well ... there is carnaval in town this year again (i live in Aalst-Belgium)

there are a few rules... our mates have a sound-installation that goes to 12.000 W rms, my question here is ... how much decibels would that produce .. when u stand on about 2 meters distance (its in open space .. on the streets)
the police has set a limit of 95 decibel ..

how loud will the 12000W installation go ?

The sound from a loud motorcycle is about 100 dB, so my guess is that this sound system will reach upwards of 120 dB or louder. For example, standing about 2 meters away from a gun shot (if you've ever fired a gun) is about 140 dB.

But sorry i forget all the physics stuff regarding root mean square to actually give you a real number.

You'd need to know the reference used for dB calculation because it's not absolute, it's like a ratio of things. You can look up some formulas for power to dB conversion and reference value used. Then you'd need to consider the decreased sound pressure at 2 m. And you still cannot be sure if you'll be correct...
Why not just regulate the power output, it's not like you have to blast at max power...