EEVblog Electronics Community Forum
Products => Other Equipment & Products => Topic started by: edpalmer42 on March 02, 2023, 07:17:04 pm
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Has anyone seen a teardown and analysis that explains what design features are responsible for the reduced noise level of the new 'quiet' air compressors?
I recently bought one of the newer types of air compressors that is advertised as 'quiet', 'ultra quiet', 'silent' or other similar marketing babble. There are endless videos about how quiet they are. Instead of being so loud that you can barely shout over them, the quiet(er) ones make claims of < 60 dBA for small portable units (typically 1 gallon tanks) up to < 80 dBA for larger units with 20 gallon tanks. These numbers are almost worthless because they rarely describe the measurement environment i.e. distance between compressor and microphone or indoors/outdoors, etc. Even so, these compressors are a lot quieter than older designs. The 2 gallon / 2.5cfm@90psi unit that I bought claims 62 dBA with no more details. My old Radio Shack sound level meter reads about 65 - 66 dBA at 1 meter in my living room with wall-to-wall carpeting and typical furniture. Move it outside and the level will drop significantly from that.
But what design features accomplish this magic? AFAIK, the first company to produce these was California Air Tools. The air pumps on all of these units, regardless of brand, share an obvious family resemblence to this brand. Licensed copies? Outright design theft? No idea. But there's something about that design that results in a significant reduction in noise.
Any thoughts?
Ed
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When I saw these on the market, I got a Lowe's Kobalt unit with wheels. And yes, all such units seem to use the same 2 cylinder design. I don't know much more, but figured that the RPM is halved but gets the same air moved with 2 cylinders, and some attention to other details quiets them to a reasonable level.
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I don't know what they're calling "a Quiet air compressor" but I expect that it's nothing more than a sales slogan. But the quietest air compressors that I have seen are the ones that use an 1800 (or less) RPM motor instead of a 3600 RPM one. AND the compressors are belt driven to run significantly slower than the motor. The old industrial built Ingersol-Rand (IIRC) only run their compressors at 180 RPM. Not only are the motors and the compressors much quieter but they also last MUCH longer. The down side is that for a given size compressor, the slower driven ones take much longer to deliver any given volume of compressed air.
A two cylinder compressor may not be running both cylinders in parallel but may be operating them in series. A so called "two stage compressor". Most true industrial compressors are two stage since it lets the compressor develop higher pressures. 160 PSI instead of 90 to a MAX of about 110 PSI.
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https://www.eevblog.com/forum/reviews/quiet-electric-compressor-like-an-aerosol/ (https://www.eevblog.com/forum/reviews/quiet-electric-compressor-like-an-aerosol/)
https://www.eevblog.com/forum/projects/small-quiet-compressor-to-exclusively-run-a-syringe-dispenser/ (https://www.eevblog.com/forum/projects/small-quiet-compressor-to-exclusively-run-a-syringe-dispenser/)
https://www.californiaairtools.com/ultra-quiet-series-of-air-compressor-contractor-grade/ (https://www.californiaairtools.com/ultra-quiet-series-of-air-compressor-contractor-grade/)
https://www.youtube.com/watch?v=ldfCQV-LuSo (https://www.youtube.com/watch?v=ldfCQV-LuSo)
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I was intrigued by Cal air compressor so looked more. I found this review and from the tilting piston/rod design would never buy one unless I considered the purchased a non-repairable throwaway like many cheap Harbor Freight compressors that new can't buy parts for.
https://youtu.be/sG9Hl5iaL3M?t=352 (https://youtu.be/sG9Hl5iaL3M?t=352)
Makita has a similar compressor design with proper pistons that stay aligned with the cylinder bore. Of course they cost a lot more.
https://www.ereplacementparts.com/makita-ac310h-25hp-high-pressure-air-compressor-parts-c-97_1547_39919.html (https://www.ereplacementparts.com/makita-ac310h-25hp-high-pressure-air-compressor-parts-c-97_1547_39919.html)
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I've got a Portable 12V DC unit from VIAIR for the car, its just a powerful clean hum when operating compared to every other unit I've had before. Definitely a brand I would recommend if they have a product that fits the need.
