The working component in an audible sound transducer usually is a thin disc of piezoelectric ceramic bonded to a similarly thin metal membrane (see figure 5.3). When a voltage is applied to the ceramic disc, the disc deforms, causing the metal membrane to bend. When an alternating voltage is applied the ceramic / metal element vibrates at the frequency of the applied voltage, producing audible sound. (The resonance frequency of the ceramic disc alone is too high to produce audible sound, hence the need for the metal membrane.)
The construction of a buzzer greatly affects the level of the sound it emits -- and the cost of the device. Nodal support mounting (see figure 5.3) minimizes mechanical restriction on the movement of the sound-producing mechanism, so the amplitude of the vibrations is highest. On the other hand, signal originating from beyond the nodal ring will be in antiphase with signal from the central part of the element, and can reduce the sound output. To prevent this interference, the housing must absorb all output from the periphery of the element.
In clamped edge mounting, the entire surface of the flexing element can vibrate in phase. Relative to nodal support mounting, greater interaction between the sound-producing element and the housing reduces the amplitude of the vibrations. Furthermore, unit-to-unit variations in clamping can affect the uniformity of product performance, and a more substantial housing generally is needed to retain the element.
In flexible edge mounting, the flexing element is restrained in a pliant material, such as a rubber. This ensures excellent signal characteristics, at a resonance frequency half that for the same element in one of the other mounting alternatives. Again, however, careful construction of the device is critical to effective performance.