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Small sources, loud noise

05.08.2013

Ultrasonic equipment, such as, e.g., ultrasonic cleaning baths, can cause very loud noise. High-frequency airborne ultrasound was measured up to 100 kHz on a solid metrological basis for the first time, and different assessment quantities were determined quantitatively.

Airborne ultrasound is becoming increasingly used in our everyday environment, not only at work, but also frequently at home and in mobile applications. For example, certain animal-repelling systems rely on ultrasound to keep moles and other undesirable guests out of gardens. Also, parking assistance systems on cars measure the distance to objects located in the vicinity by means of ultrasonic sensors, and glasses, jewellery and even golf clubs can be cleaned using ultrasonic cleaning devices. In spite of their widespread use, the noise caused by such airborne ultrasonic sources is generally unknown. This is mainly due to the fact that ultrasound has always been difficult to detect metrologically. There are no standardized measurement procedures and hardly any suitable measuring techniques. Only little is known about the way man perceives ultrasound. Correspondingly, only few limit values are laid down in technical regulations to protect human hearing. From a metrological point of view, airborne noise measurements in the ultrasonic range are not verified as the measurement techniques used are currently traceable only up to the upper limit of the normal human hearing range, i.e. up to approx. 20 kHz.

Within the scope of the EU-wide EARS Project [1], which is funded via the European Metrology Research Programme (EMRP) [2], some basic issues with regard to airborne ultrasound are being investigated. Among other things, a measuring system has been set up which ensures the traceability of airborne sound measurements in the ultrasonic range up to approx. 100 kHz. Different noise sources which are frequently encountered both in private and in work environments have been investigated with the aid of this system. Based on the example of a marten-repelling system, Figure 1 shows the evolution of the sound pressure level (coloured scale) in a time interval of 10 s (horizontal axis) and a frequency range between 10 kHz and 150 kHz (vertical axis). The fundamental frequency of approx. 20 kHz is modulated in the manner of a siren. The higher-harmonic signal components which can be distinctly observed up to more than 100 kHz are typical of ultrasonic sources.

Figure 1: The spectrogram shows the time and frequency evolution of the airborne ultrasound emission of a marten-repelling system (photo).

From the sound pressure levels measured, several quantities were determined quantitatively which are used to assess the noise. For only a few of these assessment quantities, limit values have been laid down to date. Hence, VDI 3766 [3] specifies limit values for the maximum peak sound pressure level (LZpeak = 140 dB) and for the maximum exposure at the workplace (LAUE = 85 dB). It turned out that even devices which were developed for domestic use only can reach, and even exceed, these limits – depending on how they are used. If the assessment is carried out with quantities which are normally used in the hearing range, then the noise of a small ultrasonic cleaning bath can be compared to that produced by a jackhammer.

Given the current state of the art, it is difficult to tell how well the measuring quantities and limit values defined until now assess the noise of ultrasonic sources. The strain-effect ratios which are still lacking for this are currently being elaborated within the scope of the project. The measurement results which have been obtained to date can be used for an assessment and to find evaluation quantities which evaluate the noise quantitatively. However, even though airborne ultrasound still cannot be fully assessed, the first investigation results show that caution is recommended when using ultrasonic sources.

References:

[1] Project website: www.ears-project.eu

[2] The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union: www.emrponline.eu/

[3] VDI 3766:2012-09: Ultraschall - Arbeitsplatz - Messung, Bewertung, Beurteilung und Minderung [in German]

Contact person:

Christoph Kling, Dept 1.6, WG 1.63, e-mail: christoph.kling@ptb.de