What size air conditioner is it? And what style of unit? Small portable workshop aircon, large industrial job, split system, wall unit/window shaker?

One thing to check is the integrity of the compressor rubber mounts. They can go hard or even disintegrate over time, leading to noisy operation. And check that none of the compressor pipework inside the cabinet is touching the tin work and transmitting vibration.

I had a brand new Fujitsu spit system that rumbled the whole house at certain rpm that defied all efforts to quieten it down and that Fujitsu warranty techs said did not exceed the decibel limits so was all ok. Weights and padding made no difference as it was so low pitched it was more of a vibration than a noise.

Eventually I mounted the compressor/condenser cabinet on 50mm thick rubber mounts and it quietened down tolerably.

Thin sheet lead has been widely used for sound insulation in the likes of boat engine installations. It's normally sandwiched between layers of foam. The lead deals with the lower end of the sound spectrum. With more recent materials the lead is replaced by a fairly dense plastic. Not cheap.

Fix depends on whether the sound is being transmitted through the panels and vents or comes via excitation of vibration modes in the sheet metal panels.

Acoustic foam and similar absorbing / deadening materials are good for direct air transmission through vents and panels as they reduce the energy bouncing around in the air inside the cabinet before it can escape.

Panel vibration modes need the panel stiffened to prevent it shaking. Although lead sheet is the traditional way pretty much anything reasonably stiff will do. Lead works primarily by adding mass so iut takes much more energy to excite the panel and, usually, alters the frequency response. Biggest advantage is that a thin layer makes a decent difference. Direct stiffening by gluing a layer of plywood or similar works fine but does need to be thicker. Couple of inches of rigid foam sheet combines both strategies and is effective but usually the thickness needed cant be accommodated.

Rubber feet and similar mounts are primarily to reduce coupling into the structure that the unit is standing on. Structural coupling can give vey unexpected effects.

A few months back I moved my hydrovane compressor from the hutch outside into the sectioned off garden shed part of the workshop building and hit a resonance. Despite still being on the same concrete slab. Probably due to the sectioning wall being only single skin chipboard. Fixed by standing on a pallet style structure. The wood sections and joints absorbed the vibration before it got to the floor. Old tyres laid flat on the floor with a plywood top for the offending unit to stand on are really good at this job but hardly practical for your situation. Small pallet style base sitting on a carpet off-cut could probably be made aesthetically acceptable if structural coupling is the issue.

Clive

Maybe, but what was it???

Cars are fitted with self adhesive pads of a rubbery/ pitch like material to stop the sheet steel from drumming. It must be effective or they wouldn’t spend money fitting them but the size shape and location will have been tested for effective control. I suppose there are two approaches, solve the source of the trouble or control the effect.

Mike

I suspect fixing the 4th foot mount down to something solid to start with will make a fair difference.

Used to fit paper/cardboard/pitch laminate panels inside Hi-Fi speaker cabinets to deaden the box carcass.

Regards Ian.

Rubber does not last long in the Tropics, nor plastic. It's a constant battle here in Cairns, Qld. Rubber bands turn to goo in a couple of months. Very annoying when getting carefully bundled small taps out of their drawer, only to find they are stuck together with congealed rubber mush. And rubber and plastic parts on cars and bikes give up long before the engine and drivetrain is worn out. Everything from door trims to main crankshaft seals seem to let go at about 15 years. Newer cars its down to 10 years.

So 7 years for hard-used compressor rubbers is probably about average. Most of those better brand ACs have a five year warranty and I reckon you are on borrowed time after that.

Edited By Hopper on 12/03/2022 11:20:46