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 the sound being conveyed through the 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 sounds.

Overview

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 was famously rumored to be using 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. This method was first discovered by Italian physician Hieronymus Capivacci.

Hearing aids and implants

History

The first bone conduction hearing aids were invented in the 15th century. Italian physician Girolamo Cardano realized that when a rod was placed between someone's teeth and attached the other end to a musical instrument, the person could hear the music despite their hearing loss. This method was used by Beethoven, as his hearing deteriorated towards the end of his life. In the 1820s, French physician Jean Marc Gaspard Itard improved on this device by attaching the other end of the rod not to a musical instrument but to the mouth of another speaker. This invention was known as the Rod of Itard. In 1923, Hugo Gernsback created a new kind of bone conduction hearing aid called the "Osophone", which he later elaborated on with his "Phonosone". Bone conduction hearing aids have also been fitted to glasses, which fit tightly to the side of the head.

In the 1970s, a team of doctors in Gothenburg, most notably Anders Tjellström, had the idea to implant a bone vibrator plate into the mastoid bone with an adjoining screw that allowed an external audio processor to be attached to conduct sound. The first three patients were implanted in 1977. The device gave good results and became known as a bone-anchored hearing aid, or BAHA. In 2012, this idea was taken a step further by the introduction of the BONEBRIDGE device. Whereas a BAHA implant is a percutaneous device that requires the screw abutment to protrude through the skin, the BONEBRIDGE is a transcutaneous device and is fully implanted under the skin. In this case, the audio processor is held in place by magnets.

Candidacy

Bone conduction devices are suitable for patients with conductive or mixed hearing loss, with a functioning cochlea but problems with the outer