Sound dampening enclosure for air compressor

Aug 14, 2010

Our workshop is currently located in a residential loft community. To support the laser cutter and other tools, we have a 3 HP, 21 Gallon, 120 Volt 115 PSI Cast Iron Vertical Air Compressor from our favorite store, Harbor Freight. If you’ve ever used a compressor, you know they are REALLY LOUD! For our sanity, as well as our neighbors, we needed a solution…

IMG_0013.PNGThis is the sound level measurement for the compressor in the closet without the enclosure. The frequency range shown is the range of human hearing. According to Wikipedia, normal conversation averages between 40-60dB, and your TV is about 60dB. The compressor has many frequencies in the 70dB and 80dB range. The total sound level measured 95dB.

IMG_0012.PNGWith the compressor in the box, the sound levels are drastically different. A total of 78.2dB. Sound level comparison is confusing, but this site , and our experience with it both indicate that the perceived sound reduction is quite dramatic.  The operating sound was reduced from something that prevented conversation anywhere in the 2300 sq ft loft, and clearly audible in the hallway, to something that is barely audible in the low range from less than 15 feet away.


The obvious solution to air compressor noise is to build a box and put it inside. The box I built here is 3/4″ MDF lined with QuietBrace sound deadening sheathing. It’s a material I haven’t used before. It comes in 4×8 sheets at Home Depot for about $9. It’s pretty lightweight, it’s basically a 1/2″ paper board soaked in some kind of oil/tar. You can score it with a shop knife and break it by hand. Don’t use a good knife as the blade gets all gummy. I used a pneumatic staple gun to secure it to the inside of the box. For completeness, I added some weather stripping around the door opening. The door is supported by piano hinge since it’s pretty heavy.

IMG_1531.JPGUnfortunately, while sealing a compressor in a box makes it quiet, the success will be sort lived as the compressor will overheat in a short time. Compressing any gas gives off a tremendous amount of heat that must be dissipated and,  as an air compressor, it needs a source of air to compress. So  we had to add a chimney… this one is approximately 6″ deep and 18″ wide.

Of course, while we need air flow, putting a big hole in the box is going to let a significant amount of sound out which is not what we want. So, we employ an acoustic filter known as a photonic crystal. I discovered the concept in an article about a sculpture in Spain by artist Eusebio Sempere that exhibited unusual audio qualities. The pipes in the sculpture are in an evenly spaced pattern. It turns out that sound waves in wavelengths similar to the spacing of the pipes will run into the pipes and fail to pass through the pipe array.The Navy is researching the concept using a grid of spheres instead of pipes.

To build my photonic crystal, I started with the sound readings taken with an iPhone app called RTA. I picked out some of the frequencies that were the loudest and used an online calculator to determine the wavelength. In frequencies between 900-7,000hz I got wave lengths of 1.9, 7.4, 14.8 inches. I was using 1/2″ Rigid Metal Conduit, so I picked 1.75″ on center spacing.  That means that wave lengths at 1.75″, 3.5″, 7″, 14″ plus or minus 1/4″ or so should be blocked.

IMG_1533.JPGI used the laser to cut holes in a sheet of MDF and nearly blistered my hand cutting 60 or so 6″ pieces of pipe. I inserted the pipe sections into the holes and slid the whole assembly into the chimney. That served to keep the pipe sections separated at just the right distance. As you can tell from the sound measurements before and after, it clearly does the job… even better than I expected! Meanwhile, there is plenty of room for air to flow around the pipes, both for heat exchange, and to feed the compressor. I had originally planned to install some PC fans at the top of the chimney to force some air exchange, but found that the convection currents were more than equal to the task. I’ve gone in and felt the box and checked the air inside even after extended laser sessions and it didn’t feel overheated.

