As can be seen from the name plunger coil microphone, in this microphone a voice coil is immersed in a magnetic field. The coil, which is connected to the diaphragm, is located in an air gap and is centered by a centering spider so that it can move freely in the air gap and does not strike the walls of the permanent magnet. As soon as sound pressure( Pa) hits the diaphragm, the voice coil in the air gap of the magnetic field moves in proportion to the frequency of the incident sound pressure and generates a voltage by means of induction. This voltage is calculated from the product of the velocity (v) of the diaphragm or voice coil, the magnetic induction (B) and the conductor length (l) of the coil. With a connected nominal impedance of 200 ohms, the current results in a voltage of about 1 mV/Pa to 3 mV/Pa, which corresponds to a sensitivity of about -75 dB. The frequency range of plunger coil microphones is between 30 Hz and 20 kHz.
Moving-coil microphones are classified into large-diaphragm and small-diaphragm microphones according to the diaphragm area. The diaphragm sizes are 1", 1/2", 1/4", etc.
Due to the mass of the plunger coil, the movement at higher frequencies is damped, which is reflected in a frequency-dependent frequency response, where the higher frequencies have reduced sensitivity. The frequency response curve has a maximum at low frequencies and then falls relatively linearly to the highest frequencies. Moving coil microphones have a second coil to compensate for interference magnetic fields.
The advantages of moving coil microphones are their robustness and the ability to pick up high sound pressures. Disadvantages are the poor impulse response and the steeply falling frequency response.