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PTB calibration service for airborne ultrasound

Traceability of sound pressure at frequencies over 20 kHz possible for the first time

PTB-News 3.2016
Especially interesting for

manufacturers of measuring and laboratory equipment

users of ultrasound methods

At PTB, a procedure has been developed for the calibration of microphones in the ultrasound range of up to 100 kHz. By means of this procedure, reference standards and working standards can be produced for laboratories in industry and research in order to ensure the traceability of airborne ultrasound. This new calibration service is now available.

Reference microphone for airborne ultrasound (¼-inch measurement microphone mounted on an adapter to ½ inch) in the calibration setup at PTB’s acoustic anechoic chamber.

In many areas of our daily lives, the use of ultrasound technology is increasing. With it, airborne sound measurements in this frequency range are also becoming more important, such as those which are performed to check the limits of noise emissions. To this end, employers and occupational associations measure and evaluate the sound pressure level present at workplaces affected by noise pollution, such as ultrasonic welding facilities. In accordance with DIN EN 61010-1, manufacturers of measuring and laboratory equipment must furnish proof of the safety of their products by means of an airborne sound measurement up to 100 kHz.

To date, metrological traceability has not been possible for measurements of sound with frequencies of over 20 kHz for the unit of sound pressure, the pascal. The Danish Metrology Institute (DFM) recently introduced a primary standard which allows specialized measurement microphones to be calibrated as a primary transfer standard for frequencies of up to 150 kHz. As this method is limited to a certain microphone type, a procedure has been developed at PTB which is designed to enable the calibration of all types of microphones. The procedure is based on the conventional substitution procedure in accordance with DIN EN 61094-8, by means of which a controlled sound field is generated and successively measured by the primary standard and by the test piece. The substitution procedure was adapted to the specific characteristics of airborne ultrasound.

As the wavelength of the sound propagated in the air amounts to only a few millimeters (3.4 mm at 100 kHz), very small areas and edges (which can be neglected in hearing sound) cause reflection and diffraction; this severely disturbs the sound field and, thus, the precision of the calibration. In addition, above approx. 10 kHz, the damping in air is no longer negligible and increases sharply in the ultrasound range; furthermore, it is heavily dependent on the ambient conditions. This places high demands on the sound source, and requires that measurement conditions be precisely controlled. Here, compared to calibrations for hearing sound, more effort and higher measurement uncertainties are involved; the uncertainties, depending on the frequency and type of the specimen, range from 0.4 dB to approx. 0.6 dB.

The new calibration procedure now allows ½-inch and ¼-inch (with adapter) measuring and working microphones to be traced to the Danish primary standard for frequencies of up to 100 kHz. They serve as secondary transfer standards in research and industry, as well as being used for measurements themselves. In the next step, the procedure will be further developed in such a way that any type of microphone, such as microphone/ amplifier combinations, MEMS microphones, optical microphones or sound level meters, can be calibrated.


Christoph Kling
Department 1.6 Sound
Phone: +49 (0)531 592-1730

Scientific publication

C. Kling: Microphone calibration service for airborne ultrasound. Proceedings INTER-NOISE Hamburg (2016)