Clarification about air compressor sound level specifications

[tryton8]

I am interested in buying air compressor but I don't know what to make out of these sound level specs:

https://assets.einhell.com/manuals-pdf/900_361679/te-ac-400-50-10.pdf

TE-AC 400/50/10:
Sound power level LWA: 97 dB
Sound pressure level LpA : 85 dB

https://assets.einhell.com/manuals-pdf/900_361676/te-ac-270-50-10.pdf

TE-AC 270/50/10
Sound power level LWA: 94 dB
Sound pressure level LpA : 72 dB

How is it possible that the difference in sound power level is just 3dB while the difference in sound pressure level is a lot more substantial 13 dB?

Specs also say "The noise emission values were measured in accordance with EN ISO 2151". I googled it but this standard costs $100, half the price of the compressor.

I did find something less official:

https://www.compressorsonline.com/choose-the-right-air-compressor-system/sound-power-pressure-level-explained/

And looking at the table it seems that the huge difference between sound power and pressure level should not happen.

TLDR: Is this error in the manual, was the sound pressure level measured at different distance (despite both conforming to the same standard), or could this be a valid measurement? And if it is which of the two values should I compare when trying to determine how loud they are compared to one another?

Thanks

[Nusa]

Simplified short answer: LWA is the source, while LpA is a distance measurement. Not sure what distance, but I'm sure that standard specifies.

This is as good a detailed explanation as any:

http://www.sengpielaudio.com/calculator-soundpower.htm

[Awesome14]

Are you concerned about how much noise the device will create? If so, you need the dB at a certain distance. The figures stated are quite noisy; about as loud as a gasoline-powered lawn mower. But there is no distance given. So, it's probably 1 meter.

[tryton8]

Simplified short answer: LWA is the source, while LpA is a distance measurement. Not sure what distance, but I'm sure that standard specifies.

This is as good a detailed explanation as any:

http://www.sengpielaudio.com/calculator-soundpower.htm

But that doesn't explain why the difference in LWa only 3 dB but in LpA 13 dB? Is (vastly) different directivity factor a likely cause when two devices are essentially the same, with one having a more powerful motor? Or is it more likely an error in specs?

[tryton8]

Are you concerned about how much noise the device will create? If so, you need the dB at a certain distance. The figures stated are quite noisy; about as loud as a gasoline-powered lawn mower. But there is no distance given. So, it's probably 1 meter.

Yes that is what I am concerned about. I would rather buy a bigger unit if the difference is only 3dB, but I won't if it is 13 dB. I will be wearing ear protection so I am mostly concerned how much it will annoy my neighbors (I live in an apartment).

[Nusa]

Remember that decibels is a logrithmic scale, not a linear one. A 3 dB difference in sound power is a significant change. The smaller unit will be noticeably quieter.

I wouldn't consider either unit quiet enough for the typical apartment, to be honest. Your neighbors will likely hear it every time it kicks on.

[ciccio]

I believe that the manufacturer is lying....
Both are very noisy, indeed, for home use.
Best regards

[nuno]

There a few "silent" models, but it's better to search around youtube for comparisons as "silent" is just a relative term.

If you are DIY inclined, I guess the most silent compressor you can get is to make one out of a refrigerator compressor. Disadvantage is that it takes some time to charge the vessel, as it's a low power compressor. You'll find several such projects around.

[tryton8]

Remember that decibels is a logrithmic scale, not a linear one. A 3 dB difference in sound power is a significant change. The smaller unit will be noticeably quieter.

You are right that 3dB is difference noticeable. But 13 dB is more than twice as loud. I'm still not sure which numbers I'm suppose to be comparing, and why they are so different.

[Dubbie]

I have a screw compressor which is nice and quiet for home. (62 dB ) It's slightly overkill @ 81 CFM output, but I got it as part of a contra deal.

[Nusa]

Remember that decibels is a logrithmic scale, not a linear one. A 3 dB difference in sound power is a significant change. The smaller unit will be noticeably quieter.

You are right that 3dB is difference noticeable. But 13 dB is more than twice as loud. I'm still not sure which numbers I'm suppose to be comparing, and why they are so different.

You don't appreciate the steepness of the scale. 3dB is double the power. 10dB is times 10. Which isn't quite the same thing as human perception of sound, but that's a subject you can read up on.

The first number is a number the law requires the label contain, as it's independent of environment and very hard to manipulate in testing. The second number appears to be (after reading that link you put in your base note) a measurement at 4m from the unit. No shock that something that starts out with twice the power is noticeably louder at 4m.

In a house garage it would probably be fine, and you would only annoy yourself. In an apartment with shared walls/ceiling/floor's, it may be unacceptable. But you know your situation better than me.

[tryton8]

Remember that decibels is a logrithmic scale, not a linear one. A 3 dB difference in sound power is a significant change. The smaller unit will be noticeably quieter.

You are right that 3dB is difference noticeable. But 13 dB is more than twice as loud. I'm still not sure which numbers I'm suppose to be comparing, and why they are so different.

You don't appreciate the steepness of the scale. 3dB is double the power. 10dB is times 10. Which isn't quite the same thing as human perception of sound, but that's a subject you can read up on.

