The “level” of an audio signal is measured relative to “full scale” (also called “0 dB”). In Audacity, “full scale” is the height of an audio track, and is shown on the track’s vertical ruler as +1 to -1. It is measured as “the distance from the (silence) centre line”.
“Peak level” is the absolute maximum distance from the centre line of the waveform. Thus if the waveform peaks at +0.5 on the vertical scale, or peaks downward to -0.5, then the peak level is “0.5”.
There are two common ways that levels are measured in Audacity:
- “Linear scale”. This is the scale on the vertical track ruler. It has a “valid” range of +1 to -1.
- “dB” (decibel). This scale represents a “logarithmic ratio” of the “level relative to 0 dB”. If the waveform has a peak of +1.0 (or -1.0), then the level is “0 dB”.
Conversion between “linear scale” and “dB scale” can be a bit confusing, so here are some common conversions (close approximations):
1.0 linear = 0 dB
0.5 linear (1/2) = -6dB
0.25 (1/4) = -12 dB
0.125 (1/8) = -18 dB
0.0625 (1/16) = -24 dB
0.0 linear = -infinity dB
Note that the dB values are negative. That’s because the reference level is defined as 0 dB, and valid signals are always less than this reference level.
Note that halving the linear amplitude is a decrease of -6 dB. Similarly, doubling the linear level is an increase of 6 dB.
RMS (“root mean square”) is a kind of average level. RMS is usually measured in “dB”, which is very convenient as this relates directly to dB as used in peak level measurements. Changing the RMS level by 1 dB will cause the peak level to change by exactly 1 dB.
Example:
If you have a waveform with an RMS level of -20 dB and a peak level of -4 dB, and you amplify it by +4 dB, the resulting waveform will have an RMS level of -16 dB (-20 + 4 = -16), and a peak level of 0 dB (-4 + 4 = 0).
Limiting:
A “Limiter” effect is a kind of “dynamic compression”. It “compresses” (squashes down) peaks in the waveform so that the waveform has a more even level.
Obviously, by squashing the peaks, the “peak level” is reduced, but there is only a small change to the “average” RMS level because it is only the highest peaks that are affected - most of the audio will normally remain unaffected by limiting.
Example, using Limiting and RMS normalizing together.
Say that you have a recording with the following measurements:
Peak level = 0 dB
RMS level = -25 dB
and you want the audio to have:
Peak level = -3dB
RMS level = -26 dB
Clearly we want to reduce the peak level by -3 dB, but we only want to reduce the RMS level by -1 dB.
From the above, we can see that amplifying (or normalizing) won’t do what we want - amplifying by -3 dB will give us the required “peak” level, but the RMS level will go down to -28 dB (-25 - 3 = -28), which is lower than what we want.
How to do this:
-
RMS normalize to -26 dB. This reduces the level by -1 dB, so the RMS level is now -26 dB (-25 - 1 = -26). It also reduces the peak level from 0 dB to -1 dB.
-
Apply a “limiter” to squash down all peaks that are higher than -3 dB. This will reduce the peak level to -3 dB, but because it only acts on the peaks, there is very little effect on the average level. Measuring the RMS level after limiting may show a small reduction in average level, but it is likely to be close enough to not matter.