Time Constants and Time Weightings

Definitions, Terms, Units and Parameters

Fast, Slow, Impulsive and Peak sound levels are extensively used in acoustics

Fast, Slow and Impulse

Sound level meters have two conventional time weightings, F and S (called fast and slow in earlier days) with time constants of 125 and 1000 ms respectively.

Some also have impulse time weighting which is a quasi-peak detection characteristic with rapid rise time (35 ms) and a slow decay.

Back to the days of analogue meters they were used to give the operator chance to 'follow' the meter fluctuations by eye.

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

Peak

: to measure the True Peak values of impulsive sound levels a meter must be equipped with a

Peak Detector

.

Peak should not to be confused with Lmax which is usually measured with a

Fast

Slow

weighting.

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 impossible.

Peak Detector

: a peak detector responds in less than 100µs (microseconds), according to the sound level meter standards. A typical response time is 40µs.

Peak Hold

: peak detection process retaining the 'true' maximum value of a signal.

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.

Peak-to-Peak

: 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 periodic waves like sine, square etc., but ambiguous when the waveform is asymmetric. See also Crest Factor and RMS - Root Mean Square.