Not sure what makes you think I haven't read about it, everything I have said above is correct. 3dB is double the power but perceived as just noticeably louder. 10 dB is ten times the power but is perceived as just twice as loud. But this isn't what my question is about.

The first number is a number the law requires the label contain, as it's independent of environment and very hard to manipulate in testing. The second number appears to be (after reading that link you put in your base note) a measurement at 4m from the unit. No shock that something that starts out with twice the power is noticeably louder at 4m.

The specs don't say that it is noticeably louder at 4m. They say it is 13dB louder. How does something start 3dB as loud but becomes 13dB as loud at 4m? It seems logical that if devices are similar, and have similar directivity factor (and it seems like a fair assumption that two compressors, one with bigger motor, shouldn't different significantly here), difference should remain about constant, and the table in this link shows that.

https://www.compressorsonline.com/choose-the-right-air-compressor-system/sound-power-pressure-level-explained/

So why doesn't it remain constant in this case? That is my question, not about human perception of dB scale.

[Nusa]

As for the vendor's numbers, they may or may not be completely correct, which doesn't mean their testing scenario matches your scenaro. I'd certainly believe the unit will not be quieter than their claimed numbers. Any skepticism should be in the other direction.

Let's try an analogy that's easier to see, pun intended:

You have a light that's emitting 1000 lumens. If you view it from 4m, how many lumens is the light emitting? If you said 1000, you're right! The bulb performs the same regardless of your distance from it. That's the POWER of the bulb, unaffected by the location of the observer. Now pull out your light sensor and see how bright it is at your 4m location. That's the EFFECT (loudness if you will) the bulb has on your location. Further, that result will be affected by environmental reflections. The bulb will illuminate better in a room painted white than a room painted black.

[macboy]

Both of those will be loud compressors.
Mine is rated at "60 dB" but I don't know to what standard. You can easily carry on a conversation standing next to it, without raising your voice. The loudest thing about it is the unloader valve that goes "psshh!" at the end of a cycle. I keep my compressor in my basement, connected to the workbench and garage 24/7, and never hear it unless I'm in the room with it when it happens to cycle.

[tryton8]

As for the vendor's numbers, they may or may not be completely correct, which doesn't mean their testing scenario matches your scenaro. I'd certainly believe the unit will not be quieter than their claimed numbers. Any skepticism should be in the other direction.

Let's try an analogy that's easier to see, pun intended:

You have a light that's emitting 1000 lumens. If you view it from 4m, how many lumens is the light emitting? If you said 1000, you're right! The bulb performs the same regardless of your distance from it. That's the POWER of the bulb, unaffected by the location of the observer. Now pull out your light sensor and see how bright it is at your 4m location. That's the EFFECT (loudness if you will) the bulb has on your location. Further, that result will be affected by environmental reflections. The bulb will illuminate better in a room painted white than a room painted black.

So would you say that I should stick to sound power level and ignore sound pressure level? Now that I look at the numbers again perhaps they tested sound pressure level of one compressor at 4m and other at 1m.

[tryton8]

Both of those will be loud compressors.
Mine is rated at "60 dB" but I don't know to what standard. You can easily carry on a conversation standing next to it, without raising your voice. The loudest thing about it is the unloader valve that goes "psshh!" at the end of a cycle. I keep my compressor in my basement, connected to the workbench and garage 24/7, and never hear it unless I'm in the room with it when it happens to cycle.

What airflow does your compressor have? The smaller of the two I linked is rated at 270 l/min intake, 100 l/min output at 7 bar, larger at 400 l/min intake and 170 l/min at 7bar. They cost 137 and 270 euros. 65 dB compressor that I found with similar airflow as the smaller compressor costs 799 euros, almost 6 times as much.

[macboy]

Both of those will be loud compressors.
Mine is rated at "60 dB" but I don't know to what standard. You can easily carry on a conversation standing next to it, without raising your voice. The loudest thing about it is the unloader valve that goes "psshh!" at the end of a cycle. I keep my compressor in my basement, connected to the workbench and garage 24/7, and never hear it unless I'm in the room with it when it happens to cycle.

What airflow does your compressor have? The smaller of the two I linked is rated at 270 l/min intake, 100 l/min output at 7 bar, larger at 400 l/min intake and 170 l/min at 7bar. They cost 137 and 270 euros. 65 dB compressor that I found with similar airflow as the smaller compressor costs 799 euros, almost 6 times as much.

l/min ? bar ?   
Canada is metric, but PSI and CFM are still used here. It is rated for 3.3 CFM at 40 PSI and 2.4 CFM at 90 PSI.  That would be 93 l/min at 2.8 bar and 68 l/min at 6.2 bar.  It keeps up to what I use it for. 

[james_s]

If you're concerned about noise, don't buy a cheap oilless compressor, those things are awful. They are driven by a universal (brushed) motor at high RPM and make a hell of a racket and it's a noise that I find particularly annoying. Additionally the brushes wear out after a few tens of hours of running and the compressor is pretty much scrap. A decent belt drive reciprocating compressor with an induction motor will be much quieter, and you can make it quieter still by putting a muffler on the intake.