Actually I don’t know how they measure level in SPL competitions.

I can only speak to IASCA competitions. Other sanctioning bodies may do things differently.

The microphone is placed in a designated location, something like 4" from the windshield post on the passenger side and 1" away from the glass. From that position it can be oriented in any direction. The microphone is filtered such that only frequencies of 90Hz and lower are received.

Specified tracks from the official IASCA disk are played, and highest decibels win. Interestingly enough, it isn't a matter of just cranking up the gains until you either get loud or blow something up. Your maximum decibels will be reached somewhere well short of maximum gain on most systems. Far before you reach maximum gain, you start "clipping" or distorting and even though you can continue turning up the gain, your decibels start going down. Shortly after that, well... electronic equipment runs on smoke. The proof is, once the smoke leaks out, it stops working.

tanstaafl.

"There Ain't No Such Thing As A Free Lunch"