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108 CHAPTER 6. NOISE CONTROL6.2.3 Sources of structure-borne noiseImpact noiseImpact noise is one of the most dominant noise sources in many machines.The most important parameters of impact noise are the mass and velocity ofthe impact bodies and the duration of the impact. The frequency spectrumof one single impact shows that this is a broadband noise. Repeated impactsgenerate also harmonic noise.Some practical design rules for the reduction of impact noise are :1. Increase the time of the impact,2. Decrease the speed of the impact (see e.g. in Figure 6.11),3. Minimize the mass of the impact body,4. Increase the mass of the solid body,5. Avoid loose parts with varying load.Figure 6.11: Reduce the impact speed of a conveyor belt.GearingThis is a special form of impact noise that occurs e.g. in gearboxes. Importantparameters are the contact period, the time variation of the force duringcontact and the stiffness of the teeth. Defects in the teeth may cause extraforce variations and thus more noise. A tonal noise is produced (with tonesat multiples of the tooth frequencies).Measures to reduce the generated gearing noise are :
6.2. REDUCING NOISE AT THE LEVEL OF THE SOUND SOURCE1091. Increase the contact time2. Use helicoidal gears3. Increase the number of teeth4. Improve the quality of the transmission (alignment, accuracy of thegearing),5. Use plastic gears for small loadsRolling noiseRolling noise is the result of the roughness or the irregularity of the contactsurfaces. Rolling noise occurs in roller and ball bearings, belts, rail androad vehicles. The rolling noise also depends on the flexibility of the contactsurfaces. The frequency content of rolling noise is mainly broadband.The design rules for rolling noise are :1. Provide a smooth roll surface2. Use suitable lubrication3. Use precision bearings4. Minimize the tolerances of the housing of the bearings5. Increase the flexibility of the contact areaInertiaAcceleration of a mass leads to forces that can produce noise e.g. by impact,rolling, friction or pulsation. Inertia forces can be caused by oscillatingmasses or by (non-balanced) rotating parts.Inorder tocontrol inertia noise, onehasto takeinto account the followingdesign rules :1. Balance rotors or use dynamic balancing2. Minimize accelerating masses3. Increase the uniformity of motion
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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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36 CHAPTER 3. MEASURING SOUNDFigure
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