If we were to calculate this using equation (5.3) we would get 87.6 dB SPL - try this for yourself. 85 dB SPL and then add this to the 84 dB SPL which would give us a total of approximately 87.5 dB SPL. We can add the 80.8 and 83 first to give approx. For example if we have 3 measurements of 80.8, 83 and 84 dB SPL. If we have more than two sound levels to add we can simply break them down into a series of pairs. 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, Learners are told the deciBel level of Sound Source A and are told that Sound Source B is 10 n times more or less intense. Decibels express values on a nonlinear logarithmic scale, always in relation to a fixed reference point. Now since we are talking about plane waves, our total sould pressure level = 83.01 dB SPL. Decibel Scale The Decibel Scale Concept Builder sharpens a learners understanding of the logarithmic nature of the deciBel scale. The decibel is a unit of measurement (equal to one-tenth of a bel) commonly used to measure the strength of audio signals and acoustical volume. So we now have the sound intensity of our combined signal and we can now convert this back to a dB value: Thus, sound intensity levels in decibels fit your experience better than intensities in watts per meter squared. It represents the logarithmic ratio between the measured value and a reference value, typically the threshold of human hearing. ![]() 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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