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I have built a 38"x81" pressure treated platform to host an air condenser and a heat pump (two unit house) on the roof of my house. It is positioned right above the master bedroom so I am a little concerned about noise even though both units emit noise upwards.

Is there some kind of rubber sheeting that I could spread over the platform before I set the units that would reduce the noise by impeding reverberations? I was thinking a rubber mat similar to the Oatey shower membrane.

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A thin membrane will offer very little noise reduction. Some units have optional isolation mounts specifically to reduce noise and vibration transmission. If not available, it's possible to fabricate something similar, but must be carefully detailed to facilitate sound mechanical attachment without transmitting vibrations. –  bcworkz Feb 24 at 22:39
    
Minimal contact with roof by resilient bushing mounting of a support frame to solid support members isolated as much as possible from the roof sheeting, ceiling and wall structure (any of which acts as a huge speaker diaphragm if driven in any way by the vibration). –  Fiasco Labs Aug 24 at 2:36

1 Answer 1

This is a non-trivial problem.

To be successful, you will have to determine the vibration frequencies emitted by the equipment under various conditions and do whatever it takes to dampen all those frequencies in the structure.

One strategy which is more likely to be successful than not is to place the mount points of the platform directly over load bearing supports all the way to the ground under the structure. This could be through a series of walls which are aligned atop each other, but if there is any cavity above or below them, there is a set of frequencies at which that wall will vibrate the floor/ceiling of the cavity. It would be hard to do that for more than two corners of the platform unless it is significantly oversized to reach a pair of walls.

Another technique would be to firmly attach plate steel (or similar) in the attic to the ceiling joists to a) change the resonant frequency and possibly b) dampen any vibration. This is so hard to determine by engineering computer models that it is simply faster and more accurate to do it empirically: That is, by trial and error.

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Used dense Douglas Fir 4x4 blocks of various lengths mounted with Silicone RTV to do that once. They altered the resonant frequency and the resilient mounting is an energy conversion layer that through shear, helps turn the vibration energy to thermal energy. –  Fiasco Labs Aug 24 at 2:42

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