A cochlear implant, a device that turns sound waves into electrical signals, can help many people with severe hearing loss regain hearing. A microphone picks up the sound waves and transmits them to a speech processor, a miniature computer that turns sounds into digitized electrical signals. These signals travel to a receiver placed beneath the skin and from there to electrodes that have been surgically implanted in the cochlea, a part of the inner ear. In the cochlea, the signals stimulate the auditory nerve, which carries the signals to the brain. The brain interprets the electrical signals as sounds.
Sound is a series of vibrations moving as waves through air or other gases, liquids, or solids. A ringing bell, for example, sets off vibrations in the air. Detection of these vibrations, or sound waves, is called hearing. The detection of vibrations passing through the ground or water is also called hearing. Some animals can detect only vibrations passing through the ground, and others can hear only vibrations passing through water.
Humans, however, can hear vibrations passing through gases, solids, and liquids. Sometimes sound waves are transmitted to the inner ear by a method of hearing called bone conduction. For example, people hear their own voice partly by bone conduction. The voice causes the bones of the skull to vibrate, and these vibrations directly stimulate the sound-sensitive cells of the inner ear. Only a relatively small part of a normal person’s hearing depends on bone conduction, but some totally deaf people can be helped if sound vibrations are transferred to the skull bones by a hearing aid.
Humans hear primarily by detecting airborne sound waves, which are collected by the auricles. The auricles also help locate the direction of sound. Although some people have auricular muscles so well-developed that they can wiggle their ears, human auricles, when compared to those of other mammals, have little importance. Many mammals, especially those with large ears, such as rabbits, can move their auricles in many directions so that sound can be picked up more easily.
After being collected by the auricles, sound waves pass through the outer auditory canal to the eardrum, causing it to vibrate. The vibrations of the eardrum are then transmitted through the ossicles, the chain of bones in the middle ear. As the vibrations pass from the relatively large area of the eardrum through the chain of bones, which have a smaller area, their force is concentrated. This concentration amplifies, or increases, the sound.
When the sound vibrations reach the stirrup, the stirrup pushes in and out of the oval window. This movement sets the fluids in the vestibular and tympanic canals in motion. To relieve the pressure of the moving fluid, the membrane of the oval window bulges out and in. The alternating changes of pressure in the fluid of the canals cause the basilar membrane to move. The organ of Corti, which is part of the basilar membrane, also moves, bending its hairlike projections. The bent projections stimulate the sensory cells to transmit impulses along the auditory nerve to the brain.