Crystal Microphones

What Is a Crystal Microphone?

The crystal microphone is the most widely used type in lower-cost installations. It has a relatively high output level and high impedance. A long cable will reduce the high output voltage from a crystal microphone and may affect its high-frequency response. The output level of this type of microphone is usually between -48 dB and -60 dB. Their output impedance is almost more than 100.000 ohms. Certain crystals, such as Rochelle salt and quartz, possess the property of generating small emf's when subjected to stress or strain. Crystal microphones utilize this effect.

Crystal Microphone Construction

The construction of the crystal microphone involves a finger-shaped slice of crystal. A compliant clamp secures it at one end, and the apex of a cone bears against the other. Sound pressure waves cause the cone to press against, bend the crystal, and release it alternately. Thus, corresponding voltages are generated across the slice. A pair of contacts is fixed on opposite surfaces to remove the signal. An improvement is obtained if the single slice of the crystal is replaced by two slices cemented together. Then, when pressure is exerted, one slice is compressed while the other is stretched. Thus, equal and opposite voltages are produced, which, being in series like a car of a battery, give double the output. The double-crystal unit is termed a biomorph. One of the disadvantages of this type of transducer is the mass, which must be moved by the sound pressure acting on the cone. This consists of the cone's mass plus that of the crystal or that part of it that is moved. This restricts the frequency response to its upper end to around 10 kHz and limits the transient response. In addition, there are resonances due to the cone and the crystal.

Additional Construction Types

With better microphones, the cones do not actuate the crystal directly but through a cantilever. According to the dimensions involved, the effect of the mass of the crystal and the mechanical resistance offered by its stiffness can be reduced. Still, simultaneously, the amplitude of the transmitter vibrations, hence the signal output, can be reduced. Another type of construction is the cell, where several crystal elements are sealed together; this is also termed a multimorph. The cone is often dispensed with the sound pressure waves acting directly on the crystal. The output is lower with this arrangement, but the frequency response is better, and the cone resonance is eliminated. There is no DC part through a crystal microphone, the crystal being an insulator. Having the two electrical contacts on either side of the slice, the unit behaves as a capacitor; the equivalent circuit consists of a voltage source in series with a capacitor. The crystal microphone is typically non-directional, although a pressure-gradient crystal microphone, which gives a non-directional response pattern, is also being marketed. It would be good to mention that natural crystals like Rochelle salts are not very durable. They are adversely affected by humidity and high temperature, also they fracture easily when subjected to shock.