What are Ceramic Microphones?
We can say that ceramic microphones are modern version of crystal (piezoelectric) microphones and they originated in the late 1960s. They were considerably more stable and resistant to shock than electromagnetic microphones (Madaffari & Stanley, 1996). Operation of ceramic microphones is based on sound moving a diaphragm, which is connected to a piezoelectric material by strut or pin. Movement of the diaphragm causes the connecting pin to deform the (ceramic) piezoelectric material, resulting in producing a varying voltage across it. Ceramic microphones had internal preamplifiers for boosting the microphone output signal and for providing a low output impedance, necessary conditions for their use with transistor amplifiers.
Ceramic microphones are sometimes loosely classified as piezoelectric. Their transducer mechanism uses a special polarized ceramic such as barium titanate. It generates an electrical potential. This conversion is similar to a piezoelectric effect, however, the mechanism differs in both compressive and extensive strains produce the same polarity of potential. If untreated, such a transducer would produce a highly distorted output. By a process called polarization (not identical to that for the capacitor type) the equivalent of an offset strain occurs in the material. This offset linearizes the output. Retention of polarization is temperature dependent. At a critical elevated temperature, you lose polarization.
The stressed force is divided by a linkage to a diaphragm that moves in response to sound pressure. Such microphones are more rugged than capacitor or electret microphones. Even when used in dosimeters, they can survive rough treatment. They are also significantly less susceptible to impairment by condensation of atmospheric moisture.
Ceramic microphones have greater susceptibility to excitation by vibration, have higher noise filters floors, and have a more limited high-frequency response when compared to the capacitor and electret types. Though they can be made in higher grades, they commonly are available for type 2 instruments. These limitations are acceptable for dosimetry, and people use them in such applications because of their desirable properties.
As mentioned above the Barium titanate was the material of choice for a ceramic mic element. (See U.S. Patent 2,640,165, issued May 26, 1953.) Lead Titanium Zirconate was also used. A crystal mic element used a Rochelle salt crystal. This type of salt crystal could not withstand temperatures above 130 degrees F. Also direct moisture, or a very humid environment, would eventually damage the salt crystal. A ceramic element was far more tolerant of high temperature and high humidity. The advantages of the ceramic element eventually led to the elimination of the Rochelle salt crystal as a mic element.
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