Sound Level Meter

The IEC 61672-1 specifies “three kinds of sound measuring instruments”.[1] They are the “conventional” sound level meter, the integrating-averaging sound level meter, and the integrating sound level meter.
The standard sound level meter can be called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by a root-mean-square (RMS) circuit and thus it must have a time-constant of integration; today referred to as the time-weighting. Three of these time-weightings have been internationally standardised, ‘S’ (1 s) originally called Slow, ‘F’ (125 ms originally called Fast and ‘I’ (35 ms) originally called Impulse. Their names were changed in the 1980s to be the same in any language. I-time-weighting is no longer in the body of the standard because it has little real correlation with the impulsive character of noise events.