Somewhat simpler, lower cost calibrators are also available that work on the driven‐diaphragm principle. Some commercially available types produce 114 dB at 1000 Hz. A stabilized 1000‐Hz oscillator feeds a piezoelectric driven element that vibrates the metallic diaphragm creating a pressure in the front coupling. The diaphragm is driven by an oscillator powered by a 9‐V battery. Some calibrators generate five selected frequencies: 125, 250, 500, 1000, and 2000 Hz at 94 dB. With such calibrators, corrections should normally be made for changes in ambient temperature and pressure (altitude). Newer types of calibrator have a built‐in microphone that measures and controls the sound pressure generated in the coupler cavity via a feedback loop [7].
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