The function of the outer ear is to transmit the sound waves, which are vibrations to the eardrum, which vibrates like the skin of a drum when the sound reaches it. It then converts those sound waves into mechanical energy which is passed onto the tiny bones of the middle ear, collectively called the auditory ossicles. As they vibrate to the rhythm of the eardrum the sound is amplified because the stapes is attached to the cochlea by the oval window, which acts as a transmitter of sound between the bones of the middle ear and the inner lining of the inner ear, the cochlea. The cochlea is filled with fluid and has a lining of tiny hair extending towards the auditory nerve. Once the sound signal reaches the inner ear, it causes ripples in the fluid and the hairs react by moving. Different pitches of sound and different volumes will vibrate various hairs and different numbers of hairs. There are over sixteen thousand hairs, but once they are damaged, the damage is permanent. The agitation of the hairs zoning into the sound creates an electrical impulse in the auditory nerve, which is then transmitted, to the brain, through the eighth cranial nerve. Once these nerve impulses reach the brain the brain recognizes them as sound and it can differentiate between noises, such as a dog, or a bell, or the crack of thunder.

Air and Bone Conduction of Sound

Air conduction of sound is the name of the process by which the sound normally reaches the inner ear. Sound waves reach the eardrum from the outside, the visible part of the ear the auricle and with the concha, which is the external cavity to the auditory canal they are pushed into the canal. In animals the auricle is a useful tool for channelling sound and also for finding the direction of the sound, but it is less effective in humans. In humans it can enhance only sounds that fall in the short-wave frequency between 2,000 and 7,000 hertz and it is at this frequency that we hear the consonants of speech.

Bone Conduction of Sound

How hearing works is the traditional way that sound reaches the inner ear, but sound can reach the inner ear via bone conduction also, using the bones of the skull, rather than the outer ear.  If you place the handle of a vibrating tuning fork on the forehead or mastoid bone behind the ear, its note is clearly audible. Equally if you hold a ticking watch between your teeth you can hear it. Not only can you hear these sounds but if you put your hands over the ear tightly to shut out as much sound as possible by the normal method, then the sound is amplified, it becomes louder. This means that the sound is producing vibrations that are not entering by the outer ear; the sound is entering the inner ear through the bone. The skull vibrates in segments and these are transmitted to the cochlear fluids by compression of the otic capsule, the bony case enclosing the inner ear. This method is known as bone conduction.

At lower frequencies below 1,500 hertz the skull movement is rigid, because the ossicles have their movements constrained by their suspension in the middle ear cavity, and their loose attachments to the skull, the cochlea can move more freely because of the margins of the oval window. In effect this means that the oval window moves, as the stapes would do, it vibrates.

Some patients with middle ear disease have hearing aids with special vibrators to deliver sound, which are then conducted by bone to the inner ear. One of the oldest types of hearing test is a tuning fork, which differentiates between conductive impairment, affecting the middle ear and sensorineural impairment of the inner ear known as the cochlea.

The concept of bone conduction means the vibrations of the inner cranium created from sound emanating from the human skull. These vibrations are not an electrical signal and that is the only way that the brain recognizes sound, the hairs of the cochlea is the only part of the ear capable of doing that. Bone conduction bypasses the outer and middle ears, the outer ear does not have to receive the signal as the skull itself is vibrating, and the sound is already mechanical energy so that the inner ear is superfluous. This means that the cranial vibrations have only to stimulate the hairs of the cochlea to move, and as in air conduction different hairs react to different sound frequencies. The most common use for the principal of bone conduction is in hearing aids. Bone conduction is used in two main types of hearing aids the external bone conduction hearing aid and the bone anchored hearing aid (BAHA).

Bone conduction has been used in the design of hearing aids for the last five hundred years. In 1550 Girolamo Cardano discovered that sound could be transmitted between the teeth, and this amazing leap led to the development of the ear trumpet. It is certain that man would have attempted to use bone shells and many other natural materials in an attempt to amplify sound. Amazingly the ear trumpet was still being produced in Europe until the 1960’s . During the concept of 1920’s electronic bone conduction devices began to be explored, but the real breakthrough came in the 1950’s when the introduction of transistors made them smaller and more practical.