Fast, Slow and Impulse meter response
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 = 125 ms up and down,
S ▷ Slow = 1 second up and down,
I ▷ Impulse = 35 ms while the signal level is increasing and 1.5 seconds when the signal level is decreasing.
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).
Peak Hold is a peak detector which retains the 'true' maximum value of a signal.