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You don't add the fans togehter that way to get total decibels. There is an equation for it, but I don't remember. I think the actual noise level doubles every 2-3 decibels.
EDIT: Look at this page. >>
No, sound doubles in amplitude every 3dB, precieved sound (what you hear) doubles every 10 dB (or one bel).
That page makes working with decibel quantities more complex than it really is because it's combining more than one step into one equation and using some notation some readers might not be familiar with. I suggest that anyone who's interested in finding the total value of dB their system makes learn the method(s) below at the very least (it wouldn't hurt to read some general info on the subject too), because it's faster than using the web calculator linked to above when you need to add up more than two quantities once you've done it a few times. It's also more accurate (the website rounds to the nearest tenth). Okay, here goes:
Working with decibel quantities requires a little knowledge of logarithms and algebra, or a 100 level Physics course. If you don't meant those requirements, you can use the equations below. I've done pretty much all the work, making it a simple plug-in-values deal. So you'll only need to know how to use basic functions on a calculator, adding, log (log base 10, meaning just push the log button), exponentials, squaring, and square rooting (and not even the last two if you use the second method).
You can't add dB directly, first you have to convert it to either Sound Pressure Level or Sound Intensity Level. I won't bother defining and explaining absolutely everything and such, you people who really want to be in the know would probably go and find a better source anyway, so this'll just be the skinny. Okay, the equations below assume that the point of reference is the average human threshold of hearing, 2 x 10^-5 N/m^2 (Newtons/meter squared) is 0 dB and that the sounds are uncorrelated. Correlated sound would be two sounds of exact amplitude and period equidistant to the listener, ie. an eletrical signal split in two, fed to two exactly same speakers, both equidistant to you, you won't get this with fans. Basically, what you're going to do is either:
Convert dB values to SPL (Sound Pressure Level)
Add the squares of the SPL values
Square root the result
Convert back to dB
OR
Convert the dB values to SIL (Sound Intensity Level)
Add the values together
Convert back to dB
For SPL:
dB = 20 * log(P/(2 x 10^-5))
With some Algebra, we get:
(10^(dB/20)) * (2 x 10^-5) = P
P being the SPL
Then, just take all the
P values and square them individually, add them together, take that result and square root it, then plug it back into the original SPL equation (the total SPL value goes into
P).
For SIL:
dB = 10 * log(I/(10^(-12)))
Throw in some Algebra:
(10^(dB/10)) * 10^(-12) = I
I being the SIL
Then, take all the
I values, simply add them together (no squaring involved), then take the total SIL value and plug it back into the original SIL equation, substituting
I for that total SIL value.
After learning that, I recommend learning about SPL and SIL, what they are, how they relate to each other, how they differ from each other. With the subject of sound, understanding of Trigonometry is a must (sine waves, logarithms, etc.).
