I found this on a forum called LS1. It was posed in a discussion about this very issue back in 2008. Hope this helps.
N.J. Stat. @ 39:3-70 (2002)
@ 39:3-70. [Mufflers]
Every motor vehicle having a combustion motor shall at all times be equipped
with a [muffler] in good working order and in constant operation to prevent
[excessive or unusual noise and] annoying smoke, AND NO PERSON SHALL MODIFY THE EXHAUST SYSTEM OF A MOTOR VEHICLE IN A MANNER SO THAT THE EXHAUST SYSTEM EMITS IN EXCESS OF NINETY-FIVE DECIBELS AS MEASURED BY THE SOCIETY OF AUTOMOTIVE ENGINEERS (SAE) STANDARD J1169 (May 1998). IT SHALL NOT BE A VIOLATION OF THIS SUBSECTION UNLESS PROVEN BY PROPER AUTHORITIES THAT THE EXHAUST SYTEM MODIFICATION RESULTS IN NOISE AMPLIFICATION IN EXCESS OF NINETY-FIVE DECIBELS UNDER THE PRESCRIBED SAE TEST STANDARD.
A COURT MAY DISMISS ANY ACTION FOR WHICH A PERSON IS PROSECUTED FOR OPERATING A VEHICLE IN VIOLATION OF THIS SUBSECTION IF IT IS FOUND THAT THE DEFENDANT HAD REASONABLE GROUNDS TO BELIEVE THAT THE VEHICLE WAS NOT OPERATED IN VIOLATION OF THIS SUBSECTION.
[and] n No person shall use a [muffler] cut-out, bypass or similar device upon a motor vehicle on a highway.
And the SAE test procedure for this is
The Society of Automotive Engineers (SAE) with an established noise limit of 95-decibels (SAE J1169). Under the SAE standard, 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...