There are three different time constants in use for generating the time weighted signals.
The time constant for Fast is
125 milliseconds. This is the most used time constant.
The time constant for Slow is
1 second.
The time constant for Impulse is 35 milliseconds. This time constant is followed by a max hold function that allows the level to drop with 3 dB per second. This time constant is suited for characterizing noise with rapid level changes.
The time constants Fast and Slow are now defined in the IEC61672 standard, and the proper names are simply F and S. The I time constant is defined in IEC60651.