Sound deadening a compressor?

[T i m]

Hi All,

I've bought a little (9cfm / 50l) workshop compressor and as with all the cheaper jobbies it's not exactly quiet (I've also heard noisier).

So, I thought of making a cover / enclosure for it, probably 18mm MDF that will probably just sit over the top, plenty of clearance, with a foam seal where it touches the floor (to make it sit still and steady more than anything else).

I was going to make suitable ventilation holes with sound baffles at the back and a lift off front panel at the front (also with holes and baffles) and possibly a Perspex viewing window (to see the gauges).

The front will easily lift off (magnetic catches?) to allow the compressor to be wheeled out for use elsewhere, oil checks and water draining etc.

If it seems to get too warm in there I could also make it thermostatic / fan assisted (120mm PC style but 240V fans etc).

So, assuming someone hasn't already tried similar and found it doesn't work, or has an easier alternative [1], is there a cheap but good sound absorbing material I could line this box with please?

All the best ..

T i m

[1] There is nowhere else (like outside) I can easily put this (atm anyway).

[Steve Firth]

About the cheapest is to use a double-skin plywood construction and fill the gap with dry sand. It makes it very heavy, so you'd not want to move it.

A lot of noise will still escape via the ventilation and you would need baffles to suppress the noise.

Less cheap, but more effective, is the acoustic foam/lead sandwich used to soundproof engine compartments on boats. There's also a version with a plastic interlayer rather than lead, less effective but cheaper and lighter.

You could, at a pinch, try the bituminous "anti-drumming" panels fitted to cars to reduce noise. I've tried using the butuminous self-adhesive underfelt that goes under roofing felt and while it makes a compartment "dead" it doesn't provide much isolation.

The basic requirements are mass, some ability to move with the sound wave but to damp that movement rapidly, and a substance that doesn't reflect sound.

Oh, BTW, when you design your enclosure, check the flywheel of the compressor. These are usually designed to act as a cooling fan as well as a pulley/flywheel. You will need to exhaust the hot air and provide for admission of cold air, preferably directed onto the warm bits.

[T i m]

Hi Steve,

Hmm, that sounds like it would work well but might be a bit ott for now.

Understood. I was going to just place another piece of (say) MDF over the exit hole so the air could get out freely but the sound might not (so much). What I'll probably design by mistake is a tuned port AT the main frequency of the compressor! :-(

Oh, not heard of that one before? So would this still be used as a lining Steve or is it something I could actually build the enclosure with?

Ah, I've got some of them and have successfully used them to 'deaden' PC / Servers etc.

Ah, and that's the bit I guess I don't fully understand.

Ok, right, so the last bit (reflection) could be dealt with by using egg boxes type of thing? Is there an alternative to egg boxes that would be in more of a panel form .. like that 1/2" thick pin board material (or is that still too flat)?

I understand the mass bit (concrete bunker etc) but I wasn't aware of the 'ability to move with the sound' bit (although it makes sense). This is like a crumple zone on a car, absorbing the energy before it can get to the important bits (but non sacrificial)?

Understood (and good thinking) but this is one of the direct drive types with no obvious flywheel Steve. Coweled motor with built in fan, air from that generally also blowing over the twin cylinder pump. I would arrange the exhaust at the top back (in this exhaust airflow) and the inlet at the front bottom, so convection would also help keep things cool. If not then extra fans to be added to push or extract (or both).

All the best and thanks. ;-)

T i m

[Steve Firth]

Good Morning/Afternoon T i m.

[snip]

It's just a lining so you would have to build an enclosure.

Has some examples of the material.

It's really not cheap, so probably won't meet your requirement.

[snip]

There are things like strawboard (which used to be used as pin board) which absorb some sound. Like egg boxes they could be a fire risk.

[snip]

Yes that's what you're after. Some movement but quickly damped so it doesn't travel from the inside to the outside of the enclosure. An old technique where there's room to do it is to have a series of hanging curtains of sound absorbent material. That won't work for an enclosure though.

[Bill]

In message , Steve Firth writes

We built a boat, so I was familiar with this lead or plastic foam sandwich lining material. Didn't actually use it on the boat, but did use some to assemble a soundproof box for a PC. The 'design' was basically a chimney of lined ply with top and bottom 'labyrinth' air intake/outlets that could be lifted off for access. Normal sound level meters don't work at low levels for this sort of application, but we measured the attenuation with a decent microphone into some audio software. I can't remember the figures, but they showed that it worked well. The material that I used was bought from a boat jumble as offcuts. I think, but am not certain, that you need to seek out big enough offcuts to use a single sheet for each panel. I certainly didn't pay prices anything like those shown on the website referred to here.

This acoustic box showed that it could be done, but made access to the PC a bit awkward ( e.g. the chimney had to be lifted off if access was required to the CD-writer). Now when I need to run a noisy PC, I remote control it from a laptop in the studio and put it as far away as necessary.

Hope this is of interest, if not help.

[meow2222]

I've done this several times with various equipment, and its not hard to get very good NR figures.

A really effective sound absorbing box needs a few features.

