The Carbon Microphone is an obsolete type of microphone, one of the first to be invented, and which was the standard in telephones for almost a century.

Its operating principle is different from modern microphones. It has a small chamber filled with carbon granules, positioned between two metal plates. One of these plates acts as the diaphragm.

1. An electric current (DC) constantly passes through the carbon granules.

2. When sound waves hit the diaphragm, it compresses and decompresses the granules.

3. This pressure variation alters the electrical resistance of the granules: the more compressed, the lower the resistance; When fewer are compressed, the resistance increases.

4. This variation in resistance modulates the electric current passing through them, creating an electrical signal that is an analogy of a sound wave.

The Carbon Microphone has a very limited sound quality, noisy and with a narrow frequency response (typical "old telephone sound"). Although obsolete for recording, it was fundamental to the development of telephony and early radio transmissions, as it was robust, cheap and had the advantage of amplifying the signal acoustically.