Goteborg, Sweden —  Chalmers University of Technology has announced that new technology to hear vibrations through the skull bone has been developed. Besides investigating the function of a new implantable bone conduction hearing aid, Sabine Reinfeldt, PhD, of the university's Biomedical Engineering Division, has studied the sensitivity for bone conducted sound and also examined the possibilities for a two-way communication system that is utilizing bone conduction in noisy environments.

"This hearing aid does not require a permanent skin penetration, in contrast to the bone-anchored hearing aids (BAHAs) used today. Reinfeldt said of the bone conduction implant (BCI) hearing system she investigated. Measurements reportedly have shown that the new BCI hearing system can be a realistic alternative to the BAHA.

Sound is normally perceived through air conduction, which means that the sound waves in the air enter the ear-canal and are transmitted to the cochlea in the inner ear. However, sound can also be perceived via bone conduction. Vibrations are then transmitted to the cochleae through the skull bone from either one's own voice, the surrounding sound field, or a bone conduction transducer. In two-way communication systems, bone conduction is believed to improve the sound quality when used in extremely noisy environments which require hearing protection devices in the ear-canals.

Several studies were performed to investigate the possibilities for a bone conduction communication system and to increase the general knowledge of bone-conducted sound perception. The low-frequency increase in perceived bone-conducted sound when wearing earplugs and/or ear-muffs is called the occlusion effect. This effect was studied by different methods and it was found that it is lower for deeper insertion of ear-plugs and for larger ear-muffs, and that it varies for different stimulations.

The difference in sensitivity of the bone conduction and air conduction parts of one's own voice was estimated, showing that the former component dominated for most sounds between 1 and 2 kHz. To be able to measure the bone condution component of a person's own voice, a large ear-muff was developed to attenuate the air-conducted sound and to minimize the occlusion effect.

The study also showed that the sensitivity difference between the BC and AC parts of one's own voice were different for different kinds of sounds, depending on where in the mouth the sound is produced and on the influence from the vocal cords.

Also estimated was the difference in sensitivity between bone- and air-conducted sound from a surrounding sound field, demonstrating that the bone conduction part was 40 - 60dB lower than the air-conducted part. This measure gives the maximum attenuation achievable with ordinary hearing protection devices, like ear-plugs and ear-muffs. It also shows the possible noise reduction from the surrounding noise by using a done conduction microphone, instead of an ordinary air conduction microphone in front of the mouth, to record one's own voice in a noisy environment.

Moreover, the amount of bone-conducted sound reaching the cochleae from different positions of the skull bone was examined with the conclusion that relative bone conduction hearing can be estimated from ear-canal sound pressure and cochlear vibrations

SOURCE: Chamers University of Technology