What are Crystal Microphones?
The introduction of the crystal microphone created a new era of cheap better quality microphones. Astatic Corporation began when two Ham Radio enthusiasts, C. M. Chorpening and F. H. Woodworth, found that they could make a microphone out of Rochelle salts. Rochelle salts are one of three different salts found in household Cream of Tarter. It has the strange ability to squirt out an electric current when you bend a crystal. This is called the piezoelectric effect. The trick is to bend the crystal without cracking it.
Chorpening and Woodworth took a thin piece of Rochelle salt and placed it so that an armature could push on the ends of the crystal causing it to bend on a fulcrum.
Sound vibrates the diaphragm that moves the armature that bends the crystal. Small copper foil strips attached to the Rochelle salt crystal transmit the electricity out to the amplifier connector.
Crystal and Ceramic microphones are considered low quality and although they are better than carbon microphones are usually used where low cost is more important than good sound reproduction.