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PTB calibration service for airborne ultrasound

09.05.2016

For measurements of sound with frequencies above 20 kHz, metrological traceability has so far not been possible for the unit of sound pressure, the pascal. Now a procedure for the calibration of microphones in the ultrasound range up to 100 kHz has been developed at PTB. With this procedure, reference standards and working standards can be produced for laboratories from industry and research to ensure the traceability for airborne ultrasound. The new calibration service is available as of now.

With the increasing use of ultrasound technology in all fields of daily life, the number of quantitative measurements of airborne ultrasound has also increased. Noise immission at workplaces must, for example, comply with both limiting values in the ultrasound range and in the audible hearing range. In this connection, employers and occupational associations measure and evaluate the sound pressure level also above 20 kHz at correspondingly affected workplaces such as ultrasonic welding facilities. In addition, manufacturers of measuring and laboratory equipment must, in accordance with DIN EN 61010-1, furnish proof of the safety of their products through an airborne sound measurement up to 100 kHz. 

Up to now, metrological traceability has not yet, however, been possible for measurements of sound with frequencies above 20 kHz for the unit of sound pressure, the pascal. At the Danish Metrology Institute (DFM), a primary standard has recently been developed which allows measurement microphones to be calibrated as a primary transfer standard up to 150 kHz. The procedure is, however, limited to special microphones (combination of a WS3 ¼-inch microphone mounted on a ½-inch adapter without a protection grid, see Figure 1) which are not used in this way in practice. Another calibration step is required to transfer the unit of sound pressure from the primary transfer standard – the special reference microphone – to any microphone whatsoever.

Such a procedure has now been developed at PTB. The new procedure is based on the classical substitution procedure according to DIN EN 61094-8 which has been adapted to the specific properties of airborne ultrasound. For that purpose, a controlled sound field is generated and successively measured by the reference and by the test piece). In the case of a stable sound field, the level difference measured by the two microphones is only due to their different sensitivities. The sensitivity of the test sample can be calculated on the basis of the known sensitivity of the reference microphone.

The specific properties of airborne ultrasound place special demands on the procedure. The wavelength of the sound propagated in the air amounts to only a few millimeters (3.4 mm at 100 kHz). The smallest areas and edges which can be neglected in hearing sound cause reflection and diffraction in ultrasound which severely disturb the sound field and, thus, the precision of the calibration. From approx. 10 kHz upwards, the damping in air is no longer negligible, increases drastically in the ultrasound range and depends, in addition, strongly on the ambient conditions. This places high demands on the sound source and requires the precise control of the conditions of measurement. Compared to calibrations for audible sound, these special requirements lead to an increased effort and to measurement uncertainties between 0.4 dB and approx. 0.6 dB. The actual measurement uncertainty depends on the frequency, but also on the design of the test piece.

The new calibration procedure now allows measuring and working microphones of the sizes ½ inch and ¼ inch (with adapter) up to 100 kHz to be traced to the Danish primary standard. They serve as secondary transfer standards in research and industry or are used themselves for measurements. In the next step, the procedure will be further developed in such a way that any microphones whatsoever, such as, for example, microphone-amplifier combinations, MEMS microphones, optical microphones or sound level meters, can be calibrated. 
As a start, the new procedure, however, allows airborne ultrasound to be measured quantitatively also in practice.

 

Figure 1: Reference microphone for airborne ultrasound (¼-inch measurement microphone mounted on an adapter to ½ inch) in the calibration setup in the acoustic anechoic chamber.

 

Contact person:

Christoph Kling, FB 1.6, AG 1.63,Opens window for sending email christoph.kling(at)ptb.de