Carbon Hearing Aids—General Information

The first electrical hearing aids appeared on the market at the turn of the 20^th century. These were made possible by the invention of the carbon microphone.

The carbon microphone was originally invented for use in the telephone and had the properties of being able to modulate current. It is this property that made it useful in the early electrical hearing aids.

The carbon microphone used in most hearing aids differed from that used in the telephone. The telephone microphone used carbon granules packed in a cylinder with a metal diaphragm. The compression of the carbon by the action of sound waves on the diaphragm varied the resistance of the carbon in relation to the sound and permitted the reproduction of the sound in an earphone.
 

In contrast, the carbon microphone used in hearing aids employed a carbon block with several cupped openings which were filled with carbon shot (very tiny carbon balls). These were held in place by a thin carbon diaphragm. This arrangement permitted the control of larger currents and produced a perceived amplification of the sound received in the earphone.

When more amplification was required, carbon microphones were wired in parallel to increase the current flow. The early hearing aids came with one, two, three or even four microphones.

Carbon hearing aids were slow to catch on with hard of hearing people. As a result, acoustical hearing aids (ear trumpets, conversation tubes, etc.) were still used well into the first quarter of the 20^th century.

Carbon hearing aids only helped people with mild to moderate hearing losses. They produced noisy and scratchy sound due to the action of the carbon balls on the diaphragm. They also produced a very limited frequency response.

You can read more about the carbon microphone in the Wikipedia article on the carbon microphone.

The inside of a carbon microphone showing the carbon block—the grey "doughnut" in the center with the 6 beveled "cups" in it.

The diaphragm fit close enough to the top of the "cups" that the carbon shot couldn't fall out, yet loose enough that they could "rattle around".

One of the interesting things about carbon microphones is that they wouldn't work if they were laying flat on their backs. This is because the carbon shot wouldn't touch the diaphragm (which was one side of the circuit; the other side being the carbon block).

When the microphone was held upright,  gravity pulled the carbon shot downwards so that they spread out in the lower half of the "cup" and thus touched both the diaphragm and carbon block and current flowed.
 

When sound waves hit the microphone they pushed it inwards slightly, which slightly pushed the carbon shot tighter together. This reduced the resistance between them, which increased the current flow. When the diaphragm moved out in response to fainter sound waves, the pressure lessened and thus the current flow likewise reduced. In this way, the current flowing through the diaphragm/carbon shot/carbon block was modulated in sync with the sounds striking the diaphragm. This produced an electrical representation of sound which was then amplified.
 

Carbon hearing aids continued to be the most sold hearing aids up to about 1940. At this time, it became possible to manufacture vacuum tube hearing aids small enough to be worn on the body.

The relatively large size of vacuum tubes in the early 1930’s limited their use in hearing aids to what could be referred to as "table model" hearing aids. Many of the early table model vacuum tube hearing aids used carbon microphones in conjunction with a vacuum tube amplifier.

Because of the limited amplification of carbon microphones and the need for more powerful body worn aids, inventors tried to increase the amplification by using a mechanical amplifier (carbon amplifier).

The carbon amplifier was basically an earphone and carbon microphone closely coupled back to back where the diaphragm of the earphone also served as the diaphragm of the carbon microphone. This provided a secondary circuit to the hearing aid in which the current flow was increased.

Side view of the Acousticon carbon amplifier.

Mechanical amplifiers were previously used in the telephone industry as repeaters prior to the invention of the vacuum tube. They were not very successful and were replaced by vacuum tube amplifiers as soon as these became available.

If you want to learn more about mechanical amplifiers check out the Douglas Self website on mechanical amplifiers. This link takes you to the part of this webpage where the carbon hearing aid amplifier is discussed.