An explanation of the decrease in the earplug noise reduction when combined with an earmuff on an acoustic test fixture

Abstract

For people working in high noise level environments, typically above 105 dBA, double hearing protection (DHP) systems (earplugs worn in combination with earmuffs) are always recommended. However, it is very difficult to predict the DHP overall sound attenuation due to the complexity of the system. A recent experimental study has shown on human subjects and an acoustic test fixture (ATF) that the noise reduction (NR) of the earplug decreases considerably when an earmuff is worn over it but the reason is still not fully understood. In this work, specially designed experiments using an ATF are proposed in order to explain this observation. The focus is put on the NR of the single earplug and that of the earplug in the DHP system. The respective effects on the DHP attenuation of various sound transmission paths involved in the system (both air-borne and structure-borne) are analyzed by modifying the system coupling conditions or controlling the sound pressure level under the earmuff. This work will help better understand the interactions between the system components and will provide the grounds for developing a prospective numerical model to predict the sound attenuation of a DHP system.