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

6.3. TACKLING NOISE TRANSMISSION 113at the end of the system due to changes in the cross-sectional area or bychanging the rigidity of the wall by transition of pipes to tubes. Absorptionof Hydrodynamic noise is provided by accumulators. The design rules forthe control of liquid-born noise are :1. Use a combination of pipes and tubes.2. Use dampers.6.3.3 Structure-borne noise transmissionThe transmission of structure-borne noise from sources to radiating surfacescan be influenced by changing the mass, stiffness and damping of the structure.The selected strategy depends on a number of factors :Is an increase of weight possible or not? If so, an increase of mass nearthe region of excitation will be efficient.Force excitation or speed excitation? In case of force excitation, addingimpedance (mass) will be effective, in case of speed excitation, addingmass has no sense (in the latter case, the source may be isolated).Narrow-band or broadband excitation? For a narrow-band excitation,it is advisable to redistribute the stiffness or mass of the system (inorder to shift the resonance frequencies). The addition of damping canalso be effective. This has no sense for broadband excitation and abroadband reduction of the transmission needs to be obtained.Excitation at low frequencies, intermediate frequencies or high frequencies(quasi-static, resonance or multi-resonance response respectively)?At low frequencies, vibration isolation is the only possible solution.Panels with free edges radiate, in general, less noise than clamped panels(see Figure 6.15). In the middle frequency region, a number ofsolutions can be chosen :– Adding mass at the excitation point (see Figure 6.16).– Increasing the structural damping– Isolate the source (e.g. see Figure 6.18).– Reflection at discontinuities (see Figure 6.17).In the high frequency region, the following measures can be effective :– Increasing the mass or stiffness of the excitation region.