PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Airborne ultrasound noise at workplaces
Christoph KLING1; Robert SCHÖNEWEIß1; Christian ULLISCH-NELKEN2; Andrea WOLFF2;
Christian KOCH1
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Physikalisch-Technische Bundesanstalt (PTB), Germany

Institut für Arbeitsschutz (IFA) der Deutschen Gesetzlichen Unfallversicherung (DGUV), Germany

ABSTRACTAirborne ultrasound noise in the workplace is an increasing problem concerning occupational safety andhealth. Industry applications found most frequently are cleaning, cutting and welding. National regulations
exist, often non-consistently, providing limit or guidance values and only a few give advice on themeasurement procedure itself. But in general, ultrasound noise measurement has several basic problems, e.g.
a lack of instrumentation, traceability, tailored procedures and even knowledge on its impact on humans.
Among other issues, the EARS II project, funded by the European metrology programme for innovation andresearch (EMPIR) of EURAMET, has addressed the development of an appropriate measurement procedure
for the assessment of noise at work sites. In the daily business of occupational health and safety engineers,
there is no time for intensive studies. Thus, a reference workplace has been set up in the laboratory to
examine sound fields in detail. These studies gave a detailed view into the specific characteristics ofultrasound fields and allowed to derive an adequate and practicable measurement procedure for ultrasoundnoise assessment.
Keywords: Airborne Ultrasound, Noise, Assessment

1. INTRODUCTION
Ultrasonic devices are used in a wide-spread area of applications in industry. The most prominent
examples are ultrasonic welding of plastics such as foils or solid parts, ultrasonic cutting of cheese andcake, or ultrasonic cleaning of dirty engine parts or even sensitive electronic circuit boards. These
applications usually base on a resonant sound delivering techniques leading to high acousticalamplitudes with a fundamental frequency in a typical range between 20 kHz and 50 kHz. This
unavoidably leads to airborne sound which is emitted into the surrounding of the ultrasonic device
with very high amplitudes. Since the ultrasonic devices or the working process in many cases have tobe applied, controlled, cleaned or managed by a human, there is always a working place associatedwith them. The worker then moves in an area of high-frequency, tonal and loud noise.
Classical noise assessment is limited to a frequency range up to 20 kHz, due to restrictions of both,measurement instrumentation and assessment guidelines. Nevertheless, occupational health and safety
agencies register a growing number of complaints at working places equipped with ultrasonicmachinery. In general, any possible hazard to a worker’s health must be assessed. Thus, there is a needfor a practical and reliable ultrasound noise assessment.

2. ULTRASOUND NOISE ASSESSMENT IN PRACTISE
A number of national guidelines have been issued in the past decades all over the world, but a closer
examination shows that these basically refer to the same limited literature basis provided in the 1970’s
and 1980’s. The methods are widely borrowed from audible noise assessment, e.g. ISO 9612 (1).
Engineers following these guidelines often run into problems due to the limitations of theirinstrumentation and those methods which they have to apply. This leads to inappropriate andinconsistent noise assessment at ultrasound loaded working places.
Thus, one of the aims of the EMPIR project “Ears II - Metrology for modern hearing assessment
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and protecting public health from emerging noise sources" (3) was to develop an appropriate
measurement procedure for the assessment of ultrasound noise at workplaces. As a starting point for
the investigations, different measurement procedures have been compared. In (2), measurement results
were presented that show clear differences between stationary and spatially averaging methods tomeasure the sound pressure level. The stationary measurement method which is recommended in mostguidelines restricts the measurement to single measurement points. This leads to a higher uncertainty
in the determination of the sound pressure level. This lack of reproducibility makes it difficult forengineers to properly assess the exposure of workers at ultrasound workplaces.

3. REFERENCE WORKPLACEThe question arises why the methods borrowed from procedures for audible noise measurement
lead to inconsistent results. How does ultrasound noise differ from audible noise? An observation bymeasurement engineers is that the sound level strongly varies when moving the microphone throughthe sound field. Level differences of more than 12 dB are reported during movements of just a few
centimeters. This triggered to study the sound field of a working place in more detail. Unfortunately,such working places usually are integrated in a complex industrial production process giving nochance at all for time-consuming and extensive scientific studies.
Thus, a reference workplace was set up that simulates a typical industrial working place equippedwith an ultrasonic device. The setup consisted of a typical industrial welding machine with a driving
frequency of about 20 kHz. For better control, the machine was mounted in a free field environment inlaboratory. The setup was incorporated into the large PTB portal scanner. The scanner had three
moving axes in X, Y and Z direction and allowed computer-controlled positioning of a microphone in
the sound field with a precision of about 50 µm. The scanning area was nearly 2 m on each axis andgave the opportunity for large and highly detailed sound field studies. Extensive pretests andexaminations were performed with the sound source to ensure reliable scanning results. See (4) formore details. Figure 1 gives a photograph of the scanner.

Figure 1 – Portal scanner of PTB with a scanning volume of about 2 m x 2 m x 2 m with three axes in X, Y andZ direction with a minimum spatial resolution of about 50 µm. (4)4. A TYPICAL ULTRASOUND FIELD
The reference workplace was used for detailed recordings of the sound field around the reference
source. The scans were performed in areas where the worker typically moves. This included standing
working positions, sedentary working positions and positions in the far field and on the sides of the6334

machine. Figure 2 shows an example of a scan of the ultrasound field. More details can be found in (4).

