no this isnt a question about what exhaust i should get

just recently my bro got a fix-it ticket for having what the police said was a "modifed exhaust". now im second guessing whether or not i should change the exhaust on my car. i did some reacher and on some forums some said that its not illegal as long as it had a certain decibal level (loundess); but the cop never said anything about the decibal level of his exhaust, just the fact that it was modified?

so whats the deal?