VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

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104 CHAPTER 6. NOISE CONTROL4. Minimize the tip speed of rotors5. Avoid obstacles in the flow or adapt the obstacles (see Figure 6.7).6. Do not point the flow outlet at the panels.7. Improve the geometry of the flow (minimize bends, narrowings). SeeFigure 6.8.8. Use special nozzles (e.g. Figure 6.9).Figure 6.5: Slicer with cavity near the blades.Shocks and pulsationsShocks are generated by a rapid discharge of a compressed medium in anarea of low pressure. This happens, for example, when opening and closinga valve in a pump. A single shock produces a broadband noise, but periodicshocks result in a tonal noise. The noise generated by this phenomenon canbe reduced by either slowing down the pressure variation or reducing thepressure difference.
6.2. REDUCING NOISE AT THE LEVEL OF THE SOUND SOURCE105Figure 6.6: Sharp corners in channels produce turbulent flow and the associatednoise production.Figure 6.7: Obstacles in the flow create flow noise.6.2.2 Sources of hydrodynamic noiseSources of hydrodynamic noise can also generate turbulence, vortices pulsationand shocks. Therefore, the design rules are the same as in previoussection.Furthermore a peculiar effect, named cavitation, can also be produced.Cavitation occurs when the static pressure is lower than the vapor pressure.Cavitation bubbles are created which implode during re-compression, so highpressures can arise. This can occur for instance invalves and pumps. Cavita-
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VRIJE UNIVERSITEITBRUSSELAcousticsB
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iiWhen sound becomes noise, it will
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ContentsI Introduction to acoustics
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CONTENTSvii4.3 Measuring the acoust
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Part IIntroduction to acoustics1
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4 CHAPTER 1. FUNDAMENTAL CONCEPTS O
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44 CHAPTER 3. MEASURING SOUNDfactor
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50 CHAPTER 3. MEASURING SOUNDFor ex
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52 CHAPTER 3. MEASURING SOUNDderiva
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Page 162: 152 BIBLIOGRAPHY[12] Bruel and Kjae
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