Sound / Audible Levels

[jcrouse]

All you sound gurus out there ... answer me a question. If I have 5 fans that are 20dBA's each do I have a total of 20dBA or 100 dBA. I am guessing somewhere in between (on the lower side) like 30 dBA but an explanation would be educational.

Thanks,
John

[Crankyhobo]

Once upon a time i thought '6x 20dBA = 20dba', but the more fans you add the more noise it makes.

"Each doubling of identical noise sources results in a 3 dB increase in noise. "

Check out this Article http://www.silentpcreview.com/Primer_on_Computer_Noise

[jcrouse]

Excellent article and explanation. That is exactly what I was looking for.

Thanks a ton,
John

[Church]

Crankyhobo: yeah, but perceived by human ears doubled loudness is noise increase for arround 10db.

[Martinm210]

I think its even more complex than what dbA tells us. Noise level is one thing, but irritable level is another. I noticed here recently in testing noise of a pump plus noise of fans that while the meter may read higher, the mix of two sound types can result in a more diversified frequency which per my ears can actually be perceived as better.

The opposite can be also true when you have two rythmic sound sources like fans or pumps. Two fans turning at just the wrong differet speeds can create an undulating harmonic that is unpleasant, yet the noise level from a dbA defined standpoint may paint a different picture.

Bottom line is noise is complex, much more complex than what sound level alone is capable of. I've pulled many a hair out trying to feel comfortable with the tools we have to measure noise and have been left with trusting my ear first and formost. Sound pressure levels even when tuned to A weighting still doesn't represent what I would call "irritating sound" quantity.

[Serephucus]

With dB(A), it's a little different. Example:

2 people talking: 15dB(A)
4 people talking: 18dB(A) (+3dBA approx.)
20 people talking: 30dB(A)

In other words, you need ten times whatever's making the noise, for it to sound doubley as loud.

Edit: Martin, QFT.

[Martinm210]

FYI
Here is a pretty good resource and includes an online calculator:
http://www.sengpielaudio.com/calculator-spl.htm

Although I haven't quite seen it add up that fast, so it probably depends on the noise sources. If they are exactly they same, these equations probably apply ok, but if they vary in spectrum, it may be a different story.

[Church]

Serephucus: bad example. With 20 people talking simultaneous (remembering study breaks at class in oold school days) they all also often rise voice to be heard so they are not anymore multiplied noise sources of same loudness :)