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I was intrigued by Cal air compressor so looked more. I found this review and from the tilting piston/rod design would never buy one unless I considered the purchased a non-repairable throwaway like many cheap Harbor Freight compressors that new can't buy parts for.
https://youtu.be/sG9Hl5iaL3M?t=352 (https://youtu.be/sG9Hl5iaL3M?t=352)
Makita has a similar compressor design with proper pistons that stay aligned with the cylinder bore. Of course they cost a lot more.
https://www.ereplacementparts.com/makita-ac310h-25hp-high-pressure-air-compressor-parts-c-97_1547_39919.html (https://www.ereplacementparts.com/makita-ac310h-25hp-high-pressure-air-compressor-parts-c-97_1547_39919.html)
That's hardly a fair comparison. The Makita AC310H is a 400 psi, two stage, 2.5 HP unit. It also doesn't mention what the noise level is.
Unfortunately, the manuals for the Makita quiet compressors only show the pump as a single unit with no internal parts shown. I haven't been able to find any repair or teardown info for them.
Makita's only small compressor outside their quiet line (model AC001) is only 1 gallon, 0.45 cfm @ 90psi. They say it's quiet, but don't give any numbers. I suspect that since it's not part of their quiet series, it isn't very quiet. It uses a fixed piston/connecting rod system similar to the California Air Tools models.
I suspect that most small oil-less compressors would use the fixed piston/connecting rod system and they are known to be stupidly loud.
Ed
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60dB(A)? they are extremely noisy!!!
I'm the lucky owner of a JUN-AIR (https://jun-air.com/products/) 6-4 (https://jun-air.com/wp-content/uploads/2021/10/6-4-Rev-C.pdf) compressor:
PRO:
- only 45dB(A): I can use it at 3AM without any worry to wake up my neighbors
- sturdy constructed: "only" 18kg for a 4 liter unit!
CON:
- exaggeratedly expensive, but I got mine for less than 100€
- not so easy to find spare parts
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Want quiet, you could always mod an old fridge compressor into a bench air supply for small tasks.
EG: https://www.mattmillman.com/projects/building-a-fridge-compressor-air-compressor/ (https://www.mattmillman.com/projects/building-a-fridge-compressor-air-compressor/)
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60dB(A)? they are extremely noisy!!!
I'm the lucky owner of a JUN-AIR (https://jun-air.com/products/) 6-4 (https://jun-air.com/wp-content/uploads/2021/10/6-4-Rev-C.pdf) compressor:
PRO:
- only 45dB(A): I can use it at 3AM without any worry to wake up my neighbors
- sturdy constructed: "only" 18kg for a 4 liter unit!
CON:
- exaggeratedly expensive, but I got mine for less than 100€
- not so easy to find spare parts
Interesting. I see that further down the page you linked to, they have models that look like the models from California Air Tools. The specs for these models are in the range of 20 dB higher than yours. But I can't find any info on how they work. Why is yours so much quieter than these other models?
Ed
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Want quiet, you could always mod an old fridge compressor into a bench air supply for small tasks.
EG: https://www.mattmillman.com/projects/building-a-fridge-compressor-air-compressor/ (https://www.mattmillman.com/projects/building-a-fridge-compressor-air-compressor/)
I've seen lots of those projects. A fridge compressor isn't a good fit for an air compressor. Here's some reasons.
1. The one you linked to says that he fills a 6L tank to 120 psi in 59 sec. If I did the conversion right, that's only ~0.2 CFM. That's part of the reason it's so quiet.
2. The compressor expects to be inside a perfectly clean, oil-filled system. It's not designed to be sucking in dirty, moist air. The longevity of such a solution is questionable.
3. The compressor is capable of putting out pressures of hundreds of PSI. If something goes wrong, it can go really wrong!
