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Generator Enclosure Sound Reduction Analysis

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engpes

engpes

Mechanical
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I have a generator inside a sound reduction enclosure and am trying to determine the decibel level directly outside of the enclosure. I know there are quite a few variables to nail down, but is there a method to calculating an estimated noise level if I determine the following:

1.). Enclosure steel construction
2.). Insulation material
3.). Generator dB level and frequency

Is there a way to determine a noise reduction coefficient? Can this be used to estimate levels?

Obviously this not a type of application we deal with every day, but would appreciate any advice on the above. Thank you.
 
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There is no reasonably accurate cookbook method. You may get within 10 dB in each third octave using a transmission loss approach. Even if you knew the exact vibration of every part of the enclosure then estimating the sound field around the enclosure is tricky.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Is there any analytical way to estimate? Is testing the only way to determine the sound reduction?

I really don't need the estimate to be extremely accurate. Thanks.
 
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Can I do an FEA frequency simulation of the frame and use this to derive the dB level around the generator enclosure? Again I don't need extremely accurate results.

Thanks.
 
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If the enclosure walls were perfect there would still have to be a couple of large openings for combustion air, cooling ventilation and exhaust.

I'd use a radios shack SLM as a microphone and freeware FFT software on a laptop to analyze the sounds I'm dealing with before getting too deep into the enclosure design.
There may well be some low hanging fruit that would add considerably to the enclosure's job.

What is the generator make and model?
Is it constant speed?
Is it already as quiet as the equivalent Honda generator?
Are you sure the exhaust system is the best commercially available version?
As a first cut, is the volume of the muffler at least 10X the engine cubic inch displacement?
Folks have adapted automotive exhaust mufflers in pretty creative ways.


All my generators' intakes are pretty crude. Just wide open air filters really.
Listen to what happens when a "tuner" replaces their OEM intake silencer with an open cone filter.

Similarly, simply adding a giant aftermarket expansion chamber intake "silencer" to my old Curtis air compressor made about a zillion dB improvement. It is almost pleasant to be around now.

 
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Taking the problem as stated, ignoring intake and exhaust, you can use a transmission loss method as described in Harris or Beranek. FEA won't help unless you spend a lot of money modeling similar cases.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Greglocock,
Do you know the name of the book by Harris or Beranek? Thanks!
 
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Thank you very much Greg. This is a big help!
 
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In addition to having exhaust muffler with the proper attenuation, silencers are required at the ventilation air intake and at the ventilation air exhaust. Silencers with rated attenuation property at each frequency bands should be used to achieve the desired noise level. Ventilation fans must be sized to overcome the pressure drop across the sound attenuators. Last project we specified the engine/generator manufacturer to also provide the heat recovery steam generator, air condition electric control room, ventilation system and complete acoustic enclosure to not exceed 72 DBA sound power level at any point 10' away outside the enclosure. Owner to provide concrete pad built per engine generator requirements and all external utilities, electrical, steam and condensate connections.
 
 http://files.engineering.com/getfile.aspx?folder=e98f3c7e-5b87-41a3-9024-44ec09edb5a1&file=IMG_0738.JPG
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