How Hearing Works
To understand hearing loss, it is important to understand how normal hearing works. There are two sound wave pathways that produce the sensation of hearing – air conduction and bone conduction.
In hearing by air conduction, sound waves are collected by the outer ear and channeled along the ear canal to the eardrum. When sound hits the eardrum, the impact creates a vibration that, in turn, causes three bones in the middle ear to move. The smallest of these bones, the stapes, fits into the oval window between the middle and inner ear. When the oval window vibrates, fluid in the inner ear transmits the vibrations into the hearing organ, called the cochlea. The fluid’s wave-like motion bends thousands of microscopic hair cells, setting off nerve impulses that pass through the auditory nerve to the hearing center of the brain, where they are translated into sounds the brain recognizes.
Hearing by bone conduction works much the same way. The only difference is that the sound originates from the mechanical vibration of bone instead of from the acoustic vibration of a travelling sound wave through the air. With a bone oscillator placed firmly against the head, the vibration causes the bones of the skull to vibrate including the temporal bone that houses the cochlea. The vibrations are transmitted to the fluid surrounding the cochlea thereby setting in to motion the same chain of events as with air conduction including the bending of the hair cells with subsequent stimulation of the auditory nerve.