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Understanding Damping Techniques for Noise and Vibration Control

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Understanding Damping Techniques for Noise and Vibration Control Understanding Damping Techniques for Noise and Vibration Control By Jennifer Renninger Applications Engineer E-A-R Indianapolis, Indiana Introduction only resonant motion. Forced, nonreso- Effective control of noise and vibra- nant vibration is rarely attenuated tion, whatever the application, usually by damping, although application requires several techniques, each of of damping materials sometimes has which contributes to a quieter environ- that effect because it increases the ment. For most applications, noise and stiffness and mass of a system. vibration can be controlled using four A damping treatment consists of any methods: (1) absorption (2) use of material (or combination of materials) barriers and enclosures (3) structural applied to a component to increase its damping and (4) vibration isolation. ability to dissipate mechanical energy. Although there is a certain degree of It is most often useful when applied to overlap in these classes, each method a structure that is forced to vibrate at may yield a significant reduction in or near its natural (resonant) frequen- vibration and noise by proper analysis cies, is acted on by forces made up of of the problem and application of the many frequency components, is subject technique. The principles behind the use to impacts or other transient forces, of absorption materials and heavy mass or transmits vibration to noise- barrier layers are generally understood, radiating surfaces. so this paper will focus on the third Although all materials exhibit a certain and fourth methods, which deal with amount of damping, many (steel, alumi- reducing structural vibration. num, magnesium and glass) have Structural Damping so little internal damping that their Structural damping reduces both resonant behavior makes them effective impact-generated and steady-state sound radiators. By bringing structures noises at their source. It dissipates of these materials into intimate contact vibrational energy in the structure with a highly damped, dynamically before it can build up and radiate as stiff material, it is possible to control sound. Damping, however, suppresses these resonances. Page 2 Of the common damping materials in heat-liquefied material that hardens use, many are viscoelastic; that is, they upon cooling. Energy is dissipated as are capable of storing strain energy a result of extension and compression when deformed, while dissipating a of the damping material under flexural portion of this energy through hystere- stress from the base structure. Damping sis. Several types are available in sheet increases with damping layer thickness. form. Some are adhesive in nature Changing the composition of a damping and others are enamel-like for use material may also alter its effectiveness. at high temperatures. An example of the effectiveness of Free-layer or extensional damping is an extensional damping treatment is one of the simplest forms of material shown in Figure 2. The curves represent application. (See Figure 1.) The material five extensionally damped systems. In is simply attached with a strong bond- 1 30 ing agent to the surface of a structure. A Alternatively, the material may be B troweled onto the surface, or the structure may be dipped into a vat of 1000 Hz 0.1 C 20 AT D Damping Material ACTOR 0.01 E 10 TED NOISE REDUCTION, dB Base Layer SYSTEM LOSS F ESTIMA STEEL THICKNESS, IN. A BCDE Extension 1/32 1/16 1/8 1/4 1/2 0.001 0 20 30 40 50 TEMPERATURE, C Bending (Flexure) Resulting From Vibration Figure 2 Figure 7: Extensional Damping Figure 1 The relationship between system loss factor, A free-layer damping system is the simplest estimated noise reduction and temperature for form of damping material application. Energy several free-layer damping systems is shown by is dissipated as result of extension and com- these curves. The damping material type and pression of the material under flexural stress thickness (3/16-inch) are the same in each case. from the base structure. Page 3 each case, the damping sheet is 3/16-inch than the bending wavelengths of the thick, whereas the steel base layers are vibration being radiated as sound. The from 1/32- to 1/2-inch thick. From such effect of temperature is clearly seen. data, it is possible to obtain the overall The peak noise reduction for the 1/2-inch system loss factors (measure of energy steel plate occurs at a lower temperature dissipated per radian of vibration at than for the 1/32-inch plate. resonance) and the resulting estimated Constrained-layer damping (CLD) “large” panel impact noise reduction— systems are usually used for very stiff the reduction in noise level that results structures. (See Figure 3.) A “sandwich” from damping a panel that is being is formed by laminating the base layer struck many times per second. Large, to the damping layer and adding a third in this case, means that the panel constraining layer. When the system