Ed
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I thought I could provide some insight by gathering data on the three compressors I have. After looking over the data I am not sure that much clarity has been provided.
My hunch was that the perceived noisy quality was a combination of component robustness, sound insulation and frequency content. My three compressors are:
1. A very large and very old school shop compressor. 80 gallon tank, 160 psi and more cfm than I have ever been able to consume. Two stage compressor, one inch plumbing, radiator vanes on output pipes to cool output air and several hundred pounds total weight. Another noise reduction feature is a muffler/filter on the air intake. It is perceptually the quietest of the three, no issue standing right next to it.
2. A 1980s 1 HP portable compressor from Sears. Also two stage, 6 gallon tank, 100 psi and just a few cfm.
Far lighter construction, the whole thing probably weighs about 60 pounds. Second quietest. A little noisy to stand next to, but basically inaudible from outside the building it is in.
3. A 1990s airless shop compressor. I haven't opened the molded housing to see how many stages, but I believe it is single stage. 125 psi, mounted on 10 gallon tank, a little lower cfm than unit 2. By far the noisiest of the two. Impossible to share the room with it without hearing protection and easily audible from my well sound insulated house from its location in a freestanding shop.
I took two measurements. SPL using that feature on one of my DMMs and spectrum gathered with a laptop microphone. The SPL was done casually, approximately 1 meter distance and the meter has never been calibrated or evaluated. The laptop gathered data is totally uncalibrated for amplitude and is only useful for comparing relative frequency content.
The three SPLs were 78dB for compressor 1, 80dB for compressor 2, and 94dB for compressor 3.
While the SPLs generally track the subjective experience the noise from compressor 1 is significantly less annoying than compressor 2 and seems there would be more than the indicated difference.
The graphs for the spectrum are attached.
I will let others judge whether they support the theory that the lower speed of the older compressors is a key in their lower noise.
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Not surprising that the old compressor seems quieter due to most of the energy being in the lower frequency spectrum.
Something to do with the resonance of the auditory canal: your hearing has peak sensitivity around 3500-4000 Hz. So the spectra definitely makes a difference and your noisy one has more energy in that range making it sound extra loud.
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Not surprising that the old compressor seems quieter due to most of the energy being in the lower frequency spectrum.
Something to do with the resonance of the auditory canal: your hearing has peak sensitivity around 3500-4000 Hz. So the spectra definitely makes a difference and your noisy one has more energy in that range making it sound extra loud.
I agree as far as that goes, but doesn't explain the big difference in spectral shape between the two older compressors. And the overall lower noise of the oldest compressor while doing several times more work than compressor number two. I suspect that shear mass is a lot of the answer
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Compressor noise will be a combination of multiple noise sources. Some I can think of are:
1. Bearing noise on the compressor and motor. Shouldn't be a big issue unless they're shot.
2. Air noise. Whooshing & turbulance noise as the air is drawn in.
3. Valve noise. Nasty high frequency, metallic clicking/snapping sound. AFAIK, all normal size compressors use reed valves. Huge industrial compressors are likely different.
4. Vibration from an out-of-balance or worn system. Low frequency, but could excite higher frequency resonances.
5. Random mechanical noises from resonances that are transmitted and/or amplified by the metal mounting plate or the tank itself.
6. Mechanical piston noise as it's scraped up and down inside the cylinder.
Any others?
If you look on Youtube for info on reducing compressor noise, it mostly comes down to "build a better intake muffler" because that's the most effective thing that a hobbyist can do. Assuming that the other parts are in good condition (i.e. bearings and isolation mounts in good condition, fans & pulleys not bent), there really isn't much 'low hanging fruit' to work on. A better muffler would help deal with items 2 and 3 above.
Catalina, your large compressor has the luxury of being fixed so a large muffler has no penalty. Also, the larger pipes will reduce the air noise. It probably requires oil so that might reduce various mechanical noises. The portable compressors have to minimize size and weight so they tend to use the smallest mufflers and pipes they can get away with. It seems like the oil-less ones are the noisiest or most annoying as confirmed by your tests.
