Bone Conduction Hearing in Human Communication - Sensitivity, Transmission, and Applications

Doctoral thesis, 2009

Sound perceived via Bone Conduction (BC) consists of vibrations transmitted to the cochleae through the skull bone from either one's own voice, the surrounding sound field, or a BC transducer. In two-way communication systems, BC is believed to improve the sound quality when used in specific environments, e.g. extremely noisy environments which require hearing protection devices in the ear-canals. Several studies were performed to investigate the possibilities for a BC communication system and to increase the general knowledge of BC sound perception. The difference in sensitivity of the BC and Air Conduction (AC) parts of one's own voice was estimated, showing that the BC component dominated for most sounds between 1 and 2 kHz. Also estimated was the sensitivity difference between BC and AC sound from a surrounding sound field, demonstrating that the BC part was 40 to 60 dB lower than the AC part. A combination of these sensitivity differences provides information about the signal-to-noise improvement by using a BC microphone to record a person's own voice, instead of an ordinary AC microphone in front of the mouth, in a noisy environment. However, high-frequency losses occur in the signal when recording through the skin. In many of the investigations the occlusion effect was studied by different methods in order to estimate the low-frequency increase in perceived BC sound when wearing ear-plugs and/or ear-muffs. Moreover, the amount of BC sound reaching the cochleae from different positions of the skull bone was examined with the conclusion that relative BC hearing can be estimated from ear-canal sound pressure and cochlear vibrations. Finally, a new transcutaneous Bone Conduction Implant (BCI) hearing aid was investigated. This hearing aid does not require a permanent skin penetration, in contrast to the Bone-Anchored Hearing Aids (BAHAs) used today. Measurements showed that the new BCI hearing aid can be a realistic alternative to the BAHA.