Bone conduction

Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content without blocking the ear canal. Bone conduction transmission occurs constantly as sound waves vibrate bone, specifically the bones in the skull, although it is hard for the average individual to distinguish sound being conveyed through the bone as opposed to sound being conveyed through air via the ear canal. Intentional transmission of sound through bone can be used with individuals with normal hearing - as with bone-conduction headphones - or as a treatment option for certain types of hearing impairment. Bone generally conveys lower-frequency sounds better than higher frequency sound. Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content without blocking the ear canal. Bone conduction transmission occurs constantly as sound waves vibrate bone, specifically the bones in the skull, although it is hard for the average individual to distinguish sound being conveyed through the bone as opposed to sound being conveyed through air via the ear canal. Intentional transmission of sound through bone can be used with individuals with normal hearing - as with bone-conduction headphones - or as a treatment option for certain types of hearing impairment. Bone generally conveys lower-frequency sounds better than higher frequency sound. Bone conduction is one reason why a person's voice sounds different to them when it is recorded and played back. Because the skull conducts lower frequencies better than air, people perceive their own voices to be lower and fuller than others do, and a recording of one's own voice frequently sounds higher than one expects. Musicians may use bone conduction using a tuning fork while tuning stringed instruments. After the fork starts vibrating, placing it in the mouth with the stem between the back teeth ensures that one continues to hear the note via bone conduction, and both hands are free to do the tuning. Ludwig van Beethoven used bone conduction after losing most of his hearing, by placing one end of a rod in his mouth and resting the other end on the rim of his piano. It has also been observed that some animals can perceive sound and even communicate by sending and receiving vibration through bone. Comparison of hearing sensitivity through bone conduction and directly through the ear canal can aid audiologists in identifying pathologies of the middle ear—the area between the tympanic membrane (ear drum) and the cochlea (inner ear). If hearing is markedly better through bone conduction than through the ear canal (air-bone gap), problems with the ear canal (e.g. ear wax accumulation), the tympanic membrane or ossicles can be suspected. 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. The first description, in 1923, of a bone conduction hearing aid was Hugo Gernsback's 'Osophone', which he later elaborated on with his 'Phonosone'. After the discovery of Osseointegration around 1950 and its application to dentistry around 1965, it was noticed that implanted teeth conducted vibrations to the ear. As a result, bone anchored hearing aids were developed and implanted from 1977 on.