As quiet as it is, it was still a bit jarring if the compressor came on in the middle of the night.  So I installed a heavy duty mechanical timer rated for the compressor load. It kills the power at night and turns it on in the morning.
Lastly, when setting up a compressor to run mostly unattended, it’s important to remember to drain moisture from the tank periodically. Harbor Freight sells a auto-drain kit for this purpose, but after going through three of them, I can say confidently that they are worthless.

The theory of the valve is that a pressure actuated valve is installed in place of the tank drain. A pressure line is connected between the other side of the valve and the tank dump line. When the pressure in the tank drops quickly through tool use, or rises due to the compressor kicking on, it momentarily actuates the drain.

Unfortunately, it has several flaws:

The instructions assume you can cut a rubber line between the pressure switch and the tank to insert the provided T fitting. Unfortunately, all the compressors Harbor Freight sells use copper lines for this, so you can’t install this without buying a different fitting.

Even with that fitting, the pressure line provided with the kit is a plastic hose. The fittings are designed only to be used with that hose. While the hose is rated to withstand the pressure, it’s not rated to withstand the heat generated by the compressor. As a result, it melts and springs a leak. This leads to the compressor running non-stop until you notice. I had a mini 2 gallon compressor fail when this happened, and had an old 21g unit overheat from this.

To their credit, Harbor Freight replaced the compressor even though it hadn’t completely failed, but because I was concerned about the overheating (gave me new oil and auto-drain and everything!) This time, I purchased proper brass fittings and plumbed it with flexible copper tubing instead. When I was leak testing, I found the pressure valve leaked really bad.I disassembled it to find that the actuator is done via a piece of rubber and the rubber was pinched in the threads. I re-seated it and got rid of the leak with some significant wrenching. Unfortunately, the T-fitting supplied was only threaded half way to the bottom of the fitting. When I tightened it past that point, it split the brass fitting. So I had to replace that. After all that, it still didn’t seem to actuate as it should. I believe that this compressor is too good at keeping the pressure constant, so it’s never allowed to actuate.

This compressor actually failed on me too. Since I had the copper pipe installed, it wasn’t due to that. I had to dig deeper. The YouTube video below shows that the problem was much more significant. The actual cast metal head on the compression chamber was leaking:

Harbor Freight replaced the compressor again (I didn’t even have to show them the video I brought!) This time, I performed significant preventative steps before I even turned it on. I removed all the fittings and regulator, cleaned the goop off the threads and reinstalled a good valve but no regulator. I removed the pressure switch cover and upped the pressure cutoff to 125psi so it would run less often. I installed valves at the various distribution points so I could use different pressures for different applications. After seeing how much air leaked around the compressor fittings when diagnosing the last compressor, I also removed the housing around the compressor engine, removed, cleaned and reinstalled all the fittings using proper teflon tape and teflon paste. I was able to confirm later that there were no leaks. I did find leaks elsewhere in the distribution system and fixed those. As a result, it doesn’t run when it’s not in use. The old compressors used to run periodically due to several small leaks in the system.

IMG_1539.JPGI didn’t bother with the auto-drain this time, instead, I installed a T fitting and connecting it to a 3 foot section of hose and terminated it with a simple ball valve. Every day or two, I open it to drain the moisture in the tank. I cover the valve with a towel when doing this because otherwise it sprays the moisture all over me.

So far, so good on this setup!

I am working on a “intercooler” which will be a spiral of copper pipe that air will enter after it leaves the tank. The copper should encourage the air to cool and give up the moisture in it. At the moment, I’m using rubber hose to distribute the air through the workshop and because it isn’t metal, it doesn’t cool the air. So while I have water separator filters at the various use points, they never get any moisture in them. Yet, if I disconnect the line from the compressor, tons of water sprays out of the rubber hose. The theory I read online is that the moisture is never condensed from the air during distribution and therefore isn’t separated out. For the laser this isn’t good, but it’s not terrible either, so it’s still on the list to complete.

Update 8/16/10: Some people requested diagrams. Here is the drain diagram, followed by the plan for the intercooler (not completed/tested.)