The default initial values are -60 (minimum amplitude, silence) and 10 milliseconds (minimum allowed fade time). The fade is approximately exponential, which is what we want, but with less computation.Īny amplitude less than or equal to -60 dB (0.001) gets switched to 0 (–infinity decibels) for complete silence. The difference between those two methods is imperceptible, but this way is more efficient. Conversion from decibels to amplitude is a computationally expensive operation, so we do it only 100 times per second (every 10 ms) rather than for every single sample, and then we interpolate to each new amplitude with line~. It might seem a bit redundant to use both a line and a line~ (with a dbtoa between them), but in fact this is more efficient than using just a line~ and a dbtoa~. The line~ object then interpolates to each new amplitude. Those values are converted to amplitude with the dbtoa object. We use a line object to interpolate to the new decibel value, outputting a new value every 10 milliseconds. You can also send an amplitude-time pair in the left inlet to achieve the same result. Perceptually, a linear fade in decibels (which yields an exponential fade in amplitude) sounds smoother than a linear fade in amplitude.Ī number in the right inlet supplies a fade time in milliseconds, and a number in the left inlet supplies the desired amplitude, in decibels, and triggers a fade to that new amplitude. The abstraction fades to the new amplitude in a specified amount of time. The amplitude factor is expressed in decibels, which is generally a more intuitive way to refer to the volume of a sound. The output of this abstraction is intended to go to a *~ object to scale the volume of a sound. If we were to calculate this using equation (5.3) we would get 87.6 dB SPL - try this for yourself.This patch is an abstraction for supplying an amplitude factor to control the level of an audio signal. 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, 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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