Bone-conducted audio is a sound technology that relies upon vibrations resonating through bone surrounding the ear.
A transducer converts sound to vibration, which travels along the user’s bone structure to the cochlea, which in turn transmits the information along the auditory nerve as a signal to the brain. In effect, the user’s skull is the device’s speaker.
Bone-conducted audio is often used in mobile devices like Bluetooth earpieces, heads-up display technologies and specialty headphones (sometimes called bonephones). Bone conduction is particularly well-suited to mobile devices because it can be very compact and because it is inaudible to people in the vicinity of the user.
Because the sound doesn’t enter through the outer ear or pass through the ear canal or eardrum it may not contribute to the hearing loss caused by portable audio listeners that use earbuds. Also, because bone conduction bypasses some parts of the ear, it can make it possible for people with certain forms of hearing loss to listen to audio that they couldn't hear otherwise.
Bone-conducted audio has to be adjusted for sound correction because sound heard through bone vibration sounds deeper and fuller, just as your own voice sounds deeper and fuller when you hear it as you speak than it does when you hear your recorded voice.
Bone-conducted audio is appearing in many new devices but patents exist going back to the 1930s. As far back as the eighteenth century, the composer Ludwig Van Beethoven created a bone-conduction audio device. Beethoven rigged up a rod from his piano to his head, which allowed him to continue composing even as he became increasingly deaf.