Microphones, earphones, vibrators, and vibration pickups are needed for precise measurements on sound waves and vibrations in the audio-frequency range. Condenser microphones are calibrated with a reciprocity technique; the response to sound pressure is obtained over the range 10 Hz to 20 kHz by means of 20 cm/3 and 3 cm/3 couplers, and the response of a condenser microphone-preamplifier combination in a plane wave of sound is measured up to 20 kHz. The reciprocity technique is also used to calibrate vibration pickups in the range 10 Hz to 2 kHz. At higher frequencies up to 10 kHz, an optical interferometer is used for calibration of vibration pickups. An essential element of the instrumentation is a modulated photometer. Earphones used for measurement of hearing are calibrated on the NBS type 9-A couples, in which the sound pressure is measured between 100 Hz and 10 kHz with a reference standard condenser microphone. Vibrators for measurement of hearing by bone conduction are calibrated with artificial head-bones, the latter incorporating calibrated vibration pickups.
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