If you want the "official" method, you might want to talk to the "officials" who write the rules. Of course different officials will probably have different rules.... For car stereo competition there are these people (I'm sure there are other organizations)
http://www.iasca.com/
I think there are three settings on Db meter. The "B" setting is supposed to be closest to what the human ear hears. I've been told that position and "aim" of the meter affect the readings.
Be kind to your ears. I personally want to listen to my stereo (and everything else) for many years to come.
Corvette Store
How to "officially" measure Db level of car stereo?
