Occlusion effects of hearing protection device on ear canal transfer function and cochlear injury from impulse noise exposure

Abstract

The objective of this paper is to present a new finding on the effects of hearing protection devices (HPDs) on the ear canal (EC) transfer function (TFEC2ED). Specifically, we show that when an earmuff is worn, the TFEC2ED is not the same as that for the bare head, leading to a significant difference in cochlear injury prediction. We used shock tube blast with an acoustical test fixture (ATF) fitted with the recovered earmuffs from the historic blast overpressure project (BOP) to measure the TFEC2ED. A microphone was placed at the EC entrance under the earmuff (undermuff) to measure the pressure at the EC entrance, while the built-in ATF’s microphone was used to measure the eardrum (ED) pressure. The TFEC2ED is defined as the ratio of the pressure at the ED to the pressure at the EC entrance. We compared the cochlear integrated energy (ICE) values, when the TFEC2ED obtained from bare head (Case-1) and that measured using the ATF fitted with an earmuff (Case-2), respectively are used in the ICE-model, to estimate the effects of the difference between the TFEC2EDs on injury. The human BOP undermuff pressure data were used as model input. For Case-1, the input was at the EC. The ED pressure, reconstructed using the undermuff pressure and the TFEC2ED measured using the ATF, was used for Case-2. We found significant differences in the ICE-values as the result of the difference between the TFEC2EDs. We conclude that hearing protector models must correctly model the occlusion effects of HPDs by incorporating the change in the TFEC2ED.