noise reduction
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.
Your neighbour must be hearing it as well then, correct? Can you hear it from outside?
First culprits I would look at:
Ideally you or your neighbour could stand in his/her utility room while the sound is happening, and hopefully narrow it down. Turning off one of those at a time may also help narrow it down. Either way, you're going to have to work with your neighbour to solve it, if you are positive it isn't coming from in your house.
In California for highways and busy streets they build sound walls, which are actually just brick brick walls. They use solid bricks, not hollow bricks, and the walls end up looking like this:
If you are a good arbitrator or debater, you might even make a good case to have the city, county or state build it for your neighborhood.
Another alternative is to use trees, shrub bushes or anything that can become a hedge in you yard. Even though the old golf saying is trees are 90% air but I've never hit one through a tree. the same words for noise. Hedges and trees will block noise too. Either case make sure you follow your local code on both the block and landscaping. Pull permits where necessary.
Best Answer
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.