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102 CHAPTER 6. NOISE CONTROL6. The electrical motor is encased to reduce the radiated Aerodynamicnoise (L W = 81 dBA).From the measurements, a number of conclusions can be drawn :Themost importantsources arethestructure-borne andhydrodynamicnoise of the hydrostatic pump.The dominant structure-borne transmission paths were found betweenpump and motor and pump and reservoir.The dominant radiating surfaces are those of the electrical motor andthe reservoir.Figure 6.3: Hydraulic group.6.2 Reducing noise at the level of the soundsource6.2.1 Aerodynamic noise sourcesAerodynamic noise can be caused by :turbulence,vortices in the wake of obstacles in the flow,shocks and pulsations.These are discussed in the following paragraphs.
6.2. REDUCING NOISE AT THE LEVEL OF THE SOUND SOURCE103Figure 6.4: Different sources of noise and transmission paths of the hydraulicgroup.Turbulence and vorticesVortices can occur because of bodies in a flow. They generate pure tonalcomponents (e.g. the flow over a cylinder like a chimney pipe). Tonal noiseis also generated by a flow over a cavity (for example, the slicer Figure 6.5).In channels, noise can be generated by sharp corners or valves (see Figure6.6). Appart from the coherent vortical structures flow over objects can alsoresult in turbulent noise. Turbulent noise can also be produces due to shearstresses that exist when there is a gradient in the air velocity (e.g. in the caseof the the jet produced by an air gun). Turbulence gives rises to a broadbandnoise. The following design rules should be taken into account to reduce thenoise generated by turbulence and/or vortices :1. Reduce the workload2. Reduce the pressure drops3. Reduce the outlet flow rate (for example, use a larger opening ).
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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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10 CHAPTER 1. FUNDAMENTAL CONCEPTS
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36 CHAPTER 3. MEASURING SOUNDFigure
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44 CHAPTER 3. MEASURING SOUNDfactor
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50 CHAPTER 3. MEASURING SOUNDFor ex
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152 BIBLIOGRAPHY[12] Bruel and Kjae
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