Noise ordinances seem to be a very subjective matter when it comes to enforcement, and states with poorly drafted and/or ineffective state laws and regulations frequently cite the manufacturer's specifications or a factory installed muffler as the basis on which vehicle exhaust noise is measured.
Of the states with a test standard on the books, many ignore guidelines when handing out citations. Most states that have chosen to go the route of setting specifications choose to measure a vehicle's noise by decibels. Some states that have quantifiable noise standards on the books, but these standards often go unenforced because they are based on an in-use standard--exhaust noise is measured while a vehicle is in motion on the highway.
Other states choose not to specify a quantifiable noise standard. Typical language in these states' statutes includes prohibitions on "excessive or unusual noise" from a vehicle's exhaust system. While most motorists believe that exhaust systems should not be used in a way that causes overly loud or objectionable noise, these vague provisions fail to provide a clear and objective standard for those seeking more durable exhaust systems that enhance a vehicle's appearance and increase performance.
Legislative language that effectively limits the use of aftermarket exhausts includes sentences such as "no person shall modify the exhaust system of a motor vehicle in any manner which will amplify or increase the noise or sound emitted louder than that emitted by the muffler originally installed on the vehicle." While such language does not specifically prohibit all modification, it does not provide any means of measuring whether a vehicle has been acceptably modified. Such language also negatively affects the aftermarket industry by placing the noise limit authority in the hands of the OEMs and ignores the fact that aftermarket exhaust systems are designed to make vehicles run more efficiently without increasing emissions.
Currently, three states have enacted SEMA model legislation to provide enthusiasts and law enforcement officials with a fair and enforceable alternative. The model legislation establishes a 95-decibel exhaust noise limit based on an industry standard adopted by the Society of Automotive Engineers (SAE). Under this standard (SAE J1169), a sound meter is placed 20 inches from the exhaust outlet at a 45-degree angle and the engine is revved to three quarters of maximum rated horsepower. The highest decibel reading is then recorded.