Abstract
Exposure to airborne low‐frequency ultrasound occurs in many industrial applications such as cleaning, welding plastics, measuring distances in buildings, and by using consumer devices such as camera rangefinders, automatic door openers, parametric ultrasound loudspeakers, etc. Exposure to ultrasound in air may be dangerous to the hearing and may have negative bio‐effects on humans. Care should be taken in its use. In order to establish appropriate limits and to measure the output of ultrasound devices, there is a need to develop a sound‐pressure standard in the frequency range from 20 kHz to about 160 kHz. A calibration project for quarter‐inch microphones by the reciprocity method has been started in the PTB, and a new automated measurement setup for free‐field reciprocity calibration of quarter‐inch microphones has been established. A procedure for a free‐field reciprocity calibration of quarter‐inch microphones in the frequency range 20 to 160 kHz is described. First results of free‐field calibration of quarter‐inch microphones are presented, giving the sensitivities and the repeatability of results. The effects of reflections and cross talk on the accuracy of the measurements will be explained.
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