File:Western Electric double button carbon microphone.jpg

English: Western Electric Model 357 double button carbon microphone in 1B housing, used at WFBM, the first radio station in Indianapolis, Indiana, USA, from its founding in 1924. The double button microphone was developed around 1921 by Western Electric and was extremely widely used as a broadcast and recording microphone through the 1920s because it had significantly lower noise and harmonic distortion. It is often called the "ring and spring" micrphone, as it is often seen in period photos with its cover removed. as a small microphone cylinder suspended in the center of a metal ring by springs. Carbon microphones consist of a cell containing carbon granules between two electrodes in contact with a diaphragm, but they have high harmonic distortion because the change in resistance of the carbon is nonlinear, different under compression than under tension. The double button design used two carbon cells in contact with the diaphragm on either side. They were connected to a battery and center-tapped audio transformer in a "push-pull" circuit, which canceled even-order harmonic distortion. The stiff duralumin diaphragm had a small excursion which further reduced distortion and high resonant frequency and air damping which gave it a flatter frequency response. However the cost of this higher fidelity was that it had very low output.