Bone conduction
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Bone conduction is the conduction of sound to the inner ear through the bones of the skull.
Bone conduction is the reason why a person's voice sounds different to him/her when it is recorded and played back. Bone conduction tends to amplify the lower frequencies, and so most people hear their own voice as being of a lower pitch than it actually is.
Some hearing aids employ bone conduction, achieving an effect equivalent to hearing directly by means of the ears. A headset is ergonomically positioned on the temple and cheek and the electromechanical transducer, which converts electric signals into mechanical vibrations, sends sound to the internal ear through the cranial bones. Likewise, a microphone can be used to record spoken sounds via bone conduction.
Bone conduction products are usually categorized into three groups
- Ordinary products such as EARS-FREE headset or headphone
- Hearing Aids and Assistive listening devices
- Specialized communication products (i.e. underwater & high-noise environments)
Bone conduction products have following advantages over traditional headphones:
- Ears-free, thus providing extended use comfort and safety
- No electromagnetic waves (EMW), eliminating the potential effect of EMW on cerebrum (if any)
- High sound clarity in very noisy environments, can be used with hearing protection
- Can have the perception of stereo sound
There are some disadvantages:
- Some implementations require more power than headphones
- Less clear recording & playback than traditional headphones and microphone due to reduced frequency bandwidth
One example of a bone conduction speaker is a rubber over-moulded piezo-electric flexing disc about 40mm across and 6mm thick used by SCUBA divers. The connecting cable is moulded into the disc, resulting in a tough, water-proof assembly. In use the speaker is strapped against one of the dome-shaped bone protrusion behind the ear. As would be expected, the sound produced seems to come from inside the user's head, but can be surprisingly clear and crisp.
Bone conduction transmission can be used with individuals with normal or impaired hearing. There are several products on the market that exploit the bone conduction transmission pathway for sound. Some of these include the AquaFM and the SwiMP3 which are devices for transmitting sound to swimmers.
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References: MacDonald, J., Henry, P. and Letowski, T. (2006). Spatial audio through a bone conduction interface. International Journal of Audiology, 45, 595-599.