First it needs as much rigidity as possible. Any bending that occurs conducts sound from inside to outside, so you want to minimise that. So the first thing is to start with a stiff material. MDF is poor in this respect, 1" chip would be better.

Now no liftable wooden box is going to be fully rigid, so the next thing to do is to damp it. The cheapest material I've found works well for this is triplewall cardboard. Its free and very easy to work with.

2 layers of standard twinwall also works similarly. Another option is 2 layers of carpet, which does an additional job described later, so is more effective.

Now that you've got your stiff box, the next job is to make the input and output airpaths go round as many corners as possible. Each time it turns a corner the sound level drops. Baffles are the main way. You can also add something I dont know how to describe in words... its a ring of thin (eg half x half) wood around the inside of any air port, and this wood is all quarter to half an inch away from the box rather than against it. It makes much of the interior noise turn another corner or 2 to get out.

Finally, and importantly, you want the air in the box to go through a sieve like structure to further quieten it. This can be achieved by lining the box interior with 2 layers of carpet. Where its ok appearance wise, another exterior carpet lining creates further small scale baffling, adding more NR.

Last time I did this I was very surprised by how much NR resulted, and my best guesstimate was about 40dB. I had designed a more elaborate structure, but once this first box was in place it was clear nothing more was needed.

Where fire risk is an issue, rockwool is fireproof. Improved box rigidity is achieved by gluing bits fo sheet material onto the box, inside or out. These can be just scrap pieces and don't need to be in any way accurately cut, if you have an offcut and it goes on, just glue it on. A few screws hold it while the glue sets.

Perspex has litle rigidity and cant be lined. The thicker the acrylic the better, and a double glazed window would perform better than single.

NT

[meow2222]

PS you can also get more NR by damping the parts of your equipment that dont need to dissipate heat away. The pressure vessel acts as a sounding board for the compressor, and damping can much reduce that.

I cant imagine mag catches doing the job, not tried them though in this app

[T i m]

Well, thanks very much Steve, Bill and NT (so far) ;-)

I think I've got the idea of what's needed now and will get some bits together and experiment.

Luckily a mate is a carpenter and will have his table saw up soon so I'll be able to get some nice panels cut up easily. I'll make a point of not throwing away the off cuts. ;-)

Also it seems I can do this in stages to some degree, starting with the box then looking at internal and possibly external coverings.

I'm not looking to make this thing silent and I'm also aware I still need to let it breathe and have access to it myself, so as normal it's bound to be a compromise.

Anyway, thanks again and I'll post back my real life results as / when I can. ;-)

All the best ..

T i m

[Andy Dingley]

Listen to your compressor. Why is it noisy? Which part of your compressor is making the most racket? No point in silencing something that's already quiet in your case.

Remove the air filter (if it's a nasty plastic thing, throw it away)

Replace it a length of steel pipe into a large heavy vessel (recycled fire extinguisher shell works) then a length of pipe into a filter box, either the original or something recycled from a car scrapyard.

What you've made here is a buffer chamber to even out pressure changes in the inlet. Much of the inlet noise is because of the "pulsing" effect of this on the inlet holes of the filter from the atmosphere. Less pulsing, even though the total airflow remains the same, can be much quieter.

Unbolt the compressor / motor from the tank and fasten them to a welded steel subframe (unless it's a direct drive) and mount this subframe on the old tank-mounted frame via some anti-vibration. I use strips of rubber / canvas coal mine conveyor belt. This stops the tank being a soundboard. Make sure the subframe is rigid, as a compressor that rocks back and forth is not only noisy, it can also fracture pipe.

As for a soundproof enclosure, then that's a last resort. Needs to be big if it's not to overheat. If you have a solid brick wall handy, put it on the other side of that and run some extra plumbing. It's easier to make an outdoor shed than an indoor sound enclosure.

[T i m]

The motor and pump mainly, although the cooling fins on the pipe between the pumps and cylinder buzz a bit (but that is less energy and could be silenced quite easily).

Indeed.

There are two, with foam inserts.

I understand your logic here but part of why I bought this model was that it was portable (in the wheelbarrow use of the term).

I think it's the pump itself making most the noise in this case.

I did think of isolating the dd motor / pump from the cylinder but would then need a flexi hose on the outlet from the pump.

Understood.

Yup. I was going to make a box as big as the space at the end of my bench allowed. It would roll into it like a dog going into a kennel then the door goes on after.

I have some solid pre-fab concrete garage panels but no space around it for anything external (it's at the bottom of the garden and the full width of said garden). The back goes onto a private access way.

The good thing about this particular model is it's pretty fast, so from switching on to being up to pressure is less that a minute.

low pressure back to full about 10 seconds. My mate has a much bigger cylinder and once his has cut back in it seems to clatter away for hours, even with no air being drawn off.

No, all I'm trying to do here is just take the edge off the sound as much as possible without making it project 1874532 in my ever lengthening list. :-(

All the best ..

T i m

I agree, and external brick built enclosure would probably be the best noise wise (for me in the garage anyway) ;-)