I noticed that all the California Air Tools style quiet compressors, even the smallest, are two cylinder, single stage units that use direct drive and have the pistons arranged so that when one is compressing the air, the other is drawing in fresh air. They also state that the lower speed of their system reduces noise levels. The few teardowns that I've seen show that the piston isn't sealed with a metallic ring. It uses a teflon seal. Also, they always connect the two cylinder heads with two small pipes so that air can pass between the input sides and between the output sides. But they still use the same crappy little mufflers that the loud compressors do. Smaller units have one muffler and larger units have two. I've also noticed that the design doesn't scale up very well. If you need more CFM, they use two or three smaller units in parallel rather than one big unit.
The direct drive arrangement means fewer bearings. The teflon seal will be very quiet. The opposing pistons might help cancel out some vibrations. The crappy muffler(s) might be a candidate for an upgrade. The pipes between the heads would allow a continuous air flow in and out without stopping and starting the flow.
The only unique thing I see is the pipes between the heads. Is it possible that smoothing out the airflow with these pipes and the opposing pistons is enough to drop the noise by 20 or 30 dB?? Or is it a collection of small improvements that add up to a big noise drop? :-//
Ed
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A surprising percentage of the noise from my air compressor comes from the air intake, if you remove the filter/muffler thing it gets quite a lot louder.
The other noise is largely down to RPM, a compressor with more displacement that turns slower will be quieter, all else being equal. Or you can contain the noise, I've seen compressors that used refrigeration style hermetic compressors, those sit on springs inside a steel dome which makes them very quiet.
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Yep quiet compressors are mostly about running at a lower RPM and filters on the air intake.
Single piston compressors can't be balanced properly, so they always produce vibration, the faster they go the worse it gets and the more audible it gets as it moves up the frequency range. Same for the noise coming out of the intake, it gets louder the faster it runs.
However running at a higher RPM gets more airflow out of the same size piston. So the manufacturers will want to run there compressors as fast as reasonably possible to get better specs out of the same lump of metal. So the small cheap compressors tend to be the loudest.
Fridge compressors are very quiet indeed, thanks to being suspended by springs and closed in a metal box. However they are pumping very small amounts of air volume. So they can't be directly compared to shop compressors (tho are still useful when you do only need small air volumes at high pressure)
Scroll compressors are very quiet because they have no reciprocating pistons or valves. But those are pretty rarely found for use as air compressors, they mostly are used as the large multi kW refrigeration compressors. Why that is i have no idea, perhaps it is more expensive to make those precise spirals, or maybe they don't deal with contamination so well.
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Scroll compressors are very quiet because they have no reciprocating pistons or valves. But those are pretty rarely found for use as air compressors, they mostly are used as the large multi kW refrigeration compressors. Why that is i have no idea, perhaps it is more expensive to make those precise spirals, or maybe they don't deal with contamination so well.
They spit a lot of oil out with the air, not a problem for a refrigeration system but not a desirable trait for a shop compressor. Also they are expensive, I haven't priced one recently but they used to command a substantial premium over a similar size reciprocating compressor.
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Compressor noise will be a combination of multiple noise sources. Some I can think of are:
1. Bearing noise on the compressor and motor. Shouldn't be a big issue unless they're shot.
2. Air noise. Whooshing & turbulance noise as the air is drawn in.
3. Valve noise. Nasty high frequency, metallic clicking/snapping sound. AFAIK, all normal size compressors use reed valves. Huge industrial compressors are likely different.
4. Vibration from an out-of-balance or worn system. Low frequency, but could excite higher frequency resonances.
5. Random mechanical noises from resonances that are transmitted and/or amplified by the metal mounting plate or the tank itself.
6. Mechanical piston noise as it's scraped up and down inside the cylinder.
Any others?
The normal compressors take they air in as pulses, at best in a kind of half sine. That is a lot of low frequency noise, especially if the compressor runs fast (e.g. 2800 / 3400 RPM).
