The Crystal Microphone

Author: N.H. Crowhurst

How a piezoelectric crystal works

A piezoelectric crystal is another kind of transducer. When such a crystal is put under mechanical strain, a voltage is set up inside the crystal structure. (Similarly, when a voltage is applied to the crystal, mechanical bending occurs - this makes the crystal useful for loudspeaker use.) The best or most efficient method of getting a voltage from a crystal is by changing its shape. By cutting pieces of it in a particular manner, the crystal can be made to produce its best voltage by stretching or compressing.

Even this type of cutting does not make an efficient transducer, because the amount of force needed to produce any voltage is great and the force imparted by sound waves is small. The action can be improved by cementing two crystals together, so that when one is compressed and the other is stretched, the voltage between surfaces adds up in the same direction. Bending this combination gives increased output for a smaller applied force.

The bending crystal used in a microphone

This double crystal is still much too stiff for use as a microphone. For this reason, the diaphragm is coupled to the crystal by a lever action, which transforms a larger movement with smaller force to a smaller movement with a larger force at the crystal. In this way, the tiny movements of air particles in contact with the diaphragm are efficiently coupled to the crystal element to produce as sensitive a microphone as any other.