Acoustic Definitions

LAeq
The average equivalent sound pressure level

Definition
The easiest way to think of it is as an average sound pressure level. The L stands for sound pressure level, the A stands for A-weighted and the eq means 'equivalent'. So, it is the A-weighted, equivalent (average) sound pressure level. This figure is always given in decibels. In BS 4142 assessments, it is used to measure the ambient noise level.

LA90
The sound level exceeded for 90% of the time

Definition
The LA90 measurement is used a lot in environmental acoustics. It is used to express the background noise level. The background noise is the level which is exceeded for 90% of the time, or put another way, the sound level is only lower than the LA90 for 10% of the time. You could think of the LA90 as a backstop noise level below which the sound rarely goes.

Lw
The sound power level

Definition
The sound power level (Lw) referrs to the total sound power emanating from an object. It is sometimes seen as SWL, but more frequently as Lw. It is always given in dB. It is very important to understand that is not the same as a sound pressure level (e.g. LAeq). The sound pressure level is something that a microphone measures at a distance from the object, but the sound power level cannot be directly measured. If you could put the sound inside a giant beach ball, the sound power is all a measure of the total sound energy passing through the skin of the ball every second.