Effects of noise exposure on intelligibility of bone-conducted speech during earplugging

Bone conduction (BC) is a method of perceiving sound via living tissue. Some components of BC sound reach the middle or inner ear directly, so it can be heard even when wearing earplugs. Therefore, BC is used in speech communication devices in noisy environments that require wearing earplugs. However, the perception of BC sound under high-intensity noise is generally complicated by the temporal and frequency characteristics of the noises, the noise reduction rate of the earplugs, and the "occlusion effect," a phenomenon where the loudness of low-frequency bands increases due to wearing earplugs. Although former studies have investigated the effects of noises, the findings are varied and inconclusive. Moreover, no study has reported the systematic analysis of the effect of noise. Therefore, in this study, we investigated the effects of noise characteristics on the intelligibility of BC speech with earplugging from psychological (intelligibility test) and physical (ear canal sound pressure (ECSP) and head vibration measurements) aspects. White, pink, and Japanese babble noise were used and set to 60, 70, and 80 dB A (A-weighting), respectively. One hundred Japanese monosyllables (FW03, NTT-AT) set to the clearest sound level for each participant were used as stimuli and presented using a BC vibrator. The highest intelligibility was recorded with babble noise and the lowest with white noise. The result indicates that the more high-frequency components the noise contains, the more it impedes the hearing of consonants attenuated by the propagation characteristics of BC, thus reducing intelligibility. Physical measurements confirmed the validity of these results.Clinical Relevance- Our results indicate that the intelligibility of BC speech varies significantly depending on the characteristics of the ambient noise, and the intelligibility may decrease to as low as 50% under poor conditions. The results provide useful information for the development of BC devices, including hearing aids.