Ludwig van Beethoven is rumored to have invented this technology. The famous composer who was also deaf used to press one end of the rod on a piano while having the other end of the rod in his mouth. This would transmit the vibrations allowing him to hear. Rumor or not, but that is a gist of how bone conduction works.
Sound is perceived by our ears through the variation of pressure in air. The ear drum vibrates based on these pressure differences transmitting the vibrations to a set of small bones followed by a fluid filled cochlea.
Bone conduction used the bones of the skull to transmit the vibration to the cochlea directly. Bypassing the ear canal, ear drums and the set of bones in our ear. Bone conduction earphones usually sit on the cheekbones. The isolation and audio quality on these headphone are bad due to the fact that the ear is fully exposed. However, it provides people with hearing deficiencies and hearing loss another method to perceive sound.
Google Glass used bone conduction technology in it's devices to transmit information to the wearer. X, Alphabet's moonshot lab, is reportedly working on super hearing technology. They are working on technology that will allow the separation of voices of specific individuals from that of the group.
