Fast, Slow, Impulsive and Peak sound level descriptors are extensively used in acoustics.
Fast, Slow and Impulse
Some also have Impulse Time Weighting which is a quasi-peak detection characteristic with rapid rise time (35 ms) and a much slower 1.5 second 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 or 1,500 ms while the signal level is decreasing.
Back to the days of analogue meters, Time Weightings were introduced to give the operator chance to 'follow' the rapid meter fluctuations by eye.
Peak : P-P : True Peak : Lpeak : Lpk : etc.,
Peak should not to be confused with Lmax which is usually measured with a Fast or Slow weighting.
To measure the True Peak values of impulsive sound levels a meter must be equipped with a Peak Detector.
The Peak Detector should responds in less than 100µs, according to the Sound Level Meter Standards, a typical response time, for a Class 1 meter is 40µs (40 microseconds).
Peak Frequency-weighted Sound Pressure Level also known as Peak Sound Level is listed and defined below
Peak Hold is the peak detection process retaining the 'true' maximum value of a signal.
Peak Sound Level IEC Definition, greatest instantaneous value of a standard frequency weighted sound pressure level, within a stated time interval
● Note : if frequency weighting is not specified, the A-frequency weighting is understood,
Also known as the Peak Frequency-weighted Sound Pressure Level