To understand how bone conduction works, you first have to understand how we hear sounds, which we do in two ways:
Sound travels in waves through the air. Normally, sound waves travel through several structures in the ear, before being translated and transmitted through our nervous systems to our brains. First, the waves enter the outer ear, or pinna, which is the big flappy piece of cartilage that helps to focus the sound. From there, the sound goes into the air-filled middle ear, which includes the auditory canal and the eardrum, a flap of skin that vibrates when exposed to the energy from sound waves. On the other side of the eardrum, there are three small bones, the ossicles, which are attached to it. They transmit the vibration to the cochlea, a fluid-filled structure that takes those vibrations and converts them to electrical impulses that are sent along the auditory nerve to the brain [source: Hass].
But that's not the only way our body can process sound. Sound waves can also be transmitted through the bones in your head. When the bones vibrate, the sound reaches the cochlea, just as it would by going through the middle ear and eardrum, and results in the same sort of nerve impulses being transmitted to your brain. This method of sound transmission is called bone conduction [source: Walker and Stanley].
The great 18th- and early-19th-century composer Ludwig Van Beethoven, who suffered hearing loss apparently caused by thickening of the structures in his middle ear, may have been one of the first people to develop a bone-conducting device help him hear music. He attached a rod to his piano and then connected it to his head, so that it transmitted the vibration of his playing directly to his cochlea [source: Mai]. Bone-conducting headphones are built around this same concept.