The loud noise of car, truck and motorcycle mufflers has prompted legislation in many areas that limits the number of decibels allowed by vehicle exhaust. In such localities, police are enforcing noise reduction by measuring exhaust decibels or sending suspected offenders to testing sites for exhaust noise evaluation. Some states and cities have adopted laws requiring motorcycle mufflers to bear an EPA exhaust noise stamp. If you have an exhaust system that might cause concern, you should learn to measure the noise from your muffler as the authorities will to avoid running into problems.
Check State Laws
Check the legal limit for loud noise from vehicle exhaust in your locality. Ask about the decibel limit and the RPM level for an engine during an exhaust noise test. Some laws in the U.S. are enacted and enforced by municipalities, or by entire states. Other nations have exhaust noise reduction laws in effect that apply to the whole country.
Buy a Decibel Meter
You'll need a decibel meter to conduct an exhaust noise test. These are available to buy from online retailers including Amazon and stores like Home Depot; your local auto mechanic might rent them out so ask what's available. You could also take a look at the various smartphone apps that measure noise levels. Many apps claims to measure noise levels up to around 130 decibels. In theory, they should be suitable for measuring exhaust noise but check reviews or speak to an auto mechanic for advice if you're not sure.
Prepare a Noise Testing Spot
Put on ear protection. Stop the vehicle on a solid flat surface and leave it running. Prepare a noise testing spot that is at least 25 feet away from any building or enclosed fence wall to avoid echo. Place the microphone of a decibel meter even with the level of the center of the muffler exhaust opening. Locate the microphone 19 1/2 inches behind the tail pipe at a 45-degree angle to the opening. The microphone cannot be in the direct gas path from the exhaust.
Set the Decibel Meter
Set the decibel meter weighting for human ear frequencies, not the low frequency setting used to measure music. Several meters use "A" for ear frequencies and "C" for lower frequencies. Set the decibel range for the legal limit. That sets the maximum sound allocation in the center of the needle sweep on the meter. Most localities set an absolute maximum between 90 and 100 decibels. Set the timing for either fast or slow for metering exhaust sound.
Rev the Engine
Pull the throttle or depress the accelerator to rev the engine to 50 or 75 percent of the safe RPM limit. Some localities test muffler noise with the engine at 50 percent capacity while others test for excessively loud noise with the engine at 75 percent running capacity.
Take the Reading
Read the decibel meter for number of decibels. The exact number shown is subject to a determined variable of plus or minus 10 or more decibels. Your sound level is within legal limits if the figure displayed falls between the variable determined for the meter. The exhaust noise is well within legal limits if the meter shows the range set is too high to measure the decibels. The sound is over the legally allowed limit if the range shows it is set too low.
Calculate the RPM percentage for running a motor to perform an exhaust sound test. Divide the red line RPM number by 2 to figure 50 percent. Multiply the red line number by 75, then divide the answer by 100 to figure 75 percent. If the red line is 16,000 RPMs, then 75 times that is 1,200,000. Then 1,200,000 divided by 100 is 12,000, so 12,000 RPMs is 75 percent of 16,000 RPMs.