The air-conduction (AC) pathway is the normal hearing mode, in which acoustic environmental signals entering the ear canal cause eardrum vibrations to transmit through the middle ear and the cochlear fluids that in turn excite the sensory cells located on the delicate cochlear tissues. An alternate mode of hearing is through the bone-conduction pathway, where vibrations of the skull cause the cochlear fluids to be stimulated directly.
The bone-conduction hearing pathway is finding increasing applications in modern technologies. Google Glass, for example, makes use of bone conduction to allow audio to be heard without requiring the user to wear acoustic earbuds. Several hearing devices also use bone conduction to treat deafness in people who cannot use acoustic hearing aids due to congenital problems with the structures of their outer or middle ears, or in some cases, no or limited cochlear function on one side. These include the BAHA, SoundBite, BoneBridge, Ponto, and Sophono devices. In addition, knowledge of the bone-conduction hearing pathway is important for protecting the sensitive structures of the inner ear from damage in extremely noisy environments. On the deck of an aircraft carrier, for example, many workers complain of serious hearing loss in spite of wearing earplugs or ear mufflers that block sound transmission via air conduction. Such hearing loss, likely caused by too much sound reaching the inner ear via bone conduction, is one of the most prevalent medical problems for armed-forces personnel. Research on the bone-conduction hearing pathway could potentially lead to new technologies for reducing the amount of bone-conducted sound reaching the cochlea.