When calculating the L-min and L-max levels, the instantaneous RMS levels are first low-pass-filtered with an adjustable time constant.

 

A shorter time constant should be chosen to be able to track rapid changes. Otherwise the measurements will be smoothed out. On the other hand, when the RMS levels are slowly varying, a longer time constant will provide more precise values.

 

Typical fast time constants are 125 ms and under. An extra fast time constant of 35 ms is sometimes used to capture L-max on very short transients.

 

Typical slow time constants are 1s and over.

 

Note: The time constant is not used for the calculation of Leq. In that case, the sound energy is averaged linearly over the specified log interval.