What makes a quiet air compressor quiet?

[edpalmer42]

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

[GLouie]

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.

[Stray Electron]

  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.

[thm_w]

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.californiaairtools.com/ultra-quiet-series-of-air-compressor-contractor-grade/

[BlackICE]

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

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

[Algoma]

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.

[edpalmer42]

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

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

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

[eliocor]

60dB(A)? they are extremely noisy!!!
I'm the lucky owner of a JUN-AIR 6-4 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

[Algoma]

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/

[edpalmer42]

60dB(A)? they are extremely noisy!!!
I'm the lucky owner of a JUN-AIR 6-4 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

[edpalmer42]

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/

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

[CatalinaWOW]

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. 

[Kim Christensen]

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.

[CatalinaWOW]

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

[edpalmer42]

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

[james_s]

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.

[Berni]

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.

[james_s]

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.

[Kleinstein]

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.

[kermitfrog]

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

[eliocor]

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:


noise comparison:

[BlackICE]

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.

[james_s]

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.

[unknownparticle]

Any compressor can be made quiet, by using an acoustic cover over it!

[Kleinstein]

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.