The method involves the calculation of the LAeq8 equivalent unprotected free-field dose (LAeq8EUFF), which is obtained by using the insertion loss (IL) data of the HPD together with free-field pressure measurements. The method was validated against the historical the U.S. Army Medical Research and Materiel Command walk-up study data with volunteers exposed to simulated large weapon noise wearing a range of HPDs. The IL data were obtained using standard acoustical test fixtures fitted with the matching HPDs in replicated field tests and using shock tubes at conditions comparable to the actual exposure intensities. Logistic regression calculations were performed to correlate the LAeq8EUFF values against the walk-up study outcomes to determine the L(95,95) threshold for the protection of 95% of the population with 95% of the time. Data comparison shows that L(95,95) is 89 dBA, which is slightly higher than the 85 dBA criterion but falls in the 80 to 90 dBA range as used by various NATO nations. Therefore, considering the limitation of the walk-up dataset, it is conservative to adopt the 85 dBA threshold for general application.