:: GussetBLOG ::

:: Thursday, August 07, 2003 ::

dB is a logarithmic scale, given by Level (dB) = 10 * log10 (pressure/ref), where ref = 2*10^5 for normal air pressure.
So to add two decibel levels (L1 and L2) calculate, P1=10^(L1/10) and P2=10^(L2/10), then L_total = 10*log10(P1+P2). (You don’t need to worry about the ref for this calc as it will multiply out.)
Example: If L1=L2=20, 10^(20/10)=100, 100*2=200, 10*log10(200)=23dB.
So, when adding two sources of the same level, the total will always be +3dB.
As another rule of thumb, if there is 10dB or greater difference between the two levels then don’t even bother calculating it, just take the larger one.

As you probably now know, (A) is a weighting curve applied to the signal, either in octave or third octave bands, to emulate the hearing response of the human ear at low levels, there are also B, C and D weightings (rarely used) for different levels, where D is used for assessing levels like jet engine take off at close range, but the curve is almost flat at that point as human hearing is practically flat once you get over 100dB.

Remember, in acoustics, 2+2=5. That would have helped Winston Smith come to terms with things wouldn’t it?

:: Dan 7.8.03 [Arc]    ::
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