View Full Version : quieting a compressor

02-21-2008, 11:04 AM
After reading Forrest Addy's article about compressors in the July/August 2005 edition of the HSM prior to making a compressor purchase, I decided to buy a direct drive Craftsman compressor. In his article he says they "can be made much quieter, especially if the stock air filter was replaced with a shop made intake muffler and air filter."

I was wondering if anybody had done any modifications on one of these type of compressors that they are happy with, and would be willing to share their process, and what components were used?

Thanks in advance,


Alistair Hosie
02-21-2008, 11:30 AM
In one of the woodworking books I have a guy made a cabinet with air vents and insulation he eventually set it outside where the noise could not be heard in the room and the neighbours were unaffected .Don't put one in the attic or high up as the noise becomes worse.As far as the muffler idea I have heard of that too and it does work I will watch this post with enthusiasim.Alistair

Jim Caudill
02-21-2008, 11:38 AM
I can't recall reading Forrest's article, but I would never buy a direct drive compressor for "in shop" use. My dad and a friend each had one. Noisy as he11, as well as some other annoying features. The compressor turns at about 3600 rpm (full motor rpm) and vibrate something fierce. I might use one for occasional on-site use such as a nail gun, where I could get the darn thing away from me. The typical intake on these units was some kind of rectangular foam pad.

I have, and use, Ingersoll Rand T-30 compressors. I have had as many as 4 at one time. The "real" compressors have an intake port that is threaded to a pipe thread. I used a plumbing adapter to come off the port and go one size bigger in cast iron. After an extension and an elbow, I transitioned to "white PVC" and ran that piping up thru the ceiling and into the attic. I used the appropriate fittings to allow me to attach the original intake filter. I have seen other applications where the intake is routed thru a side wall and the filter installed outside (neighbor issue here).

02-21-2008, 11:56 AM
I took the hose off mine and replaced the other stuff with a belt drive two cylinder PUMA... Best thing I ever did!

I couldn't see putting a muffler on the intake would help all that much but who knows, might be worth the effort.

02-21-2008, 12:24 PM
i think the best answer is to put the compressor in a insulated doghouse outside thats attacked to the shop. in the warm of summer open the door to the dog house to keep the compressor cooler. don't forget ot drain the tank of excess moisture.

02-21-2008, 01:13 PM
A compressor outside creates a couple of problems for those of us in the midwest. In winter, temps get really cold. Most reasonable sized compressors use splash oil and you cannot splash something the consistency of molasses in January. I use synthetic oil in mine which helps but still does not alleviate this...but my compressor is kept inside. My shop cools to just above freezing on the worst cold days and warms up nicely fairly soon after I turn the heat on. That's cold...but its not *that* cold that the synthetic lube should not still flow well.

The other half of the year, an outdoor compressor will suck in really humid air, making for even more condensation. I wouldn't even think of one outdoors without an automatic drainer. The problem is that the compressor is even more apt to suffer from the "out of sight, out of mind" sort of neglect that is responsible for rusting them out.


Forrest Addy
02-21-2008, 01:21 PM
Here's my words taken from the article on the topic:

"Recently on the market are lubricated direct drive compressors. These emulate their belt driven cousins by having an enclosed crank case, piston rings, plus having the lower cost advantage of higher speeds and lighter weight. Since the crankcase is sealed and lubricated the sole dust concern is isolated to the incoming air. Otherwise their output and efficiency are roughly equivalent to an oil-less compressor. Lubricated direct driven compressors can be made much quieter especially if the stock air filter was replaced with a shop made intake muffler and air filter."

The operative word here is "lubricated:" that is the compresser has an enclosed crankcase.

In an oilless compressor the open crankcase permits the pulsations of the piston free access to the ambient air thus they are noiser and this noise cannot be easilv abated without a full dog house isolation treatment.

The noise signature of an compressor with an enclosed cranckase can be readily improved with an intake air silencer. This will do nothing to prevent the sound of the valves and the motor but this a a mere fraction of the overall noise emmission of the unit. If it's intolerable in an otherwise quiet shop there's still relocation of the compressor to an exterior doghouse.

Here's some un-published notes on intake silencing. "Industry practice on the silencing of noise having a strong pulasating component is to have an enclose volume to flywheel the pulsations so a steady flow rate can be established where the flow meets the environment. In automobiles and motor cycles this takes the form of a voluminous exhaust system and intake filter with a limited oriface. The tiny baby food jar-sized muffler found on gas powered lawnmowers is almost ineffective: it's a spark supressor more than a muffler. Adding a length of pipe and an expansion chamber having two connections greatly reduces the noise expecially if the expansion chamber is circular in cross section.

So my suggestion for inlet silencing a compressor is to add an inlet expansion chamber via hard pipe and add an air filter to the inlet of the expasion chamber. My thought is the expansion chamber should have an internal volume about 10 to 20 times of the cylinder displacement. For a small 1 HP compressor a handy place to start for an expansion chamber is an exhaused 16 oz disposable propane tank. Working out of doors, vent the residual gas, remove the Schraeder valve and insert a 1/8 diameter tube connected to a source of compressed air to the bottom of the cylinder. Let it blow for an hour or so. Heat it to 400 F to bake out the odorant, then make your connections.

Larger compressors require larger chambers and most any round steel vessel will work - like a past hydro 20 lb propane tank "de-milled" as in the 16 oz tank or a piece of deisel-sized exhaust pipe, well casing, or hollow driveline then weld blanks on it. Non-round vessels will be less than effective; the walls of the vessel may "pant" slightly from pressure variations transmitting noise.

If your money will stretch get a home water filter. It has a large internal volume and filtration besides."

02-21-2008, 01:40 PM
I bought a 3 cylinder pump that runs at about 1100 rpm from these guys:

I suspect rpm is the key. I know when this guy is running, but I can easily carry on a phone conversation while standing next to it. It's more like an aquarium pump.

Baldor 5hp 1750rpm drives it without complaint. Tank is from craigslist, mag starter from ebay.


Frank Ford
02-21-2008, 02:04 PM
The only time McMaster ever shipped me an item by mistake, it was a piece of 1-1/2" rubber hose. They just told me to keep it, and I eventually found a use for it:


By simply adding 15 feet of hose between the original filter and the cylinder head of my 5HP vertical tank Quincy, I get a fair reduction in the intake noise. I live in a densely developed urban area, so moving the noise outside is not possible.