Question about decibel-meters

Jeroen · Post by **Jeroen** » Mon Aug 16, 2004 11:23 am

Hi,

I own a decibel-meter

not so special I guess but my question about it might be.

Its range is between 30 and 80 DB. However, I own SilenX fans (www.silenX.com)That company avertises that their fans have noise levels between 11-14DB. How on earth do I measure that? The most expensive decibel-meters I could find (4000$ and more) cannot measure below 24 DB.

Can someone explain this please?

Many thanks!!!!!

Stupid boy · Post by **Stupid boy** » Mon Aug 16, 2004 12:03 pm

Move the decibal meter closer to the fan.

Post by **MikeC** » Mon Aug 16, 2004 12:22 pm

Short answer: If you want to measure at the standard 1m distance, you can't. Your meter is not sensitive enough.

Long answer: There are SLMs which will read down to 0 dBA. But they do cost >$10,000. Once in a while you see a used B&K top-of-line SLM a couple years old selling for under $3000. Once in a great while.

Caveat: Even if you have such a sensitive meter, you may still not be able to measure much below 20 dBA/1m because of ambient noise. So if a fan is rated for 14 dBA/1m, then you have to position the SLM mic 1m away. And if the general ambient background noise of your environment is higher than ~10 dBA, then your measurements will be questionable. If they are at or higher than the noise of your fan at 1m, then you can't meaure it.

Jeroen · Post by **Jeroen** » Mon Aug 16, 2004 12:35 pm

MikeC wrote:Short answer: If you want to measure at the standard 1m distance, you can't. Your meter is not sensitive enough.

Long answer: There are SLMs which will read down to 0 dBA. But they do cost >$10,000. Once in a while you see a used B&K top-of-line SLM a couple years old selling for under $3000. Once in a great while.

Caveat: Even if you have such a sensitive meter, you may still not be able to measure much below 20 dBA/1m because of ambient noise. So if a fan is rated for 14 dBA/1m, then you have to position the SLM mic 1m away. And if the general ambient background noise of your environment is higher than ~10 dBA, then your measurements will be questionable. If they are at or higher than the noise of your fan at 1m, then you can't meaure it.

thankx this really explains alot for me:-)

so, suppose, just for the sake of argument that I sit about 1 meter away from my computer and that I measure my fan with my meter, which has a sensitivity of 30 Db(a)(the meter), at 10 cm distance and it says that the fan makes 35 db(a) of noise

What is the noise level of that fan at 1 meter. simply said : what is the formula to calculate this?

Post by **MikeC** » Mon Aug 16, 2004 12:51 pm

Jeroen wrote:thankx this really explains alot for me:-)

so, suppose, just for the sake of argument that I sit about 1 meter away from my computer and that I measure my fan with my meter, which has a sensitivity of 30 Db(a)(the meter), at 10 cm distance and it says that the fan makes 35 db(a) of noise

What is the noise level of that fan at 1 meter. simply said : what is the formula to calculate this?

No such handy formula, I am afraid. It's hard even to estimate.

Here's why: (from Acoustics FAQ-- http://www.faqs.org/faqs/physics-faq/acoustics/ )

*** 2.9 How does sound decay with distance ?

The way sound changes with distance from the source is dependent on the size and shape of the source and also the surrounding environment and prevailing air currents. It is relatively simple to calculate provided the source is small and outdoors, but indoor calculations (in a reverberant field) are rather more complex.

If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance.

Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected.

In addition it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction.

Jeroen · Post by **Jeroen** » Mon Aug 16, 2004 1:29 pm

MikeC wrote:
Jeroen wrote: What is the noise level of that fan at 1 meter. simply said : what is the formula to calculate this?
No such handy formula, I am afraid. It's hard even to estimate.

Here's why: (from Acoustics FAQ-- http://www.faqs.org/faqs/physics-faq/acoustics/ )
*** 2.9 How does sound decay with distance ?

The way sound changes with distance from the source is dependent on the size and shape of the source and also the surrounding environment and prevailing air currents. It is relatively simple to calculate provided the source is small and outdoors, but indoor calculations (in a reverberant field) are rather more complex.

If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance.

Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected.

In addition it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction.

wooooooow this is way out of my league

But wouldn't the decibelmeter interpred the sounds levels correctly at my specific location?

Allow me to explain, say that I measure something at 10cm and my device gives me 35db(a)as noise level, and than I recalculate that reading for a distance of 60cm...Than that reading should be correct for my location. Suppose I measure the same thing in an other room I could get a totally other reading but that doesn't matter for me. I'm not a company that advertises or so...

Am I kinda right

sthayashi · Post by **sthayashi** » Mon Aug 16, 2004 1:41 pm

Jeroen wrote:But wouldn't the decibelmeter interpred the sounds levels correctly at my specific location?

Allow me to explain, say that I measure something at 10cm and my device gives me 35db(a)as noise level, and than I recalculate that reading for a distance of 60cm...Than that reading should be correct for my location. Suppose I measure the same thing in an other room I could get a totally other reading but that doesn't matter for me. I'm not a company that advertises or so...

