Here is a basic guidline for decibel testing for most state an other testing agencies. We use the same process when we decibel test our exhausts.
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 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.