Verifying the attenuation of earplugs in situ: variability of transfer functions among human subjects

Abstract

The use of in situ measurements of hearing protectors' (HPD's) attenuation following the MIRE‐protocol (Microphone In Real Ear) is increasing. The attenuation is hereby calculated from the difference in sound levels outside the ear and inside the ear canal behind the HPD. Custom‐made earplugs have been designed with an inner bore that allows inserting a miniature microphone. A thorough understanding of the difference, henceforth called 'transfer function', between the sound pressure of interest at the eardrum and the one measured at the inner bore of the HPD is indispensable for optimizing this technique and extending its field of application. Of particular interest is the variation of these transfer functions among humans. This was checked experimentally on 19 subjects. Differences in sound pressure were measured at the HPD's inner bore, by the MIRE‐microphone, and at the eardrum by inserting an extra tube microphone in the ear canal. All transfer functions showed a comparable shape, however variability was substantial for the exact frequency and amplitude of the resonance peaks. The link between this variability and the morphology of the individual's HPD and ear canal was addressed using FDTD‐simulations (Finite‐Difference Time‐Domain) of the outer ear canal occluded by an earplug with inner bore.