What is a Crystal Microphone?
The crystal microphone is the type most widely used in lower cost installations. It has relatively high output level and high impedance. A long cable will reduce the high output voltage available 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 subjecting to stress or strain. Crystal microphones are utilizing this effect.
Crystal Microphone Construction
The construction of the crystal microphone involves a finger-shaped slice of crystal. A compliant clamp secures it at the one end, and the apex of a cone bears against the other. Sound pressure waves cause the cone to alternately press against and bend the crystal and release it. Thus, corresponding voltages are generated across the slice. A pair of contacts is fixed opposite surfaces to take of 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 as biomorph. One of the disadvantages with this type of transducer is the mass which must be moved by the sound pressure acting on the cone. This consists of the mass of the cone plus that of the crystal, or that part of it which is moved. This restricts the frequency response to its upper end to around 10 kHz and also limits the transient response. In addition, there are resonances due to the cone and the crystal.
Additional Construction Types
With some better microphones, the cones do not actuate the crystal directly but thru 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 but at the same time so also is the amplitude of the transmitter vibrations, hence the signal output. Another type of construction is the cell where several crystal elements are sealed together, this also being termed as multimorph. Here 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 also the cone resonance is eliminated.
There is no DC part thru 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, then consists of a voltage source in series with a capacitor. The crystal microphone is normally non-directional although a pressure-gradient crystal microphone which gives a nondirectional response pattern is also being marketed.
It would be good to mention that the natural crystals, such as Rochelle salts, are not very durable. They are adversely affected by humidity and high temperature, also they fracture easily when subjected to shock.
Additional Resources and Source texts