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I agree with the statement that the dB (decibel) scale is NOT linear. In fact, it is a LOGARITHMIC scale based on log10. So a 3dB increase represents a 2X increase in perceived volume. For example 73 dB sounds twice as loud as 70dB to the human ear. A 20 dB increase represents a sound intensity that is 10X greater.

With that in mind, let's look at your question:

1) 3 x 30dB does NOT produce 90dB of sound level. It doesn't work that way, if it did, we'd all be deaf! Sound specified in dB (or dBA - basically the same) cannot be added like this. Remember you high-school algebra class and logarithms. What happens when you ADD LOGs? You are MULTIPLYING the base numbers. So by addding 30 + 30 + 30 dB we are attempting to multiply sound pressure and there is no multiplication involved here.

We need to "un-dB" these numbers, add them together, and then "re-dB" them to get the resultant sound level.

The formula for Sound Pressure Level (SPL) is:

SPL = 20 log10(P) ; where P is sound POWER

If we want the P, given an SPL in dBA, we use:

P = 10^(SPL/20)

In this example, SPL = 30 so P = 10^(30/20) = 31.62

For the other SPLs we have:

SPL P

32 -> 39.81

24 -> 15.85

27 -> 22.39

30 -> 31.62

OK, now to calculate:

Example 1:

3 x 30dBA Fans

2 x 32dBA Fans

(3 x 31.62) + (2 x 39.81) = 174.48

That, expressed in dBA is 20log10(174.48) = 44.83 dB

Example 2:

5 x 24dBA Fans

2 x 27dBA Fans

(5 x 15.85) + (2 x 22.39) = 124.03

that, expressed in dBA is 20log10(124.03) = 41.87 dB

So in your scenario, example 1 is going to be TWICE as loud as example 2.