Massachusetts is set to introduce a bill back into legislative review that would ban the sale or installation of any exhaust that falls under the following definition. The quote is best enjoyed if read aloud in a Boston accent.
An exhaust system which has been modified in a manner which will amplify or increase the noise emitted by the exhaust
SEMA has successfully fought the legislation up to this point, citing no clear standard by which an officer of the law could determine if an exhaust had been modified - or was just wicked rusty. [Modified Exhaust System Ban via SEMA.org]