Just found this....
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...
You can get electronic cutouts that you can open and close at your discretion while still going through your mufflers when needed.