Similar to the air intake the crank case housing also sees a change in volume - the crank case ventilation can thus also be noisy.
With an old worn out comressor a loose fit on the crank shaft / piston pins can be a horrible noise source. A special case or a bad bearing.
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There are generally mods that can be done to the air filters to lower noise. Other than that not much you can do short of replacing it with a quieter unit
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Here you will find an excerpt of the Jun-Air compressors technical manual with component lists and exploded views.
Maybe it can be of some help on how they did to reduce compressor noise.
Video showing the internals of such compressors: another league compared to traditional compressors:
https://www.youtube.com/watch?v=kibN1DBAvLY (https://www.youtube.com/watch?v=kibN1DBAvLY)
noise comparison:
https://www.youtube.com/watch?v=X2u0n3mnF6U (https://www.youtube.com/watch?v=X2u0n3mnF6U)
https://www.youtube.com/watch?v=X-W9ihHt1Dk (https://www.youtube.com/watch?v=X-W9ihHt1Dk)
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Those compressors are probably pretty quiet but their price reflects that and more. I think they're mostly marketed towards dental and medical offices where they need reliable quiet clean air supplies and price isn't this so much of an issue. You could probably buy 5 or 10 poor quality compressors that would cost less than one Jun compressor of a similar capacity.
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They're just modified refrigeration compressors, you could build one yourself without great difficulty. The tradeoff is they are much smaller capacity than a typical shop compressor.
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Any compressor can be made quiet, by using an acoustic cover over it!
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Any compressor can be made quiet, by using an acoustic cover over it!
The is still a relatively tricky part as compressors produce quite a lot of waste heat. In a first approximation this is as much as the electricity taken from the grid - possibly even a little more.
An acoustic cover for the relatively low frequencies that can still pass enough air to get rid of the heat is quite challenging.
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Yeah most things that insulate acoustic noise will also thermally insulate pretty well.
The motors are generally wound for maximum power with minimum material, rather than maximum efficiency so they produce a fair bit of heat. Then the compression of air is basically a heat pump too.
There are silent marine diesel generators that are put inside insulated boxes, those run coolant lines out of the box to dissipate the heat, but on a boat you got a convenient large volume of cool water right under you.
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Well of course the inlet and exhaust air ports would need some thought but I have used compressors that have manufacturer enclosures that work no problem, and have made enclosures for a couple of compressors myself, it's not that difficult. Even with inlet and exhaust ports the reduction in noise is significant. Here's a nice little unit I used for a while, for some idea.
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If you start with a noisy compressor, you have two options.
First, you use a good muffler on the intake to suppress the noise from the air movement and the valves. That's a relatively easy DIY project and is appropriate for any size of compressor.
Second, you build an enclosure around the entire unit to suppress all noise sources. As Kleinstein stated, the heat generated by the compressor has to be taken into account. Usually, the result is a fan-forced enclosure where both inlet and outlet vents are in the form of a labyrinth that has foam-lined walls. The entire interior of the main cavity is also foam-lined. This technique becomes awkward with large compressors because you're building something the size of a garden shed! I've never seen a project where the compressor was seperated from the tank and enclosed seperately.
The results of either option can be quite impressive.
But the intent of this thread was to explore how some compressors manage to be relatively quiet without requiring either of these extra steps. We've seen examples of extremely quiet units, but it looks like they achieve low noise at the cost of low air volume.
I hoped that we'd hear from someone with better information on the design of these relatively quiet units, but that hasn't happened. But the discussion has been very interesting and useful.
Ed
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Some of the newest air compressor designs no longer use pistons, but rather twin helical screws. The twin rotors don't touch each other but are of an interlocking shape so they fit perfectly and the air gets sucked in on one end and pushed towards the other end, compressing it in the process. It's a bit like a supercharger on a car, the big ones that bolt on top of the engine.
here is what the compressor looks like.
