Jim McFarland
October 27, 2010

Exhaust Noise
Imagine driving down the road and getting stopped for the modified muffler on your painstakingly restored '66 Mustang. Now imagine sitting on the shoulder, receiving a citation from local law enforcement, while a stock Ferrari overtakes your car and drives on. This is the scene being played on state highways across the country, the result of poorly drafted or ineffective state laws and regulations. The laws on the books in these states frequently cite the manufacturer's specifications or a factory installed muffler as the basis on which vehicle exhaust noise is measured.

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 amongst 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 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.

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. 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.

Enforcement of the previous law and regulations in California, for example, resulted in many drivers being pulled over by state and local police and cited for improper modified exhaust systems despite having what they believed to be legal aftermarket exhausts. To prove our point (and educate ourselves) about the widespread improper enforcement of the previous California exhaust law, SEMA conducted a series of exhaust noise tests in early April of 2001. First, we contacted California SEMA Action Network members to see how many folks had received citations for excessive or modified exhaust. We were surprised and dismayed to learn how many fit the category! We then invited them to have their cars tested to see if they actually complied with California law. Finally, we hired a board-certified acoustical engineer and did the testing according to the standards set out in California law. Long story short, of the cars we tested only one exceeded the 95db legal level.

To remedy this problem in 2002, SEMA helped enact a new enforcement procedure in California through its model bill. The new law forces compliance with an objectively measured standard in a fair and predictable test. Through this procedure, motorists who drive vehicles legally equipped with modified exhaust systems can confirm that they comply with California's exhaust noise standard. The California Bureau of Automotive Repair began operation of the motor vehicle exhaust noise-testing program in 2003. The law also allows courts to dismiss citations for exhaust systems that have been tested and for which a certificate of compliance has been issued. Under the program, the 40 Smog Check stations statewide that provide referee functions are performing the test. These referee stations are issuing certificates of compliance for vehicles when tests of their exhaust systems demonstrate that they emit no more than 95-decibels, under the SAE test procedure. However, only those vehicles that have received a citation for an exhaust noise violation are permitted to submit their vehicle for the test. A similar standard was enacted in Maine in 2003 and Montana in 2007.