Noise & Vibration Mitigation

In many cases, noise and vibration are generated by machinery e.g. pumps, fans, electrical engines etc. Instead of this equipment working perfectly smoothly, i.e. all the internal forces go into the process; a small proportion of the forces and moments excites the structure causing it to vibrate. These vibrations propagate through the structure and finally couples with the surrounding media, normally air, resulting in sound being radiated.
Generally there are three ways of reducing noise and vibration, namely: 1) at the source i.e. making the machinery run smoother 2) modifying the transmission path(s) and 3) changing the response of the receiver.

SOURCE

Reducing the problem at the source is normally the preferred option but this is not always possible. The problem can be caused by a design that although works well for its purpose, unfortunately results in the generation of high noise and vibration levels. For example a fan that provides the required cooling might do it in such a way that it results in very turbulent flow with high-pressure drop. Redesigning the layout can significantly reduce the problem. Other examples could be machinery that is badly balanced or that suffer from misalignment. Making machinery run smoother or to change its operating conditions can lead to significant reductions in the generated noise and vibration levels.

TRANSMISSION PATHS

Transmission paths from a source to a receiver can be many and complex. Determining the relative importance of the various air-borne and structure-borne paths can be difficult and might involve measurements and/or modelling e.g. using SEA and/or FE depending upon what is most appropriate. Structure-borne paths are normally more complex as they involve several wave types typically bending waves, shear waves and longitudinal waves that couples at structural junctions/boundaries.
With an understanding of the importance to the various transmission paths it is possible to define which palliatives will be most effective. The next step is to introduce them in such a way that they are not over-designed (increasing the effectiveness of a palliative will only be effective as long as the path that it is attenuating is the dominant path).
Examples of changes to transmission paths could involve vibration isolation systems, double wall constructions, floating floors, and structural modifications that reduces the strength of a transmission path (e.g. change of materials properties and dimensions to reduce the sound radiation efficiency).

RECEIVER

The response of the receiver might be particularly sensitive to excitation at certain frequencies. If such regions of high sensitivity coincide with the dominant excitation frequencies, introducing modifications can possibly reduce the response. The approach will consist of identifying modifications that results in shifting the natural frequencies away from the main excitation frequencies.
In cases where the scope for modifications is limited, a frequently employed solution is to reduce the response of the receiver by increasing the amount of damping i.e. the dissipation. The effectiveness of this approach is, however, limited by the amount of damping already present. For acoustics systems, increasing damping might involve installing layered foam constructions that are tuned to attenuate for a particular frequency range. For structural systems, reducing vibrations can be achieved by installing constrained damping layers that can be designed to provide maximum damping in a specific frequency range. If the response spectra is dominated by a single frequency then this can be reduced by installing Helmholtz resonator systems (if air-borne) and tuned mass dampers (if structure-borne).
Most passive palliatives are not very effective at low frequencies. In such cases, active noise control or active vibration control might provide a solution. Active systems employs a number of sources (loud speakers or force generators), sensors and digital processing equipment to generate a sound or vibration field that is in anti-phase with the primary field and when added to the primary field will provide a high reduction.