Is it necessary to measure Hearing Protectors Attenuation at 4 and 8 khz ?

A Microphone In Real Ear (MIRE) technique specifically designed to measure Hearing Protection Device (HPD) attenuation in the field has been recently made available on a large scale. Some detailed studies of such measurement system, often referred to as “Field-MIRE” or “F-MIRE” systems, have also made clear that the measurement attenuation for the high frequencies (typically in the 2, 4 and 8 kHz Octave Bands) could become surprisingly variable over time, for a same fit of an HPD on a human subject. After thorough investigation, and because the measurement system itself had been demonstrated to be reliable, the reason for that large variability appears to be basically related to the difficulty of objectively measuring -with a somewhat modified probe microphone- a Sound Pressure Level (SPL) in a small occluded cavity. Indeed, for these high frequencies, the sound wavelength becomes short compared to the length of the microphone probe tube used and any slight dimensional change of the occluded earcanal can dramatically change the reading of the SPL measurement. Even if such variability in the measured HPD attenuation is not caused by the measurement system itself (that simply “revealed” this change because of its high sensitivity) but really associated with the physical system under measurement (an occluded ear-canal that is constantly slightly changing in dimension, because of jaws movements and skin tissue capillary blood flow), it is still of interest to understand how this variability in high-frequency is affecting the overall reported attenuation value.

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