At the right hand of the scales, if the two sound levels differ by as much as 20dB then the lower sound level makes very little difference to the total sound level. 80+1 = 81 dB SPL).Īt the left hand side of the nomogram, if the two sound levels are equal (difference = zero) then we should add 3 dB (i.e. 1 dB) this is then added to the higher sound level (i.e. So for our previous example, we take the difference between the two sound levels (80 - 74 = 6 dB) and read the lower scale to find the correction (approx. It is equivalent to a 3 dB increase in the total sound pressure level.įigure 5.2: Nomogram for addition of decibels If we add two unrelated sounds of the same intensity together, Now since we are talking about plane waves, our total sould pressure level = 83.01 dB SPL. So we now have the sound intensity of our combined signal and we can now convert this back to a dB value: If we now add I 1 and I 2 to give I total we have: If we refer to the two sound intensities as I 1 and I 2 which are both equal, then as we have already seen: I 1 = I 2 = 10 -4 W/m 2 assumptions of a plane wave) then the first thing we need to do is convert our dB SPLs into intensities as in 5.1. If we assume that the value in dB SPL is the same as it would be if we measured it in dB IL (i.e. So, for example suppose we have two independent sound sources producing white-noise and the sound pressure level of each one measured on it's own is 80 dB SPL - our question is, what is the resulting sound pressure level when they are both turned on together?
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