Powerhouse noise

[thumbs]

I have a hydro powerhouse with two 50MW generators. The generator housing is a fabricated steel structure approxamatly 30 ft dia, 12 feet high, inside a 50ft wide by 140 feet long, by 50 ft high building. There is a lot of resonate noise with a peak of 100dBA at 125 Hz. I am looking for a fire resitant noise solution. Any tips?

 

[Hatch]

Your low frequency (125 Hertz) will not be easy to attenuate.&nbsp;&nbsp;Your best bet is to solve the problem at the source i.e. enclosure or acoustic blankets.&nbsp;&nbsp;Both solutions require lots of $$$.<br><br>

 

[noise]

You could use a fiberglass blanket with a 6 inch air gap behind it to obtain a high abosorption coefficient at low frequency.&nbsp;&nbsp;The fiberglass should be 3 to 6 pounds per cubic foot and be encased in a 1mil vapor barrier.

 

[Franko]

I just joined eng-tips today, so this may be too late. A cellulose type insulation on some of the inside walls and ceiling would reduce reverberation. One type is K-13 from: <A HREF="

http://www.internationalcellulose.com/&nbsp"

TARGET="_new">

http://www.internationalcellulose.com/&nbsp</A>;&nbsp;&nbsp;A

brochure gives 12 dB reduction at 125 Hz for a compressor house (should ask them how much of the building walls and ceiling were treated). <br>

 

[Hatch]

You can look up the absorption coefficient (alpha) for the fibreglass and space gap and then calculate the reduction in sound level from the extra absorption.&nbsp;&nbsp;However consider the following points:<br><br>1. The building not including floors has 26,000 square feet of area.&nbsp;&nbsp;If you acoustically insulate 50% that leaves 13,000 sq. ft. and at $7/sq ft. installed (my estimate), you are in the region of $70k+.&nbsp;&nbsp;Just remember, this is a low estimate, try installing absorption panels on a 50 foot high ceiling with some steel work to provide the 6 inch gap and some sort of protection cover.&nbsp;&nbsp;Fugitive fibres (loose fibreglass particles) are now becoming a concern, so you can't leave the fibreglass uncovered anymore. <br><br>2. 13,000 sq feet of uncovered plastic vapour barrier is a fire hazard!!!<br><br>3. Assuming that you don't have a strong standing wave (125 Hertz = approx 9 ft wavelength so I doubt it) , the direct sound propagation from the equipment may still be a problem.&nbsp;&nbsp;Calculations will have to be done to estimate the noise reduction at various points in the building.&nbsp;&nbsp;At any given point the total sound level is the sum of the reverberation sound and the direct sound.&nbsp;&nbsp;Depending on location the direct sound may not change much and your problem is not solved.<br><br>4. The reduction in sound level is dependant on the total absorption, therefore the quote of 12 dB reduction can't be used because it may have assumed a 100% coverage in a small room.&nbsp;&nbsp;In addition for a room as large as yours, air absorption becomes significant and adding additional absorption may require a high coverage and $$.<br><br>That is why I suggested earlier that you control the noise at source.&nbsp;&nbsp;you really need to do a room reverberation calculation to estimate the noise reduction for various amounts of additional absorption.<br><br>Hope this helps.

 

[Franko]

K-13 is a spray-on insulation, also used for auditoriums. I have no idea what the installed cost/sq. ft. is; contact International Cellulose Corp. (1-800-444-1252).

 

so if 125 hertz = 9 foot wave -length would this be linear and would 250 hertz = 36 foot wave-length

thanks
machine doctor

 

[Hatch]

Wrong way around, the higher the frequency the smaller the wavelength.

The formula is:

wavelength (lambda) = (speed of sound)/frequency

speed of sound = 344 m/s
frequency = Hertz
Therefore units of lambda is metres.

For 125 Hertz 344/125=2.75 metres or 9 feet approx.
For 250 Hertz 344/250=1.38 metres or 4.5 feet approx.