Simply put, bone conduction is the conduction of sound directly to the inner ear through the bones of the skull, thus bypassing the outer and middle ear. 

Sound waves are vibrations that travel through the air and are picked up by the ear.  When we hear via air conduction, sound travels through the air and into the outer ear, through the outer ear and into the middle ear, through the middle ear, and into the inner ear, which then transfers sound to the brain.

In contrast, bone conduction bypasses the outer and middle ear and sends sound straight to the inner ear, or cochlea. It does so through vibrations as well, but in this case, vibrations of the skull. By vibrating the skull, sound reaches the bony inner ear directly.

There is a myriad of factors that affect bone conduction and how sound reaches the inner ear during this phenomenon. Bone thickness at different points on the skull, the fact that the skull is comprised of different sections fused together, placement of the bone oscillator, as well as the depth of the compressed tissue underneath all affect sound transmission. It has been found that the closer the transducer is to the cochlear, the better the sound will be transmitted.

Bone conduction testing is invaluable when determining differential diagnosis. It can be performed during pure tone audiometry to determine the severity of loss in the inner ear, and thus, the nature of the hearing loss, whether it is conductive, sensorineural, or mixed. If an individual has a middle ear problem, sound will be diminished when travelling through that area of the ear and thus by attenuated by the time it reaches the healthy inner ear.  Where bone conduction and air conduction thresholds match, the indication is that the hearing issue is in the inner ear. Where there is a discrepancy, and the thresholds have been verified with masking, the indication is that the hearing problem lies within the outer or middle ear. Having this differential diagnosis  is important for the treatment and management of hearing loss. Issues with the middle ear will be treated very differently than those of the inner ear.

Bone conduction testing can also be performed during auditory brainstem response (ABR) testing. A bone oscillator, like what is used during pure tone audiometry, is used to transmit stimuli. ABR responses are then recorded. Audiologists can then determine hearing acuity in the inner ear alone.

Vibrations can also be transmitted using tuning forks. Tuning forks are metal implements that when struck, vibrate at a specific pitch. They can then be placed on the skull to transmit that pitch to the inner ear or cochlear. Tuning fork tests are a quick way for providers to get preliminary data on an individual’s hearing loss. These tests can also be used to support audiometric results.