Leqis the sound level in decibels equivalent to the total sound energy measured over a stated period of time.
LAeqis the sound level in decibels equivalent to the total A-weighted sound energy measured over a stated period of time.
Most community and industrial noise measurements are A-weighted so the LAeq descriptor is used
Early sound level meters had a very limited dynamic range, so if the noise levels fluctuated by more then 20 dB during a measurement the accuracy was in doubt. They also used exponential averaging circuits (Fast, Slow etc.) which helped the operator average the meter fluctuations by eye, but introduced another accuracy limitation.
Modern integrating sound level meters monitor a wide range of fluctuating levels faithfully and the digital linear averaging circuits record and display the Leq time histories and the statistical noise levels, over minutes, hours or days as required.
Leq is widely used, but not widely understood - click here for a more detailed explanation.