White Paper | February 19, 2014

Dewatering Never Sounded So Good: Noise Management At Metropolitan Utility Repair Sites

Source:
watermain

By Robert Marsh, Technical Director, Polymer Technologies

Usually what is spoken about on a construction site relates to the reality of it being on occasion a sloppy, muddy mess or the notion that it is an unsightly source of noise and inconvenience. These viewpoints can be readily dismissed when the jobsite is in a rural or industrial area. But when the site is in an urban or metropolitan area where the neighbors live and work, measures must be taken to try to minimize the perception of nuisance or harm. Utility contractors face this situation often when they are called to make repairs to a metropolitan infrastructure.

Dewatering is just one of the many challenges that may need to be addressed on an excavated site or for utility repair. Banks of large pumps may be required to run non-stop to keep an area dry enough to allow work to be done, or a contractor might need to roll up to an urban site with a tow-behind pump. When the pumps are powered by diesel engines the noise level on the site will be impacted and may need to be attenuated. Open frame pumps might not be able to be used as the upper limit of the noise level allowed is encroached upon. By taking a systematic approach to acoustical design, product offerings can support the objective of a quieter jobsite as equipment manufacturers offer quieter pumps that will help contractors meet the noise level requirements they are subject to.

Identify Noise Sources

Objects moving back and forth create pressure variations translated into sound when perceived by the human ear. When sound becomes objectionable to the hearer, it is considered to be “noise.”   Diesel powered pumps have many components that produce noise. The pumps, the diesel engine, cooling fans, and even electronics, are noise sources. Identifying the noise sources then allows them to be ranked according to their contribution to the overall noise level of the system, so that attenuation can be applied appropriately.

Rank Noise Sources: Frequency Spectrum Analysis

Acoustical testing of the individual noise sources can be performed, but more often testing is performed on the system. In addition to measuring the sound pressure level (SPL), frequency spectrum analysis of a noise source can identify the dominant frequencies that may be causing the greatest acoustical impact. Once all of the noise sources have been analyzed, the next step is to research alternative components that may be quieter.

Replace Noisiest Sources With Quieter Components

Before pursuing acoustical treatment of noisy components, research should be done to determine if other, quieter components could be substituted. The frequency spectrum analysis of each should be compared as well. Two noise sources that have the same overall Sound Pressure Level (SPL) may sound very different depending on the sound signature of each.

Isolate Noise Sources To Minimize Mechanical Transmission

One of the keys to effective noise reduction is isolating the noise source from other components. In effect, this involves engineering isolation mounts to prevent the transmission of vibration into other components or structures. Structure borne energy may cause acoustical issues if the transmitted vibrations drive another component into resonance.

Enclose Noise Sources

Sound radiates in all directions from a sound source. One of the ways to attenuate sound is to block it by fabricating an enclosure around the source. The key to an acoustical enclosure is mass. The more massive an enclosure, the more energy it takes for sound waves to transmit through it.

Openings are required in enclosures to facilitate efficient operation, but openings present

acoustical challenges since the omni-directional sound waves can easily escape through these openings without being attenuated. For this reason it is crucial that line-of-sight access to the noise source be eliminated. An effective way to do this, while maintaining the required opening, is to build baffles that force the sound waves to travel along an acoustically treated, tortuous path.

Smooth Air Flow

Turbulent air can cause acoustical issues, as well as flow concerns. Since sound is produced by pressure variations, turbulent air may result in more sound being produced than if the airflow was laminar. By designing an air path that results in smoothing the flow of the air, the noise produced by the airflow may be reduced. A smoother airflow could also prove beneficial by decreasing the load on the device moving the air.

Use Materials (As Necessary) To Obtain Desired Goal

Finally, after applying the design principles discussed above, it may be necessary to add acoustical attenuation material to further reduce the noise level of the system. The five categories of materials commonly used for noise control are: Acoustical Absorbers, Acoustical Barriers, Acoustical Dampers, Isolators, and Gaskets. Usually these materials work best in combinations, but they can be used effectively independently.

The Materials

Acoustical Absorbers: Acoustical absorption materials are typically flexible, lightweight cellular foams or fiber batting that “soak-up” airborne sound energy. These materials convert sound waves into thermal energy, thus dissipating the propagating noise. Not every foam or fiber makes a good absorber, though. Acoustical absorbers are designed for the applications in which they are employed. Enclosed dewatering pumps will likely employ acoustical foam on the inside of the enclosure to reduce the sound signature of the unit. Absorption material should also be used in the baffles or louvers that are typically installed in enclosed dewatering pumps.

Acoustical Barriers: Mass is crucial to effectively diminish the amount of energy that passes through a surface. Acoustical barriers are designed to be massive, yet flexible. They are typically used to enhance the performance of an enclosure by adding mass or introducing a layer of material with dissimilar properties to enhance transmission loss performance.

Acoustical Dampers: Damping materials are used to constrain enclosure panels to minimize the propagation of structure borne noise. Damping sheets are adhered to panels that are subjected to vibration to ensure that the panels do not resonate. They also allows for the optimization of the transmission loss of the panel.

Isolators: Isolators are used to decouple (detach) a noise source from the structure supporting it so that the structure borne noise is minimized. Isolators need to be designed specifically for the application to be fully effective.

Gaskets: Gaskets help by sealing openings to keep noise inside the enclosure. They are often used to cushion moving components as well and can reduce the structure borne noise induced by moving parts.

Manned machinery will often employ acoustical attenuation to improve the work environment for the operators or those working in close proximity. Unmanned equipment is usually not treated or even enclosed. But the noise levels of each source, manned or unmanned, combine to reach the overall noise level. The challenge of keeping your jobsite below a set noise level can be daunting at times, and the acceptable noise levels limits are getting lower. Equipment performance will have to change to meet those targets. Applying acoustical attenuation principles and engineered acoustical materials in the design of the equipment in use can help.

Image credit: "water main," © 2013 MTAPhotos, used under a Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/

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