Figure 2 – Measured sound pressure levels in front of an ultrasonic welding machine in an area of about
0.65 m in width and 0.8 m in height. The black rectangle marks the position of the driving sonotrode. (4)The scanning picture shows a highly non-uniform sound field with distinctive interference patterns.
The level differences are up to 35 dB between points with a distance of a few centimeters. The
characteristics are similar in areas at all sides of the machine. The various scans have been analyzed tofind significant parameters for the characterization of ultrasound fields. This will be used to derive aproper and applicable measurement method for noise assessment.
The scans clearly show that ultrasound fields differ from the more uniform audible sound fields.
Specific measurement methods are required to reproducibly obtain sound pressure levels that arerepresentative for the workplace and that allow to properly assess the noise exposure of the worker
even in non-uniform ultrasound fields.

5. INFLUENCE BY THE OPERATORThe non-uniformity of ultrasound fields is not the only difference to audible sound fields. The fieldstructure is quite more sensitive to any change in the acoustic properties of the surrounding or structure
incorporated in the sound field. This includes not only the machine housing but also the operator
himself. The standard procedure for audible noise assessment is to measure the noise in absence of theoperator. In fact, the sound level difference measured with and without him present is usuallyneglected in practice. This may not be appropriate for ultrasound fields.
In preliminary studies, it was observed that the operator influences the sound field. In (5), an
artificial head was placed in front of a loud speaker and a sinusoidal sound signal of 10 kHz was played.
The sound field was scanned next to the right ear of the head in rough steps with and without theartificial head present. Instead of the sound pressure level, the impulse response was determined ateach point which resulted in complex information about the sound field. The phase angle at eachmeasurement point was plotted for both situations and is shown in figure 3. In the situation with thehead present, an interference pattern can be observed that is not seen in the situation without the head.
The corresponding local level differences were about 12 dB.

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Figure 3 – Phase of the impulse response, left panel without an artificial head in the sound field, right panelwith head. Both plots show the phase in degrees as colors over an area of about 12 cm in width on axis X and22 cm in height on axis Y next to the right ear. (5)Motivated by these findings, the reference workplace studies were extended by the situation that anoperator was simulated sitting or standing in front of the welding machine. Again, an artificial head
was used as a representation of the operator. The sound field was scanned for sound pressure levels inareas around both ears. As an example, figure 4 shows a plot of a preliminary analysis of these scans.

Figure 4 – Measured sound pressure levels coded by colors in front of an ultrasonic welding machine without(left) and with (right) the head present. The scanned area is next to the right ear and has a width of about
25 cm and a height of about 29 cm.
The analysis of these measurement series is still in progress. But the first results show that there isa strong influence of the head and the torso of the operator on the sound field. The influence may affectthe level as well as the structure of the sound field. Both parameters must be accounted for whenassessing the exposure.
The minimal conclusion from these findings is that an ultrasound noise measurement is only valid
when the operator is present in the sound field. Further conclusions will be drawn when the analyses
will be finished.

6. CONCLUSIONSThe sound field at workplaces that involve ultrasonic machinery has been studied. Since real
workplaces do not allow extensive scientific studies, the investigations were performed using areference workplace that simulates a typical welding working station under controlled laboratory
conditions. Spatial high-resolution scans of the sound field were performed around the source.
Moreover, an operator was simulated using an artificial head being placed in front of the machine. First
results showed that the operator himself strongly influences the sound field and thus the result of anoise measurement. Further analysis of the data is in progress.
The aim of these studies was to derive a measurement procedure that allows the practical and6336

reliable assessment of the ultrasound noise exposition of a worker. The method should include thespecific characteristics of ultrasound fields and should be applicable even on a running industrial
production line.

ACKNOWLEDGEMENTS
This project is part of the Ears II project "Metrology for modern hearing assessment and protecting
public health from emerging noise sources". 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.

The ultrasonic welding machine used in this project was kindly provided by Herrmann
Ultraschalltechnik GmbH & Co. KG.

We gratefully acknowledge the support of the Braunschweig International Graduate School ofMetrology B-IGSM.

REFERENCES
1. ISO 9612:2009 "Acoustics - Determination of occupational noise exposure - Engineering method".
2. Christian Ullisch-Nelken, Andrea Wolff: "A measurement procedure for the assessment of industrial
ultrasonic noise". 25th International Congress on Sound and Vibration ICSV25, 8-12 July 2018,Hiroshima, Japan.
3. Website of the EMPIR project "Metrology for modern hearing assessment and protecting public healthfrom emerging noise sources": https://www.ears-project.eu/
4. Robert Schöneweiß, Christian Ullisch-Nelken, Andrea Wolff, Christoph Kling, Christian Koch: "Ears II:
Entwicklung eines metrologischen Konzepts zur Messung und Beurteilung der Luftultraschallexposition
am Arbeitsplatz". Conference proceedings of DAGA 2018 in München. Berlin 2018: Germany
Acoustical Society (DEGA).
5. Sonja Walther, Christoph Kling: "3-D-Vermessung von Luftschallfeldern". Conference proceedings of
DAGA 2015 in Nürnberg, p. 239-242. Berlin 2015: Germany Acoustical Society (DEGA).

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