Fast, Slow and Impulse sound levels
Most sound level meters have two exponential time weightings, F = Fast and S = Slow.
Some also have an impulse time weighting, a quasi-peak detection characteristic with a rapid rise time and a much slower decay.
F : Fast Sound Level = 125 ms rise and decay time
S : Slow Sound Level = 1 second up and down,
I : Impulse Sound Level = 35 ms while the signal level is increasing and 1.5 seconds when the signal level is decreasing.
Sound Level time weightings are also known as sound level time constants
Back to the days of analogue meters, these time weightings were introduced to give the operator chance to 'follow' the rapid meter fluctuations by eye.
Peak should not to be confused with Lmax which is usually measured with a Fast weighting.
To measure the True Peak values a sound level meter must be equipped with a Peak Detector, which according to the International Sound Level Meter Standards, should responds in less than 100µs. The typical response time for a Class 1 meter is 40µs (40 microseconds).
● Note : If frequency weighting is not specified, the A-frequency weighting is understood.
Peak Sound Pressure