Acoustic Glossary


Sound Level Time-weightings Terms and Definitions ...

Fast, Slow, Impulsive and Peak Time Constants are extensively used in acoustics.

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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