The Dynamic (Moving-Coil) Microphone

Author: N.H. Crowhurst

The principle of dynamic microphone operation

If a wire connected to a meter that will indicate when current flows is moved about near to a magnet, the meter will show current fluctuations. When the wire is moved, the meter deflects. Holding the wire still produces no current. The direction of current indicated on the meter depends on the direction in which the wire is moved. This is an ideal basis for converting movement into electrical current. Because movement is the essential feature for conversion, a microphone using this principle is called a dynamic microphone. The problem in making a microphone of this type is that a large movement is needed to produce even a small current, while the movement of the air particles due to sound waves is small.

Increasing the effectiveness of the magnet

This problem is overcome, to some extent, by increasing the intensity of the magnetic field. A North and South pole are brought close together, and the wire moves in the narrow space between them. To increase its effectiveness, many turns of wire move in the same gap. It is convenient to make the gap circular, because this simplifies construction of the coil, gets the poles close together, and gives the coil free space in which to move.

Converting movement into current is only part of the job. We must first move the coil by means of the sound waves, which requires a diaphragm. To move freely, the diaphragm must be light - as little heavier than air as possible. Because the coil is also attached to the diaphragm, it, also, must be as light as possible, or it would load the diaphragm down. Hence, a small coil must be used.

Cross section of a simple moving-coil microphone

The use of a small coil requires a very intense magnetic field to get the best results. To accomplish this, the gap is made very small. To prevent the coil rubbing against the magnet poles, a centering "spider" or suspension is used, which allows free movement in the direction of vibration, while preventing the coil from moving against the pole faces.