Acoustic Glossary


Time Constants and Time Weightings : Definitions, Terms, Units and Measurements

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

Fast, Slow and Impulse

Most sound level meters have two conventional time weightings, F = Fast and S = Slow with time constants of 125 ms 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 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 : 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 or 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 Level IEC Definition : 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.

Peak-to-Peak : also known as P-P, 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.

Time Constant : IEC Definition time required for the amplitude of that component of a field quantity which decays exponentially with time to change by the factor 1/e = 0,367 9…

See also :

Crest Factor
Exponential Averaging
RMS - Root Mean Square

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