How to measure volume?

[sbyatt]

Does anybody know whether there's a particular method to measure the decibel level of a bike and/or car for a track day? I don't really want to turn up to a 95db limited day if my car makes 104db! Is it checked during scrutineering? Is the sensor placed next to, behind in front of the car? At what revs? Or is there a mic on the track and you're pulled off if you're too loud? TIA, SB