The Development of Modern Hearing Aids

views updated

The Development of Modern Hearing Aids

Overview

Artificial aids to hearing may have been developed in ancient times to help hunters, warriors, and sailors send messages over long distances.

Eventually, similar devises were adapted to help individuals who had suffered hearing loss. Deafness can be caused by accidents, disease, or aging and the loss of sensitivity to sound can vary from partial to severe. A hearing aid can be thought of as a miniature amplifier, that is, a device that increases the level of sound for the user. The history of aids to hearing includes various kinds of ear trumpets, tubes, bone-conduction devices, and electric aids. Modern hearing aids use electric amplifiers to boost the original sound.

Background

The technology of modern medicine, including diagnostic devices and prosthetic devices, reflects the profound changes that have occurred in nineteenth- and twentieth-century medicine. By 1900, diagnostic instruments were being used on apparently healthy people to detect early signs of disease or as a means of detecting hidden disorders, such as impairment of vision or hearing. The foundations of the modern science of acoustics were established by Herman Ludwig Ferdinand von Helmholtz (1821-1894) in his classic treatise, On the Sensations of Tone (1862). Important twentieth-century contributions to understanding and measuring hearing loss were made by Robert Bárány (1876-1936) and Georg von Békésy (1899-1972). Bárány won the Nobel Prize for Physiology or Medicine in 1914 for his contributions to otology, the field of medicine that deals with the study, diagnosis, and treatment of diseases of the ear and related structures. His research on the equilibrium system, based in the vestibular apparatus of the ear, led to the establishment of valuable clinical techniques and tests, such as the pointing test for the localization of circumscribed cerebellar lesions and the caloric test for labyrinthine function. Bárány's syndrome was named in honor of his observations of a condition characterized by unilateral deafness, vertigo, and pain in the occipital region. Békésy was awarded the Nobel Prize in 1961 for research in physiological acoustics, especially his discoveries about the physical mechanism of stimulation within the cochlea. As a result of his basic research, he developed a new audiometer, the semi-automatic Békésy audiometer, in 1947. Also important during the early twentieth century were improvements in tests for deafness and hearing loss, including early assessment of deafness in infants.

Alterations in the functioning of the ear caused by accidents, disease, or aging can result in deafness. The loss of hearing can be partial, but in some cases sensitivity to sound is totally lost. Compensation for some forms of hearing loss can sometimes be accomplished by decreasing the distance between the hearer and the speaker. The oldest method of increasing hearing—and reducing the distraction of background noises—is to cup the hand behind the ear. This primitive approach actually increases sound energy by about 5-10 decibels. The speaker can increase the level of sound by speaking more loudly or by using a mechanical device. Artificial aids to hearing may have been developed in ancient times to help hunters, warriors, and sailors send messages over long distances. Although speaking tubes of various sizes and shapes are apparently quite old, the first written description of the use of ear trumpets and other devices as aids to the deaf appeared in the seventeenth century. By the eighteenth century, some surgeons were attempting surgical interventions for diseases of the ear.

A hearing aid is a device that increases the level of sound for the user. Various devices can be used to collect more sound energy and direct it to the auditory canal or sound vibrations can be directed into the skull. The history of hearing aids includes various kinds of trumpets, tubes, bone-conduction devices, and electric aids. Speaking-listening tubes channel sound directly from the speaker to the listener. These devices were shaped like bells and trumpets. Depending on their size and shape, ear trumpets could amplify sound by about 10-20 decibels, but much of the normal range of human speech is lost. Even the best ear trumpets could only help people with fairly mild hearing impairments. Large trumpets, which were heavy and difficult to use, could provide more amplification. Hearing impaired individuals, however, often tried to disguise or conceal their hearing aids and preferred small devices that could be hidden behind the ear, under long hair, under a beard, or even as part of the furniture. Most early trumpets were custom made until specialized business firms that manufactured and sold hearing aids were established in the nineteenth century.

Conversation tubes sometimes resembled and even functioned as monaural stethoscopes. Conversation tubes had a long, flexible rubber tube, 2-3 ft (0.7-1m) long, and hard rubber mounts on each end. The devices had an ear peg on one end and a removable, conical chest-piece on the other. The cone-shaped end could be given to the speaker or placed on the chest if a physician wanted to use the device as a stethoscope. The ear peg was inserted into the ear of the deaf person. Such tubes were quite common at the beginning of the twentieth century.

