Heres California's law about exhaust systems, because if its legal here, its legal anywhere elce.
VC 27151 Modification of Exhaust Systems: Sound level in Compliance
(a) No person shall modify the exhaust system of a motor vehicle in a manner which will amplify or increase the noise emitted by the motor of the vehicle so that the vehicle is not in compliance with the provisions of Section 27150 or exceeds the noise limits established for the type of vehicle in Article2.5 (Section 27200) No person shall operate a motor vehicle with an exhaust system so modified.
(b) For the purpose of exhaust systems installed on motor vehicles with a manufacture's gross vehicle weight rating less than 6,000 pounds, other than motorcycles, a sound level of 95 dbA or less, when tested in accordance with Society of Automotive Engineers Standard J1169 May 1998, complies with this section. motor vehicle exhaust systems or parts thereof include, but are not limited to, nonoriginal exhaust equipment.
The laws may change state-to-state, but in my opinion, the state of California has the strictist law of every state.