Acoustic Damping/Attenuation

I have a particularly noisy air conditioning compressor cabinet. A fourier analysis of the noise shows (as expected) a fundamental at twice the mains frequency i.e. 100 Hz and strong but diminishing harmonics at 2x 3x 4x etc.

The simplest solution would be to buy some sticky backed acoustic foam of 2cm thickness and apply to every panel that shakes but would this successfully absorb such low frequencies?

Should I instead be looking to add weights to the vibrating panels to move the resonance away from 100 Hz?

Ideas? Experience?

Maybe old hat Colin, but is thin lead sheet still used to surround the noise source?

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.

I've ordered some 3cm thick padding as it's really cheap from China to Thailand.

The machine is a Samsung split AC 1500W power in and 5400W cooling out single phase 220V. It's ancient, a non inverter, and appears to have 4 good rubber feet although one of them is floating.

I guess I could look for some superior rubber feet ...

The only use I've encountered for lead is nuclear shielding of X rays, gamma rays, and high energy neutrons. (Don't get me started on thermal neutron capture cross sections.)

Cheers, Colin

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.

My first motorhome had been fited by a previous owner with a "Noise Killer" sound deadening kit that used a lead foil/foam sandwich.

Seems that the product is still available but it is quite pricy.

https://www.nkgroup.co.uk/product/noisekiller-lead-sandwich-material/ for those who like to know where a link goes.

Nigel B.

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

I used some stuff for deadening engine noise in a car, it was cheap and effective.

Bob

Maybe, but what was it???

Clive,

Everything you say makes a lot of sense. Increased mass to lower the resonant frequency and I may as well throw in the foam sheets for good luck.

Thanks, Colin

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

Mass damping - to reduce at source, eg the lump of concrete in a washing machine.

De-Coupling - rubber or spring mounts

Absorption of air bourn noise - eg acoustic foam / foam/ metal sandwich/ Axminster Carpet

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

And the cigar goes to Dave Halford!

The application of a wood working G clamp to the floating 4th foot made a huge difference.

I've now replaced all 4 of the rubber pads/feet (price 50p for a set of 4) and splashed out on some stainless steel nuts and bolts and lock washers to secure each foot to the wall bracket. And now it's really quiet.

The top panel is still vibrating a little and quietens when a damping hand is placed in the middle, so, when the sound deadening foam arrives I'll add some stiffening and foam to the lid and that will be as quiet as I can easily make it.

Thanks again guys.

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

Regards Ian.

Search using the right parameters and you will find it.

sounddeadeninguk.co.uk

Follow up from killing the noise ...

The black foam tiles arrived and were glued in place. Result? The noise got worse!

So with the help of my son I removed all the panels and finally gained a view of the compressor and its rubber shock mounting feet.

The one on the left is obviously the before and the one on the right the after. The after was purchased in Phuket town where a set of 3 cost approx. 2 GBP.

With three new feet the air conditioner is much quieter, probably as quiet as when it was installed 7 years ago.

Now, this is the tropics and the air conditioner has been working hard with a compressor that gets unpleasantly hot to the touch but does 7 years seem like a reasonable life for a rubber shock mounting? It is less than I would have expected.

.

In those conditions, 7 years seems very reasonable to me

Now that you have demonstrated that the isolator configuration does the job … it may be worth looking for something with a higher specification : I would suggest looking at the BarryMount and IsoFlex offerings [at least for inspiration].

MichaelG.

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Edited By Michael Gilligan on 12/03/2022 06:23:26

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

You could try just putting your hand on the various panels while it is running to hear if it makes any difference, and if so, which panel. That would simulate the fitting of damping material at no cost.