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Time Weightings - Fast, Slow or Impulse?

Sound level measurements using any grade of sound level meter can be Fast, Slow, or Impulse time weighted. These weightings date back to the time when sound level meters had analogue meters and defined the speed at which the meter moved. Under Fast the needle would move fast to show quickly varying noise and under Slow the needle would be damped to smooth the noise out to be easier to read.

The Impulse time weighting is about four times faster than Fast, with a short rising time constant but a slow falling one. It is little used in most countries these days and so can usually be ignored.

Fast corresponds to a 125 ms time constant. Slow corresponds to a 1 second time constant. Impulse has a time constant of 35 ms.

As these time constants were set in standards, they have continued in digital meters, now being calculated or simulated before being displayed, stored or used to calculate other acoustic parameters that need the time weighting.

The following terms are used to express the sound level, clearly indicating the time weighting as well as the frequency weighting:

  LAS Slow, A-weighted Sound Level
LAF Fast, A-weighted Sound Level
LCS Slow, C-weighted Sound Level
LCF Fast, C-weighted Sound Level

What About Leq?

The Leq is the "Equivalent Continuous Sound Level" - it is the average sound level. For this reason there is no time constant applied to the Leq.

More about Leq

What About Peak?

The Peak is the maximum value reached by the sound pressure. It therefore has no time constant applied.

More about Peak