Sound Attenuation for Business Energy Applications

sound-attenuation

Credit: iStock/shipiolik

Editor’s note: This article first appeared in the November/December 2015 edition of Business Energy.

Intrusive noise is a growing concern for both the public and private sector these days. If the power equipment rattles, clanks, and booms loudly, there will likely be complaints—or worse: fines and citations. Consider this: according to the Noise Pollution Clearinghouse (www.nonoise.org), 13 states have noise pollution laws, and five provide model ordinances for municipalities to follow. These laws can be quite rigid. For example, the City of Lyme, CT, passed an ordinance that makes any noise above 55 decibels (dB) during the day and 45 dB at night a violation, punishable by a fine of $90 per offense. In Texas, the state law says that noise is too loud if it exceeds 85 dB at the property line. So, listen up. You need to make some sound decisions, and we have some experts and resources that can help.

Attenuation, Ventilation, Safety
It’s not surprising that power generation equipment has been identified as a source of sound pollution by many government agencies, considering the fact that distributed energy use continues to grow in urban environments. The result, according to Mike Witkowski, COO of enclosure manufacturer Pritchard Brown LLC, in Baltimore, MD, is that engineers are finding their projects often require a high degree of acoustic customization.

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“The ordinances for sound attenuation are getting more stringent, and at the same time, people have less space to dedicate,” says Witkowski. “Europe is well ahead of us in regard to their requirements for sound attenuation, because they are much more crowded, and they’ve been on top of one another for much longer. Here, the joke used to be that if you want quieter generator sets, you get a bigger piece of land. But that’s changing, and the installations now are so much more power-dense. It’s not uncommon to find big data centers with 20 to 30 megawatts of generator capacity located in a giant generator farm, and the sound from them is substantial. So, designers need to be more creative with mitigating this noise, yet maintaining the proper airflow and fitting within the space constraints of the site.”

When it comes to the physics and science for sound attenuation, he notes that the solutions are fairly straightforward, even though new products and materials that help absorb sound are finding their way to the marketplace. “It still comes down to getting creative with moving air. It would be incredibly easy to attenuate generators that didn’t need 100,000 cubic feet per minute of air. Just put a massive box around it and it will be quiet, but the volume of cooling air required means large devices to acoustically treat the ven­tilation air.”

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Another consideration is the fact that mufflers for exhaust are different from air handling silencers. There is some new technology in combustion exhaust silencers, but emissions regulations are getting more stringent. “Now you have devices that limit the emissions, and they make it more challenging to treat the acoustics,” says Witkowski.

Such high-performance systems typically require the best quality. “We’re talking about premium materials and attention to detail,” adds Rick Grambo, vice president of sales and engineering at Pritchard Brown. “So, fit, finish, and service after the sale are critical, because often you’re well into the project and changes are required.”

He continues: “Projects rarely go as planned. Simple things such as equipment that isn’t the size expected, or no equipment, shows up. Or, an engineer calls with a new requirement. Typically, the customers are not the end-users. They are the suppliers of the equipment inside the enclosure, and at the beginning of the project, they give us information on air requirements, space, dimensions, noise signature, and what the customer wants in terms of site installation, such as a roof where it might need to be assembled in pieces. Safety is critically important, so we design a unit that is safe for the people working inside, in terms of space and shelter, and lighting and egress. These are normally emergency power situations, and anybody working on it could be there during a storm or other emergency situation.”

For enclosures, there are safety and electrical code clearances from the National Electrical Code, and other building codes. But clearances aside, if the air handling function is not designed properly, it can create a negative pressure or vacuum in the enclosure when the generator set is running, leading to difficulty opening the doors.

Finally, Witkowski stresses the importance of having engineers think about the enclosure issues at the very beginning of the project. “When there are sound attenuation and fuel storage and emission controls involved, it’s very important to get the manufacturer involved early so subjects such as limitations can be discussed. I get calls from engineers, and they say their project is 90% done and an intern got the specs out of the manufacturer’s catalog for a one-megawatt generator, but it makes too much noise and sits at the property line. If we were involved at the beginning, we could have developed a solution that would have been less complicated and less expensive than addressing it in the field.”

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