Low drone from flat roof mounted air conditioning unit

[Phil Lucas]

I have just installed an air con unit on a flat roof of my house. Construction is fairly basic: bitumen covered ply on 7" wooden joists with 12.5mm plasterboard on the underside of the joists forming the ceiling of the room below. The air con unit is mounted on hard rubber mounts on a metal stand which has four one 8"x8"square pad feet, each foot sitting on a thick solid rubber sole. Inside, we can hear a low frequency drone. I am about to renovate the rooms below, so I would be most grateful if someone would suggest a way of reducing the fan drone noise.

Should I try to decouple the air con unit some more? Perhaps by putting lead sheet or cork board under the feet?

Should I decouple a sound insulation ceiling board from the joists using Z-bars?

Should I put as much dense firbre wool matting between the joists as possible?

Many thanks, Phil

[stoneyboy]

Phil Lucas,
Can only suggest you get hold of some 25mm thick foam rubber sheet and some 300mm square paving slabs. Use these to form large bearer for the existing feet.
end

[Perry525]

You need to identify if the sound is airborne or via the fabric of the building.
If airborne and via the windows then secondary glazing will probably solve the problem.
If transmitted via the fabric of the building, then you need to de couple the air conditioner from the roof.
Placing the air conditioner on a tray suspended by several springs from a frame, or spreading the load over a wider area using softer foam rubber will solve the problem.

[TheDoctor5]

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