The signal from a ribbon microphone is generated as a motional voltage; the movement of the ribbon in the magnetic field generates a voltage proportional to the velocity of the ribbon.

The flat, faithful frequency response of the condenser microphone arises from its mechanism. The charge on the membrane depends only upon the spacing and shows no appreciable resonances to skew the frequency response. The capacitance of the parallel plate membrane structure is given by

When the spacing changes, the charge changes, giving an electric current through the resistor R.

The voltage measured across the resistor is an electrical image of the sound pressure which moves the membrane.

Electret condenser microphones are not to be compared with the studio standard condenser microphones which have such excellent frequency response characteristics. The electret class of microphones are condenser microphones which use a permanently polarized electret material for their diaphragms, thus avoiding the necessity for the biasing DC voltage required for the conventional condenser. They can be made very inexpensively and are the typical microphones on portable tape recorders. Better quality electret condensers incorporate a field-effect transistor (FET) preamplifier to match their extremely high impedance and boost the signal.

The electret material of the diaphragm may be less than a thousandth of an inch thick. Even so, it is polarized enough to produce an active capacitive change in the voltage between the membrane and the back plate when it is moved by the pressure of a sound wave. The polarization is achieved by a combination of heat and high voltage during manufacture. Electron bombardment may also be used (Rossing). The diaphragm is backed by an evaporated metal film.