Sounds, of course, are transmitted to the ear by vibrations in the air, but also by bone conduction, that is, by the vibration of the bones in the skull. Sound energy causes hard, thin materials to vibrate. For some people, aids to hearing based on bone conduction provided the best way to transmit amplified sound. Appropriate devices could conduct vibrations to the internal mechanism of the ear via the teeth or skull. For example, a woman could hold an acoustic fan against her teeth and a man could use a pipe with a vibrating stem. Bone-conduction devices can be traced back to the sixteenth century, but the first practical one seems to have been the 1879 Rhodes Audiophone, which used a fan to pick up air vibrations and transmit them to the teeth. Electric bone-conduction hearing aids appeared in the 1920s and were a major improvement. Today, most hearing losses related to bone conduction are corrected surgically.

Impact

By the end of the nineteenth century many different hearing devices were available. These devices ranged from cheap models made of tin or hard rubber to expensive ones made from precious materials. Expensive devices, however, did not necessarily work any better than their less elegant counterparts. Effectiveness varied depending on the characteristics of the device and the specific impairment of the user, but, until the twentieth century, choosing a hearing aid was a matter of trial and error. During the nineteenth century, inventors found that electrical energy could be used to amplify speech. Alexander Graham Bell (1847-1922), the inventor of the telephone, was one of the early pioneers in this field. Although Bell did not invent the electrical hearing aid, he seems to have devised a simple transmitter, receiver, and battery system for his hearing-impaired mother. He also attempted to build an amplification system for his future wife, Mabel Hubbard, who had lost her hearing when only four years old. His experiments on what he called "speech reading" led to his invention of the telephone.

A few hearing aids that used the principle of electrical amplification appeared as table models at the turn of the century, but more practical and wearable instruments were not available until about 1902. The first electric hearing aids were essentially miniature telephones composed of a power source, a magnetic earphone/receiver, and a carbon microphone. A microphone is an energy converter that changes acoustic energy into electrical patterns. Electric hearing aids generally used a large 3-volt or 6-volt battery as the power source. Electrical current can be more easily amplified than acoustic or mechanical energy. Therefore, the electrical hearing aids, also known as carbon hearing aids, that were introduced at the turn of the century provided useful amplification for a broader range of hearing impairments.

The earliest electric aids offered the same amplification ear trumpets had but covered a wider range of sound frequencies. The amplification of sound energy provided by the early microphone-battery-receiver hearing aids was not significantly greater than that of some of the better pre-electric aids, but the range of the frequencies that were amplified was substantially extended. Later models with multiple microphones provided 25-30 decibels of amplification. During the 1920s, amplifiers were incorporated into carbon hearing aids and the amplification increased by 45-50 decibels. Volume controls that varied the amount of current flowing into the system from the battery were incorporated into the carbon hearing aids.

At first, the receivers for carbon hearing aids were large hand-held earphones. The size was eventually reduced so that the earphone could be supported by a headband. Eartips and earmolds carried the acoustic energy into the auditory canal. Custom earmolds were introduced in the 1930s. For cosmetic reasons, many devices were designed to be camouflaged as purses, camera cases, or headbands. Carbon hearing aids remained common into the 1940s and helped people with moderate hearing loss, but they did not have enough power to assist people with severe problems. More modern devices were made possible with the development of small dry-cell batteries and the electron tube.

In the early 1930s, the carbon hearing aids were being replaced by table vacuum-tube hearing aids, but, even in 1944, some 50,000 carbon hearing aids were still being used in the United States. The technology used in the vacuum-tube hearing aids can be traced back to experiments conducted by Thomas A. Edison (1847-1931) in the 1880s. More practical modifications of the vacuum tube appeared early in the twentieth century. Unlike the previous generation of carbon instruments, the new vacuum-tube hearing aids had adequate power for severe hearing losses. This type of hearing aid first appeared in 1921, but vacuum-tube devices were expensive well into the 1930s.

