Devices for the hearing-impaired have a long history. Until the late 19th century, hearing aids were acoustic and offered only modest amplification. They were cartoonish ear trumpets: tubular or funnel-shaped devices supported in one hand that collected sound waves and funneled them to the ear. The development of electrical technologies, however, opened new possibilities for amplification.
The telephone, patented by Alexander Graham Bell in 1876, was the first electrical device that could transmit speech. Since the receiver was held up to the ear, some hearing-impaired people found it easier to listen to a conversation on Bell’s telephone than listening to someone in person.
However, Thomas Edison, who was hard of hearing, did not find the telephone helpful. He later recalled that his inability to hear sounds from the receiver spurred his interest in improving it. This led to his 1878 invention of the carbon transmitter for the phone, which—unlike Bell’s device—amplified the electrical signal. Some people—including AIEE and IRE member Miller Reese Hutchison in the United States and Frederick Alt in Austria—tried to adapt telephones as hearing aids in the early 20th century. But the results were obtrusive, heavy, and only amplified speech by 15 decibels at most. The volume of a normal conversation averages about 60 dB, and a person normally raises their voice by about 30 dB if a conversation partner has trouble hearing, so the early aids were of limited usefulness.
Then along came the vacuum tube. Applying the three-element tube invented by Lee De Forest in 1907, Western Electric Co., in New York City, started producing technically superior hearing aids in 1920. These offered 70 dB of amplification and a more even frequency response. It was anything but portable, weighing in at 100 kilograms and was as big as a filing cabinet. In 1924, Western Electric had a new model: its electrical components fit in a small wooden box weighing just 4 kg. The box was attached to a receiver that, as in olden times, users had to hold up to their ears. Though portable, it was hardly inconspicuous. Many refused to use them not only because of the inconvenience but also because of the stigma attached to being visibly hearing-impaired.
In 1938, Aurex Corp., an electronics manufacturer in Chicago, developed the first wearable hearing aid. A thin wire was connected to a small earpiece and then to an amplifier-receiver that clipped to the wearer’s clothes. The receiver was wired to a battery pack, which strapped to the leg. Subminiature vacuum tubes developed in 1937 by Norman Krim, an engineer at Raytheon, allowed for amplifiers that were not only smaller but also required less power. Marketed to hearing-aid manufacturers, these amplifiers quickly gained a fair share of the market, but they still relied on a separate, strap-on battery pack.
BUTTONS AND BOARDS
In the late 1940s, manufacturers combined these tubes with two innovations from World War II—printed circuit boards and button batteries—to produce more compact and reliable models. Batteries, amplifier, and microphone were combined in a single unit that could fit in a person’s shirt pocket or even hidden in a woman’s hairdo. The unit was connected to an earpiece via a wire. But the devices were not invisible, despite users’ attempts to camouflage them by hiding the microphones in their hair or using them as tie clasps, brooches, and the like. The hearing-impaired wanted a true one-piece unit that could be worn at the ear, but, of course, this was impossible even with the smallest subminiature vacuum tubes.
A solution came in 1948 with the invention of the transistor by Bell Telephone Laboratories. Krim recognized its potential, and by 1952 Raytheon was manufacturing and selling junction transistors (under license from Bell Labs) to hearing-aid companies. More than 200 000 transistorized hearing aids were sold in 1953 by companies such as Beltone, Sonotone, and Zenith, eclipsing the sales of vacuum tube–based models.
In the late 1950s, Otarion Electronics, in Chicago, introduced the first hearing aid worn entirely at the ear—the Otarion Listener. The company did this by putting the electronics in the temple pieces of a pair of eyeglasses. Lee De Forest, himself, by then 84 years old and hard of hearing, appeared in ads in 1957 endorsing the product, saying, “It overcomes all of the objections I previously had to wearing a hearing aid.”
Other manufacturers, such as Beltone and Sonotone, introduced their own versions, and by 1959 “hearing glasses” had captured about half the market. The glasses were even worn by people with perfect eyesight.
In the 1960s, hearing glasses gave way to the smaller hearing aids familiar today. In 1964, Zenith Radio, a longtime hearing-aid manufacturer in Chicago*, sold a behind-the-ear model using an integrated circuit amplifier and a 1.2-volt button battery. It weighed just 7 grams, and Zenith claimed it was 500 times more reliable than hearing aids built with discrete transistors. The microphone, placed inside the ear, was connected by a wire to the amplifier and battery unit, which was clipped to the ear.
In the late 1980s, several companies were applying digital signal-processing chips to hearing aids, initially in hybrid analog-digital models in which digital circuits controlled an analog compression amplifier.
Fully digital models debuted in 1996, and programmable models, which allow for greater flexibility and fine-tuning of the hearing aids according to the patient's needs, became available in 2000. By 2005, digital hearing aids had captured more than 80 percent of the market. But there is still room for improvement. Today’s problem is background noise. Excelling at amplification and controlling acoustic feedback, digital hearing aids also bring in extraneous sounds that can obscure a conversation. Researchers are working on devices that filter this noise out.
For more information, read “Hearing Aids and the History of Electronics Miniaturization” [IEEE Annals for the History of Computing, 2011, Volume 33] by Mara Mills.
The IEEE History Center, which contributed this article, is funded by donations from institutions and individuals.
*This article has been corrected from its original version.