Am I kinda right

If I understand MikeC's post correctly, you're not quite right. The function for sound decay over distance varies greatly depending on environment. If you measure the sound at 10cm, then the calculated sound at 60 cm will be greatly different depending on whether you're out in a field or sitting in a hallway.

Post by **MikeC** » Mon Aug 16, 2004 1:43 pm

sthayashi has got it right.

There is simply no way to "recalculate that reading for a distance of 60cm..."

Jeroen · Post by **Jeroen** » Mon Aug 16, 2004 2:08 pm

oke oke :p I got it too now

First of all I'ed like to thank everyone for their help!!! it has been really great.

but I've come up with one final question.

I understand it's rather impossible for me to get an accurate reading..at least with my equipment.

But if I get lets say 35db(a) at 10 cm of distance that sound should be considerably less at 60 cm because of the "sound decay", correct??.

If I compare my readings with those of the silenX company(They make all kinds of silent pc stuff)I should get fairly good idea of how to recalculate all other readings from my other pc parts. (from the same company)
The rest will depend on my subjective interpretation I guess

It may not be extremely accurate but good enough for me !!!

I know I'm stuburn

Post by **MikeC** » Mon Aug 16, 2004 2:33 pm

Here is a serious problem with what you're trying to do:

1) There's the unknown decay with distance. Unknown because that decay is specific to your precise situation. Move everything by 1 meter and you might have a different decay with distance. This is what we've been talking about.

2) Then there is something called the proximity effect. It causes accentuation of certain portions of the frequency band as you put the mic closer to the sound source. I've read that you need to be at least 50cm away to make sure you dont get this effect. It is possible to compensate for this with calculations but they are way over my head and also not precise. I do know that when you get so close (10cm = 4") your voice will take on a bit of huskiness. FM announcer voice syndrome. They get real close to make themselves sound deeper, sexier. We're talking awaywhere from 3~16 dBA at frequencies under 300Hz. It's where the fan makes much of its noise. This proximity effect is a very big problem.

Here's what I suggest as a very rough technique:

A. measure something loud in your room at 2m, keeping both the noise source and the SLM awy from the walls or floor. It should be loud enough for your SLM to register clearly. Like 45 dBA.
B. measure the same sound at 1.5m and 1m. Now you have a basic idea of sound decay w/distance in your room in that position.
C. put your SLM as far away from the quiet sound source (fan or whatever) that you have) while it still registers something. Maybe 36 dBA at 30 cm? (Of course you have to make sure there are no other sound source at all...)
D. apply the .5m compensation learned from A & B to get a rough estimate of the noise farther away.

What are you trying to do anyway?

If you are trying to confirm whether SilenX fans are 14 or 11 dBA/1m, forget it, it's just not possible with your meter or your ambient noise. You have to be in a very good anechoic chamber and have access to >$10,000 equipment to verify those specs if they are for real.

Jeroen · Post by **Jeroen** » Mon Aug 16, 2004 2:52 pm

MikeC wrote: What are you trying to do anyway?

If you are trying to confirm whether SilenX fans are 14 or 11 dBA/1m, forget it, it's just not possible with your meter or your ambient noise. You have to be in a very good anechoic chamber and have access to >$10,000 equipment to verify those specs if they are for real.

Well I am trying to get an idea of the noise levels generated by each part in my computer (Harddisk, grafics card, fans, and powersupply unit) and compare those with the noise levels found in reviews and official specs, etc...

Than I'ed replace the most noisy parts first and see how much more quieter my pc would become.

Most parts will produce about 30db(a),give or take a little, exept for the fans ofcourse... those should be arround 11-14 db(a) ( I'll give up measuring the fans though

I never imagined it would be THAT difficult...

At least I have much more insight in the matter thanks to you...

hvengel · Post by **hvengel** » Mon Aug 16, 2004 7:03 pm

About the most you can hope for is to be able to get an idea of how much of a difference there is between the noise level before and after a change to the system. So you would place the meter close enough to get a solid reading on your meter above the background level. Then record that reading. Then make the change to your system (new fan or what ever) then repeat the reading with the meter and PC in exactly the same location. Then note the difference between this reading and the before reading.

This will not tell you anything about the absolute noise level of your PC but you will be able to say that replacing fan X with fan Y reduced the noise level by Z decibels. And even at that I think you would have to say by ABOUT Z decibels since there are so many things that can affect the reading that you can not control.

Straker · Post by **Straker** » Mon Aug 16, 2004 11:32 pm

i don't think most people have PCs sitting in bathtubs, so those effects probably won't be such a big deal aside from the obvious (ie loud stuff backed up right against a wall)... but considering ordinary noisemakers like fans and drives give off completely different amounts of noise in different directions i'd also agree trying to calculate anything at all is doomed to failure.
if you're just trying to compare stuff in your pc to see which is louder/quieter though, you've just solved your own problem - who cares what the actual power/pressure is? you can just measure all of your fans and drives and stuff at an arbitrary low distance, but remember even this will be subject to other factors (ie hard drive right behind front bezel vs PSU fan); what'd be best is if your meter happened to be sensitive enough to be able to measure each component while still inside the case, then the comparison would be at least a bit closer to the relative levels you're hearing.