carbon microphone
The carbon microphone is an acoustic transducer that generates electrical voltage changes from changing sound pressure. The transducer itself consists of very fine carbon particles which are compressed under pressure and thus change their resistance value.
The carbon particles are contained in a capsule covered with a thin film that forms the diaphragm. It is connected to a DC voltage source. When sound pressure hits the diaphragm, the carbon particles are compressed, the resistance value decreases, and the current flowing through them increases.
The carbon microphone with its classic transducer method was used in studios in the early years of broadcast technology. In order to avoid acoustic disturbances caused by footstep and movement sound and, good studio microphones were embedded in marble blocks, which were suspended with springs. Later, carbon microphones were installed in telephone receivers.