Bone conduction is the conveyance of sound through bones in the skull to the inner ear. There are two kinds of bone conduction: compressional and inertial. The former reacts to high-pitched sounds while the latter responds to low-pitch sounds. The principle of bone conduction is used in many hearing aids and other assistive listening devices.
Only a portion of what an individual with typical hearing ability hears comes from bone conduction: most sound travels through the outer and middle ear. Bone conduction gathers sound only from the skull, and can manifest in two ways: compression and inertia. Both kinds of sound absorption end up in the inner ear.
The vibrations in compressional bone conduction are created by the stimulation of sensory cells by high-pitched sounds. They are stimulated via compression of the bony case around the inner ear. Those cells pick up waves of sound as they travel through the air.
Inertial bone conduction involves the vibration of the entire skull as it reacts to low-pitched sound waves. In this case, the skull actually moves as it responds to the sound. Directly opposite to the process of compressional bone conduction, the inner ear remains still during the inertial conduction.
Bone conduction creates a different kind of sound than that processed by the outer and middle ear. The skull does not transmit sound with as high a frequency as the outer and middle ear. One result of this in most people is their thinking that their voice sounds higher than it actually does to others.
The principle of bone conduction is used in hearing aids and assistive listening devices. An aid using bone conduction rests on the temple and cheek and transmits the vibration of sounds in the air through to the skull bones with the use of an electro mechanical transducer. This equipment creates mechanical vibrations out of electric signals. Those vibrations transmit to the internal ear via the cranial bones. The sound produced is similar to, if not exactly like, the sounds transmitted through the outer and middle ear.
There is also a variety of headphones called bonephones that use bone conduction to carry sound to the inner ear. The headset curls around the ear, rather the covering it in the middle. Bonephones can be helpful to individuals suffering from microtia, aural atresia, and other disorders of the outer and middle ear. They may also be useful to people who wish to keep their ears open, whether as a matter of comfort or necessity. Bonephones can also provide clear sound in noisy atmospheres and give the user the option of using earplugs to block out harmful noise.