On this topic, states can generally be divided into two major categories: states with noise standards and states without noise standards. 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. States that have quantifiable noise standards on the books are shaded red in the map above. These standards often go unenforced. One reason these regulations are not enforced is that they are based on an in-use standard-exhaust noise is measured while a vehicle is in motion on the highway. The states that employ these operating standards typically divide vehicles into classes and then set separate standards: one for vehicles while driving on roads with a speed limit of 35 mph or less and a second standard for vehicles driving on roadways with a speed limit greater than 35 mph. The measurements are to be taken while the vehicle is in motion on the road, usually from a distance of 50 feet from the center lane of travel.
Other states choose not to specify a quantifiable noise standard. These states are shown in yellow in the map above. 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. Language that effectively limits the use of aftermarket exhausts can be found among both yellow and red states. Such language includes sentences such as "no person shall modify the exhaust system of a motor vehicle in any manner that 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 doesn't 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.
Green on the map identifies the three states that 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.
Previous California law allowed modifications so long as the noise levels did not exceed the 95-decibel limit. However, the roadside enforcement of this limit was chaotic, leading to subjective, selective, and improper enforcement.