How Does a Carbon Microphone Work?

A carbon mic is a simple device to turn sound waves into an electronic signal. Carbon microphones were used in telephones, radio broadcast systems and many other devices through the 80s. Although they were eventually replaced by higher-fidelity, less noisy microphones, they are still used in military installations and other applications where their durability and ability to work on low power give them an advantage over more sophisticated mics.

A carbon microphone is built inside a tube. It consists of two metal plates with a layer of small grains of carbon between. Each metal plate is attached to a wire which connects it to an audio receiver. The top of the microphone is usually covered with a metal or plastic sheet with holes in it, which lets sound in while preventing anything from damaging the mic element.

Carbon is a resistor, meaning it conducts electricity, but not very well. A current runs from one plate through the carbon to the other plate. The carbon molecules normally resist it somewhat, lowering the power flow. When a sound wave pushes down on the top plate, however, it squeezes the carbon molecules more tightly between the two plates. This increases their conductivity, creating more electric current. As the plate moves up and down with the sound wave, the current increases and decreases, creating an electric wave in the shape of the sound wave.