a cop does NOT need a decibil meter to cite someone for an excessivly loud exhaust. if the cop thinks its unnecessarily or excessively loud that is all the PC he needs to issue an infraction for a loud exhaust.
just like when a cop observes someone traveling excessivly fast he does NOT need radar, laser, vascar, airplane or any other speed measuring device to cite you with traveling too fast. if he believes ur traveling above the posted limit or simply too fast for conditions due to weather, traffic, etc... based on his training and experience he does not any official equipment to cite you.