Verifying the attenuation of earplugs in situ: comparison of transfer functions for HATS and human subjects Annelies Bockstael a and Dick Botteldooren b and Bart Vinck c Ghent University ac De Pintelaan 185 2P1 b Sint-Pietersnieuwstraat 41 9000 Gent BELGIUM ABSTRACT Recent studies have clearly demonstrated that hearing protector’s attenuation determined in labo- ratory conditions significantly exceeds the actual protection offered to the individual user. Hence, the performance of hearing protection devices should also be verified in-situ, for instance by the MIRE-method (Microphone In Real Ear). The attenuation is hereby calculated from the difference in sound levels outside the ear and inside the ear canal behind the hearing protector. To apply this technique without altering the protector’s characteristics, a custom-made earplug with an inner bore that allows insertion of a miniature microphone can be used. However, this approach does not ac- count for differences between the sound spectrum at the microphone and at the eardrum. Therefore, studies have been conducted with a head-and-torso-simulator and human subjects to determine the transfer function between these two points for protectors manufactured in acrylic and silicone. The use of different materials resulted in clearly distinguishable functions, but the characteristics of the spectrum in general correlated with the acoustical features of the earplug’s design. All transfer func- tions showed a comparable global configuration, however variability among humans was substantial for the exact frequency and amplitude of the major pressure differences. 1 INTRODUCTION Recent studies have clearly demonstrated that attenuation values of hearing protectors deter- mined in laboratory conditions significantly exceed the actual protection offered to the individ- ual user [1], [2], [3], [4]. Therefore, the performances of the hearing protection devices should also be verified in-situ. Different measurement techniques have been proposed [5]; the MIRE approach (Microphone In Real Ear) for instance offers a quick [6], [7] and objective way to evaluate the attenuation. Testing may be carried out with one or two microphones. In the single microphone technique, the receiver is placed in the ear canal during separate, consecutive mea- surements with and without a hearing protector. Using the two microphone technique, one is placed inside the ear canal underneath the hearing protector, the other measures simultaneously the sound level outside the ear. Both methods have proved to be successful with earmuffs [7], [8], but the application with earplugs often requires extra adaptations [9], [10], [11], [12]. By contrast, Voix and others [13], [14], [15], [16] describe a custom-made earplug with an inner bore that allows insertion of a miniature microphone registering sound pressure levels inside the residual ear canal portion. Since this test design becomes more widespread [13], [17], [18] a Email address: annelies.bockstael@ugent.be b Email address: dick.botteldooren@ugent.be c Email address: bart.vinck@ugent.be