(https://www.air-compressor-guide.com/wp-images/2014/05/rotary-screw-element3.jpg)
I'm not sure if this is what is being used in those "quiet" pancake compressors, but it's definitely the tech used in larger vertical compressors like the type i'm looking into buying to replace my Craftsman 26 gallon compressor. My Craftsman is about 93dba while running and these rotary compressors seem to be around 60-68dba which is a massive improvement.
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That's a screw compressor, they've been around for close to 100 years, they're more expensive than reciprocating compressors though.
The quiet pancake compressors are modified reciprocating refrigeration hermetic compressors.
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Also can look up scroll compressors they are also quieter.
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To ask what makes an Air compressor Quiet, you are asking What makes a good Circuit board... the Answer is Complicated.
:-DD However I do have a suggestion for you, which maybe NOT suitable for you.
12V Fridge compressors, aka - Single Spring Piston Pumps - You see these in Engels, EvaCool style units.
Food for thought.
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A Diaphragm compressor might what you are looking for, moderate flow, moderate noise.
Screw type compressors aka Super-Charger styles could work and they are used in Commerical Units (at a cost), you still need to spin those at Engine speeds (2000 rpm onwards)
I only mention this because I seen a post where some listed these.
:-DD There is no simple answer, just like Electronics :box:
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A Diaphragm compressor might what you are looking for, moderate flow, moderate noise.
Screw type compressors aka Super-Charger styles could work and they are used in Commerical Units (at a cost), you still need to spin those at Engine speeds (2000 rpm onwards)
I only mention this because I seen a post where some listed these.
:-DD There is no simple answer, just like Electronics :box:
I have a spare 2.3L screw supercharger from my car that I replaced with a 3L supercharger. I wouldn't think of using it for an air compressor because it would have to spin at least 5000 rpm to make pressure needing a lot of power. Also there is air leakage on the rotors, spin to slow and the leaks negate any pressure on the output. Besides it is designed to only put out about 1 to 2 bar of pressure.
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I have a spare 2.3L screw supercharger from my car that I replaced with a 3L supercharger. I wouldn't think of using it for an air compressor because it would have to spin at least 5000 rpm to make pressure needing a lot of power. Also there is air leakage on the rotors, spin to slow and the leaks negate any pressure on the output. Besides it is designed to only put out about 1 to 2 bar of pressure.
Is it a screw type or a Roots blower? Either way they are not all alike, screw compressors intended for use in industrial air compressors are fairly common, they're specifically designed for higher pressure and lower volume than a similarly sized automotive supercharger. My friend's shop has two big screw compressors, I think they're 15 or 20 horsepower, very quiet relative to the 10 hp reciprocating compressor they used to have.
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screw type.
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Is only you could 3D print such items :-DD
add some Electroplating
spin it up :popcorn:
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Practically speaking for small quiet shop type compressors, as far as I know you're pretty much looking for Jun Air (buy used) or belt drive. You can slow down the belt drive compressors to a point to quiet them further with a smaller motor pulley, but you need a minimum RPM for proper splash lubrication, and enough belt wrap on the motor pulley to actually transmit power, some stock ones I've seen are about as small as you can get already.
The automotive guys I know will typically put the main one in a closet, little enclosure outside the building (bad idea with close neighbors), shed, garage attic, basement, etc. Just separating yourself from the thing with a wall or two helps a lot, running an air line is pretty cheap.
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I don’t know the technical reasons behind a “quiet” small compressor, but we use them at work for the air injection system on our lasers. For years we have used cheap pancake compressors made for nail guns, they make enough noise to wake the dead, but are cheap. Ours are inside the work space and create a level of noise that prohibits conversation when running. We recently bought one of the Mikita Quiet series units that are about three time what we paid for the HomeDepot basic Porter Cable ones. The difference in noise level is astounding, you can stand next to it in a small room and easily talk over it. Just amazingly quiet, like at first you wonder if it is working right until you see the gauges ramp up. I have studied air gun silencers/moderators a bit and suspect a similar approach is used for the compressors, plus the expected vibration dampeners etc..