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I have a closed box fan motor above an insulated false ceiling. I asked the builders to put acoustic absorbing material around the motor to absorb its sound but am not happy with the deep resonant sound which still existed.

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Today I investigated and found that the cause of this sound is the vibration of the bottom wall of the fan box which as you can see in the photo is where the fan rests. I can actually stop the noise by pushing the wall with my hand from underneath and causing it to flex inward.

Can anyone suggest a way to deaden this vibration and subsequently this noise?

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If you can access the bottom of the fan you might be able to wedge something in there to put pressure on the box and stop the vibrations, but I've found that often with fans like this, they only vibrate at certain speeds regardless of how it was mounted. Something easy and relatively inexpensive you can try is adding a fan speed controller (not a light dimmer!) and turning it down slightly to see if this stops the vibrations.

Side note: If you want a quieter fan, a trick is to buy a larger fan then required and run it at a slower speed.

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Thanks for reply. This is what I was thinking, to deaden the vibration by having something pressed against the box. I'm just not sure how to do it. There is no space inside the box so it would have to be something pressed from the outside, either attached to the ceiling above or strapped around the box. –  mark Aug 14 '12 at 20:03
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