Acoustic Glossary


Sound Level Time-weightings Terms and Definitions ...

Fast, Slow and Impulse sound levels

Most sound level meters have two exponential time-weightings, F = Fast and S = Slow.

Some also have an impulse time-weighting, a quasi-peak detection characteristic with a rapid rise time and a much slower decay.

F : Fast Sound Level = 125 ms rise and decay time
S : Slow Sound Level = 1 second up and down,
I : Impulse Sound Level = 35 ms while the signal level is increasing and 1.5 seconds when the signal level is decreasing.

Sound Level time-weightings are also known as sound level time constants

Back to the days of simple analogue meters, these 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 weighting.

To measure the true peak values a sound level 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 a peak detector which retains 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 Definition (IEC 801-22-10) maximum instantaneous level of stated kind that occurs during a stated time interval.

Peak Sound Level is the maximum instantaneous sound pressure level during a measurement.

Peak Sound Level Definition (IEC 801-22-15) greatest instantaneous value of a standard-frequency-weighted sound pressure level, within a stated time interval, also known as the peak frequency-weighted sound pressure level.
Note : If frequency weighting is not specified, the A-frequency weighting is understood.

Peak Sound Pressure

Time Constant Definition (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

Time Constant is also known as relaxation time

See also crest factorexponential averagingrms (root mean square)

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