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Scientific news from Division 1 |
The receiving range of sound field measurements in cavitating media
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The receiving range of a sound field measurement in a bubble cloud was determined to better employ such measurements for an optimum use of cavitation.
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Sound field measurements are a versatile tool for the quantitative description of cavitation processes which is required for an optimum use in technical and medical applications. Recent measurements with high resolution showed detailed spatial structures even for subharmonics and broadband noise. This result had not been anticipated because most of the sound is produced by the bubbles themselves and is emitted omnidirectionally, and this effect had been expected to blur a spatially resolved sound field measurement. For practical purposes it is important to know how local a sound field measurement information is.
At the Sound Department a technique was developed that allows the determination of the receiving range of a sound field measurement. The sound field is detected by two calibrated, similar hydrophones, mounted face to face in the cavitation cloud. The distance between the hydrophones is varied and the correlation between the two time-dependent signals is analyzed in the frequency domain defining a local parameter. The dependence of this parameter on the distance between the hydrophones describes the receiving range of a sound field measurement by means of a characteristic distance.
The technique was applied to two different cavitation applications, and effective distances in the range from 1 mm to 3 mm were obtained. Fig. 1 shows in the upper plot the coherence between the two hydrophone signals as a measure of their correlation as a function of the frequency and distance between the hydrophones. With increasing distance, the correlation decreases, which can be seen explicitly in the upper plot which presents the local parameter. The results show that sound field measurements in cavitating media are strongly localized. In addition it can be concluded that the spatial resolution of a sound field measurement in a cavitation field is determined by the hydrophone size and does not depend on the cavitation conditions if at least a medium cavitation level is provided.
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| Figure 1: Coherence C12 (left plot) and local parameter J12 (right plot) for a 20 kHz sonotrode at a higher cavitation level in deionised, air-saturated water with detergent.
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Contact person: |
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Christian Koch, FB 1.6, ultrasonics@ptb.de
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© Physikalisch-Technische Bundesanstalt
Page created: 16.07.2007, last update: 16.07.2007,
Dirk Röske
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