I realize I might be off topic, but .... The following is from my 2000 Vehicle Code book. 27151 refers to 27150 and 27200. 27200 refers to 27206. 27200 states: "...which produces a maximum noise exceeding the applicable noise limit at a distance of 50 ft. from the centerline of travel under test procedures established by the Department of the California Highway Patrol ..." 27206 defines this limit as "After 1974... 80dBA" 27150 is vague: "...excessive or unusual noise..." But when all the references are done 80dbA at 50 ft. seems to be the defintion of excessive. The actual test procedure is not specified, but it certainly involves a sound pressure meter, etc.
The burden of proof is on the state. Your argument is that the officer didn't use the specified test procedure. He merely is assuming that your exhaust is too loud, he has to use the specified procedure to verify that your vehicle is indeed over 80 dBA.
Get the book and study all the statutes. If you do that and take it to court you will win. The statute doesn't say that if it is louder than stock it is illegal, and they can't measure it because they don't have the equipment to enforce this law.
You have to base all your arguments on the statutes. Don't discuss the matter any further with the police, you will only give them ammo to fight you with. Don't take the stand in your own defense - it won't be necessary. Go to court and observe before your trial, if you can. This is one of the few times when you have nothing to lose by pleading not guilty and going to trial; you get to ask all the questions - enjoy it!