In-Ear Noise Dosimetry : Field Method Using Earmuffs’ Noise Reduction
Abstract
In-ear noise dosimeters have recently been developed to accurately assess the residual noise dose under intra-aural hearing protectors (earplugs). In this paper, this approach is adapted to circum-aural hearing protectors (earmuffs). In order to do this, three requirements have to be met: a) determine the optimal placement for the external and internal microphones, b) develop a method to reject the measurement artefacts such as the Wearer’s Induced Disturbances (WID) and c) find a way to assess several individual transfer functions (TF) required to accurately predict the effective attenuation. Regarding the first requirement, outer-ear microphone optimal position is determined based on experimental tests conducted on an acoustic test fixture as well as on 23 human subjects in various acoustic field conditions. Regarding the second requirement, the proposed method relies on continuous measurements of the noise reduction NR* to estimate the residual noise dose. The NR* is computed from the difference between outer ear and underneath the ear cups sound pressure levels. The NR* is monitored continuously and then used to assess the residual noise dose under the earmuffs based solely on external microphone measurements, making the WIDs’ energetic contribution negligible. Finally, for the third requirement, the use of a finite element method (FEM) is considered to assess the individualized TFs required.
The proposed method is presented in detail together with some preliminary experimental results while the remaining challenges are also discussed.
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