Effect of ear canal occlusion on loudness perception
The common practice to estimate the noise exposure of workers wearing hearing protection devices is to measure the ambient noise at the worker’s position and then subtract the attenuation provided by the hearing protector. In the case of communication equipment, such noise measurement needs to be realized directly at the wearer’s ear and then converted into equivalent free field data using the transfer function of the open ear. None of these methods account for any change of sensitivity caused by the presence of the in-ear or over-the-ear device. Indeed, it is generally assumed that noise sensitivity should remain constant for a given frequency and sound pressure level at the eardrum, whatever the listening conditions.
The present study investigates the potential effect of ear canal occlusion on loudness perception. An experiment was made on human participants to compare the equal loudness sound pressure levels obtained in an open vs. occluded ear. Each subject was asked to perform loudness balancing tests at several frequencies in the open and occluded ear. Headphones were used as a source on both sides while one ear was equipped with an earplug. Both ears were equipped with small microphones for measuring the in-ear sound pressure levels.
This paper details the experimental design and results obtained on a pilot group. A clear difference appears for frequencies ranging from 125 Hz to 1 kHz, where the equal loudness sound pressure levels were measured on average to be 4 to 8 dB higher in the occluded ear. These findings, which raise questions as regards the determination of noise exposure received by workers wearing earplugs, also are a key point to consider for new emerging in-ear dosimetry applications.
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