In the year 2002, the Specialty Equipment Manufacturing Association or SEMA brought about the introduction of measures in the state of California that would limit the noise to not more than ninety five decibels. This law was applicable throughout the state and applied to all kinds of motor vehicles.
Soon, other states also adopted such rules and many of these states even proposed to enact their own laws. At the same time, there are many states that have laid down their own laws that govern the amount of sound that can be emitted by the exhaust of a vehicle.
There are a number of states in the US that have adopted the standards as laid down by SEMA. When measuring the levels from the exhaust a sound reading has to be taken at a distance of twenty inches from the pipe of the vehicle. An A weighted which is a special calibration decibel meter will placed in a certain way at the tip of the pipe.
The test has to be conducted while the car is in neutral and with the speed of its engine at three fourths of its maximum rated revolutions per minute or RPM. The procedures used during the tests have been laid down by the Society of Automotive Engineers.
Almost every state has a law in place regarding the kind of muffler that a vehicle must use in order to ensure that it does not sound louder than permitted levels. This device must always be in fully functional state. There are however no laws in place regarding which kind of device have to be used in a vehicle. This means that it is possible to use cherry bomb low restriction straight through mufflers are perfectly legal.
There are also many states that use their own set of criteria regarding different kinds of vehicles. There are different criteria that will be used depending on the year in which the vehicle was manufactured. Florida is a good example of the kind of variable amount of tolerance there is towards laws about how loud exhaust can be.
In the case of a motorcycle that was manufactured prior to first January 1979 the limit is set at 82 decibels at speeds not exceeding 35 miles per hour. If the bike is traveling at more than 35 miles per hour the maximum sound that can be produced by the bike is 86 decibels.
A new bike will have a different noise level which is set at 78 decibels for speeds of 35 miles per hour or less. For speeds exceeding 35 miles per hour the permissible noise levels for a new bike are 82 decibels.
If a vehicle does not weigh more than ten thousand pounds and which has been built prior to first January 1979 the amount of sound that it can make is limited to 76 decibels at speeds of 35 miles per hour or less. For speeds of more than 35 miles per hour, the amount of sound that it can produce is limited to 79 decibels.
A new vehicle cannot produce sound in excess of 72 decibels for speeds of up to 35 miles per hour. For speeds in excess of 35 miles per hour, it cannot produce sound that is more than 79 decibels.
If an exhaust is too loud then it means that it is not in good working condition. To limit the sound produced, many auto enthusiasts have started installing electric and even manual cutouts which are mounted into the system. When these are used these devices help in venting gases from the exhaust into the atmosphere but without sending them through a muffler.
Texas is a state which does not allow use of such cutouts and it also prohibits use of bypasses or any other similar device in an automobile. Though the state has said that such devices cannot be used in a vehicle there is no explicit instruction that prevents a person from installing such devices in their vehicles.
All that this shows that there are really no fixed set of laws about how loud an exhaust can be. However, by following the SEMA guidelines you can learn to stay within the law in most states and not have to worry about breaking any laws about how loud an exhaust can be.