USING FINITE-ELEMENT MODELING TO PREDICT THE EFFECT OF EAR CANAL MICROPHONE POSITIONING ON THE SOUND ATTENUATION OF HEARING PROTECTORS

The sound attenuation of earplugs can be measured in laboratory using "Microphone In the Real Ear" (MIRE) method or on the field using "Field-MIRE" method. These methods require a measurement of the sound pressure level at the eardrum. However, in practice, the microphone is located somewhere between the inner face of the earplug and the tympanic membrane. This measured sound pressure level is then used to calculate either the earplug insertion loss or the noise reduction if one uses an exterior microphone. When doing so, it is assumed that an error is committed since the position of the microphone inside the ear canal is neither precisely known nor at the eardrum location. This paper uses a 2D axisymmetric finite element model of the open and occluded ear canal to calculate the sound pressure field in the ear canal. The validity of this model is assessed based on comparisons with measurements on human subjects. The model is exploited to evaluate the transfer functions between the pressure at the inner microphone and that which would be measured at the eardrum for both the open and occluded ear. The sensitivity of these transfer functions to various earplugs/ear canal configurations is discussed.