A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture.

The sound attenuation of double hearing protectors (DHPs), earplugs combined with earmuffs, generally falls short of the sum of each single protector's attenuation when used independently. This phenomenon, referred to as the DHP effect, is found to be related to structure-borne sound transmission involving the outer ear and can also be observed on acoustic test fixtures (ATFs). At present, it still remains not fully understood, and no available model can help demonstrate the associated sound transmission mechanisms. In this work, a finite element model is proposed to study the DHP effect on an ATF between 100 Hz and 5 kHz. Power balances are calculated with selected configurations of the ATF in order to (i) quantify the contribution of each sound path, and study the effects of (ii) the artificial skin and (iii) acoustic excitation on the ATF exterior boundaries. The DHP effect is shown to originate from the structure-borne sound power injected from the ATF boundaries and/or earmuff cushion. The important influence of earcanal wall vibration is highlighted when the skin is accounted for. The simulation results allow for gaining more insight into the sound transmission through a DHP/ATF system.