From Sheldon Hochheisert, “The History of Hearing Aids“:
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.
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 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.