Acoustic Glossary


Time Weightings : Definitions, Terms, Units, Measurements ...

Fast, Slow, Impulsive and Peak sound level time constants are extensively used in acoustics.

Fast, Slow and Impulse

Most sound level meters have two exponential 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 : Peak-to-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, 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 may be measured with a C or Flat frequency weighting. Using the A curve, introduces it's own time constant which makes the measurement of True Peak impossible.

Peak Hold is the peak detection process retaining the 'true' maximum value of a signal.

Peak Measurements are unambiguous for symmetric periodic waves like sine, square, etc., but ambiguous when the waveform is asymmetric

Peak-to-Peak : P-P is the amplitude difference between the most positive and most negative values in a time waveform.

Peak Level

IEC 801-22-10, maximum instantaneous level of stated kind that occurs during a stated time interval

Peak Frequency-weighted Sound Pressure Level, also known as Peak Sound Level

Peak Sound Pressure

Time Constant

IEC 801-21-45, 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 FactorExponential AveragingRMS - root mean square and the IEC Definition of Level

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