Time Constants and Time Weightings
Fast, Slow, Impulsive and Peak sound levels are extensively used in acoustics.
Fast, Slow and Impulse
Back to the days of analogue meters they were introduced to give the operator chance to 'follow' the fast meter fluctuations by eye.
Most sound level meters have two conventional time weightings, F and S (called Fast and Slow in earlier days) with time constants of
125 ms and 1000 ms
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.
Peak, True Peak
: to measure the True Peak values of impulsive sound levels a meter must be equipped with a
Peak should not to be confused with Lmax
which is usually measured with a
Peak can only be measured with a
C or Flat [Z] Frequency Weighting
, and even C-weighted Peak should be treated with discretion.
The A curve
, for example introduces it's own time constant which makes the measurement of True Peak
: a peak detector responds in less than 100µs (microseconds), according to the sound level meter standards. A typical response time is 40µs.
: peak detection process retaining the 'true' maximum value of a signal.
: maximum instantaneous level of stated kind that occurs during a stated time interval.
Peak Sound Level
: twenty times the Logarithm
to the base ten of the ratio of a peak sound pressure to the
reference sound pressure
Peak Sound Pressure
: greatest absolute instantaneous Sound Pressure
during a stated measurement period.
: the amplitude difference between the most positive and most negative value in a time waveform, that is, the total Amplitude
Peak measurements are unambiguous for symmetric
waves like sine, square etc., but ambiguous when the waveform is Asymmetric
: Crest Factor
and RMS - Root Mean Square
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