Digital filters based on FDTD simulations for accurately verifying the performance of hearing protectors

In the prevention of noise induced hearing loss, the European Directive stipulates that the worker's effective exposure must take the attenuation provided by hearing protectors into account. Unfortunately, the discrepancy between the attenuation measured in laboratory conditions and the actual protection has been well-established. Therefore, individual field attenuation measurements are advisable. To enable these, an earplug has been developed with an inner bore in which a microphone can be inserted to register the sound level behind the hearing protector via the Microphone In Real Ear (MIRE) technique. However, the actual exposure level can only be accurately predicted if the relationship, henceforth called transfer function, between the sound level at the MIRE measurement microphone and at the eardrum is known. This prediction can be done for each individual by digitally filtering the MIRE microphone response. Starting from Finite-Difference Time-Domain (FDTD) simulations the appropriate filter coefficients can be found with multiple linear regression. In general, this method can predict an individual's transfer function quite accurately if the length of the earplug's inner bore and the length of the residual part of the ear canal behind the protector are known. However, the prediction is less reliable for ears with a longer residual part