smart:limit is using the median of all measured PSR values (PSR = peak to short-term loudness ratio, see formula below) for measuring the dynamics. Although the PLR value (peak to long-term loudness ratio) is a more commonly used descriptor for dynamics, we found that our descriptor better matches the actual - also short term - dynamics of a track.
Our tests showed that the PLR value can sometimes be a bit misleading since it’s (basically) simply another descriptor for the integrated loudness. For example, if multiple tracks are normalised to -1dB True Peak, the PLR value is simply their Integrated Loudness value subtracted by 1.
The PSR is the difference between the Short Term Loudness value and the maximum True Peak value measured within a window of 3 seconds.
PSR = Max. Peak - Short Term Loudness
=> computed within a 3 second window
The PLR is the difference between the Integrated Loudness value and the maximum True Peak value measured over the whole observation period (e.g. typically the whole track).
PLR = Max. Peak - Integrated Loudness
=> computed over the whole observation period