Demystifying the Double Hearing Protector Effect

Introduction

Hearing protectors (earplugs and earmuffs) are sometimes the only practical barrier available to protect workers from occupational hearing loss. Particularly for workers exposed to extreme noise, such as airline crews and military staff, the level of protection must be further increased and the use of double hearing protectors (DHPs)—earplugs combined with earmuffs—is recommended. However, it was found that the DHP overall sound attenuation is not equal but rather lower than the sum of the attenuation achieved with each device worn separately. This effect, referred to as the DHP effect, is not well understood. Thus, it is difficult to select proper devices to use in a DHP, and ensure that workers are correctly protected. Past studies attributed the DHP effect to the sound conducted by the head’s flesh and bone to the inner ear. A recent investigation has shown that this effect can be characterized by the decrease of earplug attenuation when wearing earmuffs, indicating that the outer ear path is involved. The DHP effect has also been observed on an acoustic test fixture (ATF)—a type of artificial head that mimics acoustic and vibratory features of the human outer ear. ATFs are particularly useful in measurements under extreme noise conditions, where tests on human participants are not feasible for ethical reasons. The observed DHP effect is suspected to result from sound transmitted through the structures involved in the ATF. Unfortunately, the physical mechanisms behind this effect are still not fully understood. In this article, we pursue the investigation and seek to figure out how sound energy propagates through a DHP using a combination of measurements and simulations on a human participant and dedicated ATFs.

Figure 1. (a) possible sound transmission paths through a DHP/head system; (b) test of the DHP effect on a human participant (project H20201103 approved by the Research Ethics Committee at ÉTS).

Sound Transmission Paths

Possible sound transmission paths through the DHP/head system are shown in Fig. 1(a). A path is referred to as airborne (AB) when the sound transmitted through air is more important or structure-borne (SB) when sound is mainly conducted via structures in the form of mechanical vibrations. The following paths contributing to sound pressure in the ear canal are anticipated: (i) “direct” AB path through the earmuffs, air cavity under the earmuffs and earplugs; (ii) SB path through the head caused by vibrations of the earmuff headband (SB1); (iii) SB path originating from an external source exciting the head (SB2); (iv) SB path caused by floor vibrations (SB3); (v) SB path resulting from ear cup vibrations and transmitted to the head through the earmuff comfort cushion (SB4).

The DHP effect was verified on a human participant by measuring the decrease of earplug attenuation after adding earmuffs with miniature microphones outside and inside the ear canal. This effect was well observed in the most important frequency range for communications (see difference between solid and dashed blue curves above 500 Hz, in Fig. 2).

Figure 2. DHP effect characterized by the decrease of earplug attenuation on a human participant (blue diamonds) and ATF (red).

Experimental and Numerical Analysis on Artificial Heads

First, the DHP was placed on a commercial ATF (see Figs. 3(a) ‒ (c)). The DHP effect clearly appeared with a similar order of magnitude to that previously measured on a human participant under similar conditions (see red curves in Fig. 2). Thus, the ATF was used to investigate more thoroughly the associated mechanisms. Measurements were then performed to verify the influence of structure-borne paths SB1 ‒ SB3 by modifying setup conditions (see Figs. 3(d) ‒ (f)). It turns out that the corresponding test results were nearly identical to the original configuration, suggesting that the DHP effect is more likely associated with the structure-borne path SB4.

Second, numerical simulations were carried out to illustrate sound energy propagation through the system in a more direct way by reasonably disregarding the earmuff headband, ATF tripod and sound energy impinging on the ATF exterior boundaries (i.e., SB1 ‒ SB3). An in-house ATF with a simplified geometry was used to facilitate the numerical analysis (see Fig. 3(g)). The simulation results (not presented here for the sake of brevity) confirmed the importance of SB4—at most frequencies, sound energy is transmitted from the ear cup, through the earmuff cushion/ATF assembly and into the ear canal via the earplug or ear canal lateral walls (see Fig. 3(h)). In this situation, the earmuffs are no longer a hearing protection but can be regarded as a noise generator, which constitutes a primary reason for the decrease in the noise insulation performance of the DHP.

Figure 3. Studied hearing protectors: (a) silicone earplug, (b) EAR-MODEL-1000 earmuffs; (c) test of the DHP effect on a commercial ATF; configurations to test the influence of structure-borne paths (d) SB1, (e) SB2 and (f) SB3; (g) numerical model of the DHP on an in-house ATF; (h) structure-borne path SB4 found to be responsible for the DHP effect.

The results presented in this article can provide manufacturers of hearing protectors with clues to achieve better hearing protection, namely by controlling vibration transmissions through the earmuff comfort cushion. Ultimately, this work will also help improve the design of ATFs in order to better predict DHP sound attenuation, thus properly protecting workers exposed to hazardous levels of noise in the workplace.

Additional information

For more information, please refer to the following publications:

Luan, Y., Doutres, O., Nélisse, H., Sgard, F. (2021). Experimental study of earplug noise reduction of a double hearing protector on an acoustic test fixture. Applied Acoustics, 176: 107856.

Luan, Y. (2021). Experimental and numerical study on the contribution of acoustic test fixtures to hearing protector sound attenuation: Sound transmission paths in the case of a double hearing protector and influence of eardrum acoustic impedance. (Ph.D. thesis under evaluation, ÉTS).

Luan, Y., Sgard, F., Nélisse, H., Doutres, O. (2021). A finite element model to predict the double hearing protector effect on an in-house acoustic test fixture. Paper submitted for publication in Journal of the Acoustical Society of America (2nd revision in progress).