noise reduction of axial fans mounted to sheet metal

[schexb]

Hi all,

I'm working on a project to reduce the amount of noise produced by two 34" axial fans mounted on top of a small tunnel. The fans are currently mounted to 12 gauge sheet metal, causing the sheet metal to vibrate like crazy. The fans appear to be incredibly unbalanced, just from a simple visual observation after shutting them off and watching them slow down. The area is also very dirty and the blades are caked in dirt and debris, which I'm sure further contributes to their unbalance.

In talking with our maintenance crew, they suggested simply replacing the fans. For reference, this is the model of fan:

http://americraftmfg.com/product-lineup/duct-fans/model-b

(B-34-5-TEFC). While I wanted to avoid spending 3 grand to replace them, it still doesn't fix the fact that they are mounted to sheet metal. My idea was to add a 3/8" thick sheet on top of the existing sheet metal to help stiffen up the surface, but I'm sure there are better options I am not thinking of. Any help would be much appreciated.

Thanks!

 

[GregLocock]

balance them, and then compliantly mount them. If push comes to shove mount them to a nice rigid platform and compliantly mount that to the 12g tin.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rob768]

add some dampening panels such as used in yachting industry to the construction.

 

[GregLocock]

Hmm, damping will struggle when faced with out of balance forces. It may take the edge off, but I doubt even that.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Tmoose]

Got a picture of your set up?

Is the entire tunnel "alive" if felt when a finger tip, or just the fan mount top plates, or ????

I'd run the output of a cheap sound level meter on the A scale (

http://www.amazon.com/Radio-Shack-Digital-Sound-Level/dp/B000A1EHGW)

into a PC sound card and process the signal with a free FFT program.
Then I'd kind of know the frequency of the sound. If it was not related to fan 1X rotational frequency I would be less concerned with balance and unbalance.

Using the A scale knocks down the lower frequencies so the display will "look" like it sounds, unlike this -

http://upload.wikimedia.org/wikibooks/en/d/d8/Noisespectrum.gif

 

[Tunalover]

Use rubber washers or o-rings to mount them. Extremely simple and cheap!

Tunalover

 

[Tmoose]

On another board one experienced industrial vibration analyst user reports using a guitar tuning phone app to successfully analyze a noise problem. I have not phone capable of apps, so I'm guessing there was a strong ~single frequency content.

 

[SomptingGuy]

If you're just looking at measuring audio and picking frequencies in it, try a simple telephone "app". I like this one:

https://play.google.com/store/apps/details?id=com.keuwl.spectrumanalyzer

It even averages.

- Steve

 

[1gibson]

Can you stiffen the sheet metal with some simple angle iron, or even some well placed creasing? Do you get the "fake thunder" noise from the sheet metal flexing?

 

[Tunalover]

Why not just mount the fans with rubber o-rings or gaskets? It takes minutes and pennies to try. Why consider other solutions until after you've tried the simplest solution?



Tunalover