dimensions are equal to or greater flexes during vibration, shear strains develop in the damping layer. Energy Damping is lost through shear deformation, rath- Adhesive Constraining Layer Layer er than extension, of the material. System loss factors and estimated Base Layer (Substrate) noise reductions for several CLD steel systems are shown in Figure 4. For a Adhesive Damping Constraining Layer Layer given base layer thickness, the values obtained are significantly higher than those in Figure 2, although the damping material properties and thickness are Shear Base Layer (Substrate) identical. Further, varying layer thick- ness ratios permits optimizing system Figure 8: Constrained Layer Damping Figure 3 loss factors for various temperatures without changing the material’s A constrained-layer damping system is composition, Figure 5. usually recommended for stiff, structural applications. Energy is dissipated as a result The constraint method is not critical as of shear deformation of the damping layer. long as there is adequate surface-to-sur- face pressure. The layers may be bolted Page 4 or riveted instead of glued into a sand- or deterioration. wich and still provide optimum perfor- Structural damping, whether extensional mance. Adhesives, if used, must have a or constrained-layer, provides an at-the- high shear stiffness. Shear strains in the source solution to noise control problems. adhesive will reduce the strains in the Further, it is not always necessary to use damping layer, reducing its effective- 100 percent panel coverage to achieve ness. significant noise reductions. For exam- Another advantage of CLD systems ple, 50 percent coverage will provide a is that they can be used in harsh envi- noise reduction that is ronments. The damping layer is totally typically only 3 decibels (dB) less than covered by the top constraining layer, for 100 percent coverage; 25 percent so it typically is not subject to abrasion 1 30 1 30 1000 Hz 1000 Hz AT AT 0.1 20 0.1 20 0.188 IN. ACTOR ACTOR B A TED NOISE REDUCTION, dB STEEL THICKNESS, IN. TED NOISE REDUCTION, dB DAMPING LAYER A B THICKNESS, IN. 0.250 0.125 0.500 0.007 0.250 SYSTEM LOSS F 0.063 ESTIMA SYSTEM LOSS F 1.00 0.500 ESTIMA 0.125 2.00 1.00 10 0.01 0.01 10 01020304050 0 10 20 30 40 TEMPERATURE, C TEMPERATURE, C Figure 4 Figure 5 The relationship between system loss factor, The relationship between system loss factor, estimated noise reduction and temperature for estimated noise reduction and temperature several constrained-layer damping systems is for varying damping layer thicknesses in shown by these curves. The damping material several constrained-layer systems is shown type and thickness (3/16-inch) are the same in by these curves. In each case, constraining each case. layer and base layer are 1/4-inch-thick steel. The damping material is the same in all cases. Page 5 coverage is only 6 dB less. When 40 properly used, damping can be as cost 30 effective as it is acoustically effective. Vibration Isolation 20 , dB RUBBER ISOLATOR This method reduces the transmission LIGHTLY DAMPED 10 of vibrational energy from one system UNISOLATED SYSTEM to another. Common vibration isolators 0 are steel springs, rubber pads or HIGHLY DAMPED TRANSMISSIBILITY bellows. These devices are available 10 ELASTOMER ISOLATOR in many shapes and are capable of 20 isolating masses weighing from a RESONANCE POINT few pounds to thousands of pounds. 30 LOG FREQUENCY, Hz An automobile suspension is a good example of damped isolation. Shock Figure 6 absorbers dissipate energy by pumping Transmissibility curves compare the vibra- a fluid through orifices that offer a tion isolation performance of two materials. predetermined resistance to high- Data indicate that the highly damped elasto- velocity flow. Many isolation systems mer is the better choice for a machine isolation use elastomeric materials to provide mount. Note that the amplification of the both the spring force and damping. elastomer isolator is only 3.5 dB at the Some rubbers are capable of achieving resonance point compared to 23 dB for useful damping at certain frequencies, the rubber isolator. transmissibility. Typical transmissibility although at low frequencies most exhib- curves, as shown in Figure 6, compare it loss factors less than 0.2, or roughly the vibrational acceleration response of 10 percent of critical damping. At res- materials used in isolation applications. onance, when a system dissipates the As the damping in a material increases, same amount of energy per radian as it the system amplification response can stores, it is said to be critically damped. be minimized at or near the natural fre- Loss factor is equal to the percentage of quency. This can be especially beneficial critical damping divided by 50. in applications such as stepper motors, One way to compare the behavior of which must run through a variety of various isolators is to measure their frequencies, or those applications that Page 6 frequently go through a start up or source can be realized by careful plan- slowdown as part of the operation cycle.
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