The Crystal Microphone

Rochelle salt crystals are used in crystal microphones utilizing the piezoelectric effect. These crystals produce a larger potential difference than quartz. The Rochelle salt crystals are cut into thin slabs. Suitable metallic electrodes are provided on the faces of a slab. Wire leads are attached, and the element is scaled to prevent the entrance of moisture.

Two principles are used in making crystal microphones. One type makes use of the fact that, if sound waves strike the crystal element, these feeble waves are of sufficient strength to deform the crystal, thus producing a potential difference that corresponds to the impinging sound waves. This is the "sound cell" type. The "diaphragm type" of crystal microphone uses a diaphragm to intercept the sound waves, and the motion of the diaphragm is transmitted to the crystal which is mounted so that motion of the diaphragm bends the crystal element (Fig, 16). Sound-cell crystal microphones have been built in which a number of crystals connected in series are employed, and sometimes they were connected in series-parallel combinations to give both increased output and low internal impedance.

Figure 16. Parts of a diaphragm-type crystal microphone, and assembled microphone at lower right. The output is about -50 decibels, where 0 decibels is 1.0 volt per dyne per square centimeter. This microphone works into a 5-megohm load resistance. (Courtesy Brush Development Co.)

With microphones and other devices using Rochelle salt crystals, the operating temperatures recommended by the manufacturer should not be exceeded or the crystal may be damaged permanently.