Short time Leq vs Fast LF

Short time Leq and time-weighted response L(t)

For most signals and measurement situations, the maximum time-weighted sound pressure levels will be higher than the averaged sound level Leq. However, if the measurement period is of the same range or shorter than the time constant applied for the
time weighting, this will not always be the case.
The situation may be explained by the following example:
The noise is reported as short time Leq based on a time period of 0,125 sec and as time-weighted F(ast) level LF. The level is
assumed to be low and then a steady sound appears of 0 dB. The Short time Leq will then have the averaged value 0 dB after
the end of the measurement period 0,125 sec. The F-response will have an exponential response towards the final value. After
0,125 sec the level will be 1,99 dB below the final value which is equal to the Leq value. See graph below.

leq vs fast time constance sound measurements norsonic

Leq vs Fast time constant sound measurements norsonic