Sound levels are measured by detecting the changes in air pressure created by the sound. These Sound Pressure Levels (SPLs) fluctuate quite quickly with common sound sources such as music, speech or environmental noise. Reading the sound level in real time on a display is difficult due to these quick level changes. SPL Meters thus dampen their reaction to sudden changes, creating a more smoothed display. This process is called Time Weighting.
The IEC 61672-1 standard describes two different time weightings, Fast (F) and Slow (S). They both dampen the reaction of the displayed level to a sudden change in the sound level. Fast reacts quicker than Slow. If for instance in a quiet environment a loud, constant sound is suddenly switched ON, the 'F' weighted level display would take approximately 0.6 seconds to reach the new level, while the 'S' weighted level display would reach the new level only after approx. 5 seconds. These values are given by the time constants for the 'F' (t = 125 ms) and the 'S' weighting (t = 1 s), which are defined in the standard. When the sound is suddenly switched OFF, the displayed 'F' level decays at a rate of 34.7 dB/s, while the displayed 'S' level decays at a rate of 4.3 dB/s.
The following sketch shows such a sudden event and the behavior of the displayed Fast and Slow levels.
The old superseded IEC 651 standard also included the Impulse (I) Time Weighting, which was intended for the analysis of short impulses. In contrast to 'F' and 'S', the 'I' time weighting is asymmetric, i.e. it adapts very quickly to a rising slope (t = 35 ms), while it decays very slowly after a level drop (2.9 dB/s).