RESUMO
Hearing protection devices facing high-level impulse noises provide an attenuation, generally, between 20 and 40 dB. One reason for this limitation is the direct interactions between the protection device and the impulse waves. In the case of earplugs, direct transmissions through the earplug occur. These direct transmissions combine with the already well-studied indirect transmissions arising from wave propagation in the external ear's tissues (skin, cartilage, and bone). To evaluate the transmission induced directly by the earplug, an experimental protocol using a laser Doppler vibrometer was developed. Thus, the earplug's outer lateral face (OLF) displacements and acoustic pressure at the eardrum were measured simultaneously. Two earplugs (polyurethane foam and acrylonitrile butadiene styrene) inserted in an acoustic test fixture were stimulated with impulses ranging from 137 to 180 dB-peak. A slight earplug OLF movement in the ear canal varying from 1 µm to 0.1 mm could be observed, which is likely related to ear canal longitudinal compression. The earplug's OLF displacement and acoustic pressure variation at the eardrum strongly depended on the earplug type. These direct transmissions and underlying consequences considerably alter the protection efficiency.
RESUMO
Tactical Communication and Protective Systems (TCAPS) are hearing protection devices that sufficiently protect the listener's ears from hazardous sounds and preserve speech intelligibility. However, previous studies demonstrated that TCAPS still deteriorate the listener's situational awareness, in particular, the ability to locate sound sources. On the horizontal plane, this is mainly explained by the degradation of the acoustical cues normally preventing the listener from making front-back confusions. As part of TCAPS development and assessment, a method predicting the TCAPS-induced degradation of the sound localization capability based on electroacoustic measurements would be more suitable than time-consuming behavioral experiments. In this context, the present paper investigates two methods based on Head-Related Transfer Functions (HRTFs): a template-matching model and a three-layer neural network. They are optimized to fit human sound source identification performance in open ear condition. The methods are applied to HRTFs measured with six TCAPS, providing identification probabilities. They are compared with the results of a behavioral experiment, conducted with the same protectors, and which ranks the TCAPS by type. The neural network predicts realistic performances with earplugs, but overestimates errors with earmuffs. The template-matching model predicts human performance well, except for two particular TCAPS.
