Hearing Aids and Implants
Hearing aids are devices that amplify sound so a person who has a hearing impairment can enjoy sounds from a musical symphony to a lively conversation to the rustle of leaves. Two types of hearing aids either conduct sound through the air or through bone. Usually a person who is hearing impaired can use an air-conductionhearing aid, as it amplifies sound and brings it directly to the ear. The bone-conduction hearing aid brings sound waves to the bony part of the head behind the ear and uses the bone to transmit sound waves to the nerves of the ear. A typical hearing aid contains a microphone that picks up sounds and converts them into electric signals, an amplifier, which increases the strength of the electric signals, and the receiver, which converts the signals back into sound waves that can be heard by the wearer. There are three main styles of hearing aids: those that fit behind the ear, those that fit into the ear, and those which slip into the ear canal. Power is provided by a small battery.
Devices to aid hearing have a long history. The idea of bone conduction was known in the early seventeenth century and the ear trumpet was used even before that time. The ear trumpet was shaped to gather sound and funnel it into the ear. In seventeenth century Germany, Marcus Banzer used a piece of swine bladder connected to a tube made of elk hoof to make an artificial ear drum. Later in that century the audiphone or dentiphone was invented. Made of a flexible material, the device was shaped like a fan and held at its end between the teeth as the fan was bent toward the sound. The sound vibrations captured by the fan were carried to the teeth, the jaw bones, the skull, and finally to the auditory nerve. Perhaps the largest hearing aid ever made was an imposing throne built for King John VI of Portugal in 1819. Hollow carved arms of the chair terminated with the wooden mouths of lions through which people would speak and have their voices carried by tubes to the king's ear. An artificial ear drum was devised in 1852 when an English physician, Joseph Toynbee, used a disk of vulcanized rubber attached to a rod. The Victorian era was known for some of its more elaborate concealed hearing devices, in urns, top-hats, even tiaras. In the 1870s, Alexander Graham Bell began experimenting with the conduction of sound through electrical devices originally intending to help deaf children hear. His experiments led to the invention of the telephone instead, but his work did bring public awareness to the needs of the hearing impaired. The first electrical hearing aid was made in 1901 by Miller Reese Hutchinson and he called it the Telephone-Transmitter. During the era of vacuum tubes, like those used in early radio, new hearing aids were developed starting in 1920 with Earl Charles Hanson's Vactuphone. A 1923 model produced by the Marconi Company was the Otophone, consisting of an amplifier placed in a large case weighing 16 pounds (7 kg) making it rather bulky to use. The first "wearable" hearing aid weighed 2.5 pounds (1.1 kg) and was made by A. Edwin Steven in 1935. During the 1950s transistors revolutionized electronics and Microtone introduced its compact and powerful transistor hearing aid in 1953.
For individuals whose hearing cannot be aided with a hearing aid, there is the possibility of having a cochlear implant, the first of which were done in 1973 and which involved the implantation of an electrical device which stimulated the remaining nerves in the inner ear in people suffering from nerve deafness. Although the implant did not restore normal hearing, it helped the recipient hear and interpret environmental sound. Today, multichannel electrical cochlear implants are more sophisticated and contain speech processors which allow some patients to understand speech without reading lips. Implants consist of tiny electrodes inserted in the ear and connected to a receiver implanted under the skin by an implanted wire. Robert V. Shannon of the House Ear Institute of Los Angeles has developed an auditory brainstem implant for people whose auditory nerve has been severed. The implant consists of a tiny microphone, sound processor, and transmitter all outside the ear, and an electrode that is implanted inside the head and connected to the auditory brain stem. When the microphone picks up sound, the unit converts sound energy into electric signals that are sent directly to the brain where they are interpreted as sound. Although the implant is not enough to restore hearing, it upgrades the level of environmental sound heard by the user and, in a trial situation, at least one recipient could understand limited human speech.
Computer microchips are now being used in programmable hearing aids, allowing the device to be adjusted by computer. Through programming based on the needs of the individual, signal amplification is automatically adjusted, which means softer sounds can amplified more than louder sounds. Multi-channel capability allows for programmable reception of high- and low-frequency ranges, and some devices have multi-microphone capability, which means one microphone picks up broad-range sound and the other, narrow-range sound, improving the capability of distinguishing conversation from background noise. Digital hearing aids in which a computer- controlled sound processor is contained, are also available.
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