Electronic hearing aids, also known as transistor hearing aids, appeared in 1952 and essentially replaced vacuum-tube hearing aids by the end of 1953. Because transistors only needed one battery, the size of hearing aids could be significantly reduced. The body aid (or pocket aid) and the behind-the-ear (or over-the-ear) version were useful for individuals with severe hearing loss. After Eleanor Roosevelt endorsed the use of eyeglass hearing aids, they became quite popular. Further improvements and miniaturization led to various in-the-ear models, canal aids, and aids that fit entirely within the ear canal. Further improvements included hybrid hearing aids with a combined digital/analog circuitry (1970s) and aids with a digital chip to be integrated into an analog hearing aid (1980s). The digital signal processing (DSP) chips that became available in 1982 were incorporated into digital hearing aids. The early models were quite large and expensive, but, by the end of the century, fully digital behind-the-ear and in-the-ear hearing aids were commercially available. By the end of the twentieth century, amplification had increased substantially, but, more importantly, hearing aids could be individually designed to accommodate specific hearing impairments.

A series of experiments that began in the 1950s led to the development of the cochlear implant, an electronic device that restored partial hearing to the deaf by bypassing the mechanisms of the middle and inner ears and directly stimulating the auditory nerves. The device was granted FDA approval for general use in 1984. Cochlear implants are surgically implanted in the inner ear and activated by a device worn outside the ear. Traditional hearing aids make sounds louder or clearer, but the cochlear implant works on an entirely different principle. A microphone in the cochlear implant system transforms sound into an electric signal. Electrodes in the system then use these electric signals to stimulate the auditory nerves. If the auditory nerves are healthy, cochlear implants can restore some measure of hearing in cases in which the inner ears are badly damaged.

During the early twentieth century, several surgeons introduced operations that could cure or ameliorate certain kinds of deafness. Sir Charles Alfred Balance (1856-1926) and Charles David Green (1862-1937) published their landmark two-volume treatise, Essays on the Surgery of the Temporal Bone, in 1919. The operating microscope brought the benefits of magnification to neurosurgery and to ear surgery, which made it possible for surgeons to perform novel operations on the eardrum and within the middle ear. In 1928 Walter Edward Dandy published an account of the diagnosis and surgical treatment of Menière's disease. George John Jenkins's experimental operations for otosclerosis, reported in 1917, led to the modern fenestration operation for otosclerosis. (This condition occurs when a formation of spongy bone impedes the movement of the small bones in the middle ear, causing severe and progressive hearing loss.)

During the 1930s and 1940s, many surgeons devised operations for various diseases of the ear. In the 1930s, Julius Lempert (1890-1968) introduced the one-step fenestration operation to treat otosclerosis. Lempert is considered a pioneer in the field of otology. His innovative operations led many medical schools to endorse otolaryngology as an area of specialization. Further advances in the treatment of otosclerosis occurred in the 1950s, when Samuel Rosen rediscovered stapes mobilization. In patients with otosclerosis, the stapes (a stirrup-shaped bone of the middle ear) often becomes rigid. By carefully applying pressure to the stapes, the surgeon can sometimes free the bone and restore hearing. Surgeons had attempted to treat otosclerosis by stapes mobilization in the late nineteenth century, but, at the time, the medical community regarded this as an unsafe operation of questionable value. Because of Rosen's success, however, the operation became widely accepted. Unfortunately, although stapes mobilization benefits about 70% of patients with otosclerosis, the improvement is often only temporary. In an attempt to improve on stapes mobilization, John Shea Jr. reintroduced and improved upon a procedure known as stapedectomy. In this operation, the oval window in the ear is covered with a living membrane and a prosthesis is used to reconstruct the bones of the middle ear. In the 1960s, Shea developed an improved stapedectomy procedure in which the stapes was completely removed and replaced with a prosthesis made of Teflon.

Insights into the struggles to overcome the obstacles caused by deafness can be gained by reading the autobiography of Helen Keller (1880-1968), who became blind and deaf as the result of an acute illness when she was only 19 months old, and Ruth Elaine Bender's book The Conquest of Deafness: A History of the Long Struggle to Make Possible Normal Living to Those Handicapped by Lack of Normal Hearing. Hearing aids have been collected by several museums, including the Hearing Aid Museum and Archives at Kent State University, the Central Institute for the Deaf in St. Louis; the Smithsonian Institution in Washington, D.C.; and the Oticon A/S in Denmark.

LOIS N. MAGNER