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A measurement procedure to determine the exposure to airborne ultrasound at the workplace

15.10.2018

A procedure to measure airborne ultrasound is being developed within the scope of Ears II, an EU research project. This procedure, which is designed for occupational safety, has been developed to reliably determine the exposure of staff to airborne ultrasound at the workplace. Important findings obtained during its development were provided by sound field scans with a high spatial resolution at a reference workstation at PTB.

Today, ultrasonic technologies are found in many different fields of industry. Ultrasound is used in applications such as the welding of plastics, the cutting of foodstuffs and the cleaning of objects. The side-effect of these applications is that frequently, airborne ultrasound is generated at these workplaces. Due to ultrasound being widely used, more and more people are being exposed to airborne ultrasound at the workplace. Some of these people have complained about various adverse effects related to the exposure to ultrasound at the workplace [1], which has revealed the potential health hazard induced by the use of ultrasound. According to the German regulation on the protection against noise and vibrations at the workplace (Lärm- und Vibrations-Arbeitsschutzverordnung) [2], it is necessary to determine and assess the exposure to sound occurring at the workplace. However, so far, no standardized, scientifically proven measurement procedure for airborne ultrasound has existed that would be suitable for practice. This is the reason why such a measurement procedure has been developed at PTB together with a partner, the Institut für Arbeitsschutz (IFA, Institute for Occupational Safety and Health) of the Deutsche Gesetzliche Unfallversicherung (DGUV, German Social Accident Insurance), within the scope of Ears II, an international research project [3], which is funded by the European Union.

Compared to the measurement of audible sound in the frequency range from 20 Hz to 20 kHz, the measurement of ultrasound (i.e. sound with frequencies higher than 20 kHz) is challenging. Due to the higher frequency, ultrasound has special properties with regard to its propagation and interaction with the environment; these properties must be taken into account by using dedicated measurement technologies [4] and procedures. A reference workstation was set up at PTB in order to find out what these special requirements are and, based on these findings, to develop a new procedure that does justice to these requirements. This reference workstation has offered the possibility to perform time-consuming measurements for sound field characterization on an ultrasonic source that was representative of the ultrasonic sources used in industry. The sound source used for this purpose was an ultrasonic welding machine that was made available by our industrial cooperation partner Herrmann Ultraschalltechnik GmbH & Co. KG. The reference workstation was set up in the anechoic environment of a three-axis scanner system. A four-channel microphone array that had been manufactured specifically for this purpose was used to measure the sound field with very high spatial resolution. The large measurement volume of more than 7 m3 that was provided by the measurement set-up, in combination with the high resolution in the sub-millimetre range that can be simultaneously achieved, is a novelty in this field. The components of the reference workstation are shown in Figure 1.

Figure 1: The components of the reference workstation; left: three-axis scanning system with anechoic environment; top right: four-channel microphone array; bottom right: ultrasonic welding machine (figure taken from [5])

Figure 2 shows the example of a sound field scan recorded on this test set-up. This scan represents the unweighted sound pressure level distribution of a vertical layer on the front of the ultrasonic welding machine. A finely structured interference pattern appears with, in part, very high sound pressures (depicted in yellow shades) and peaks reaching over 120 dB. However, since the sound field is interspersed with regions of low sound pressure (depicted in blue shades), individual measurement points are not representative of the total sound field. This is the reason why the measurement procedure developed at PTB makes use of grazing measurement paths to take the different sound pressure levels into account to which persons working at this workstation are exposed due to their movements.

 

Figure 2: Ultrasonic field of the reference workstation

Based on the measurement procedure developed, initial measurements have been carried out together with the Institut für Arbeitsschutz at workplaces in industry where different ultrasonic modalities are applied. These investigations will shed light on whether the new measurement procedure can be used in practice and whether it is universally applicable. Currently, further investigations are being conducted and already acquired data are being assessed in order to determine which influences the ambient conditions as well as the staff operating a machine might have on the sound field. The measurement procedure will then be subjected to revision, and it is planned to use it for standardization. Moreover, the procedure is envisaged to serve as a standardized, suitable and practicable method to determine the exposure of staff to airborne ultrasound for the purpose of occupational safety. This procedure will allow the risks to be assessed so that it will be possible to determine whether protection measures are necessary.

This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

Literature:

[1] Leighton, T. G.: Are some people suffering as a result of increasing mass exposure of the public to ultrasound in air?; Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science; 2016; 472 (2185); Opens external link in new windowLink
[2] Verordnung zum Schutz der Beschäftigten vor Gefährdung durch Lärm und Vibrationen (Lärm- und Vibrations-Arbeitsschutzverordnung vom 6. März 2008 (BGBl. I S. 261), die zuletzt durch den Artikel 2 der Verordnung vom 15. November 2016 (BGBl. I S. 2531) geändert worden ist)
[3] EMPIR 15HLT02 Ears II – Metrology for modern hearing assessment and protecting public health from emerging noise sources; Opens external link in new windowLink
[4] Wächtler, M.; Kling, C.; Wolff, A.: Entwicklung eines Ultraschall-Pegelmesssystems für den Arbeitsschutz; Lärmbekämpfung; 2018; 13 (1); S. 28-32
[5] Schöneweiß, R. et al.: Ears II: Entwicklung eines metrologischen Konzepts zur Messung und Beurteilung der Luftultraschallexposition am Arbeitsplatz; In: Fortschritte der Akustik - DAGA 2018; 44. Deutsche Jahrestagung für Akustik, München, 19.-22. März 2018; S. 255-257; Hrsg.: Deutsche Gesellschaft für Akustik e.V., Berlin 2018

Contact:

Robert Schöneweiß, FB 1.6, AG 1.62, E-Mail: Opens window for sending emailrobert.schoeneweiss(at)ptb.de