| 1890 | The electric hearing aid is patented by Alfonzo Miltmore. These devices make use of a carbon dust microphone, an earphone and a very large battery pack yet are very unreliable in efficiency due to humidity, dust particles, and the movements of the wearer. The carbon microphone consists of carbon particles in between a fixed electron plate and a vibrating electron. The current flowing between the electrons varies depending on the pressure exerted on the vibrating electrode by the carbon particles providing amplification. |
| 1898 | The first electric aid, the Akoulathon, reaches production. |
| 1900 | The carbon ball is invented by Hutchison and Kelley, greatly improving the reliability of carbon microphones. These hearing aids utilize a microphone, an earphone, an amplifier (the vacuum tube is located here), and two batteries, one to warm the vacuum tube filament and one to amplify. |
| 1934 | The vacuum tube hearing aid is introduced and produced by multiple English manufacturers. |
| 1947 | The transistor is invented and quickly adapted to the hearing aid. The transistor, along with advances in battery technology, make the devices much smaller, cheaper and energy efficient. |
| 1959 | Nicholas & Clark make an in-the-canal (ITC) device. |
| 1962 | The Zeta noise blocker is invented by Linear Technology. This device incorporates circuitry to remove environmental noise from speech. |
| 1980-1985 | Many circuits are introduced that make use of analog sound processing. |
| 1987 | A joint project between the University of Wisconsin and Nicolet Instrument Cooperation produces the first digital hearing aid. |