According to Alderman Burton F. Natarus, no sound meter is actually necessary to issue tickets to “loud pipes.”

bq. Motorcyclists who continue to defy the law may also find Chicago police officers sticking their night sticks up their bikes’ long pipes. If no baffles (noise abatement equipment) are found, they’ll be issued a ticket.

The “actual ordinance”:http://www.nonoise.org/lawlib/cities/chicago has another interesting wrinkle, in sec. 11-4-1110(2): no generated sound should be “louder than an average conversational level at a distance of 200 feet or more.” The above interpretation barring muffler alterations appears to be valid under 11-4-1160(d):

bq. “No person shall modify or change the exhaust muffler, intake muffler or any other noise abatement device of a motor vehicle in a manner such that the noise emitted by the motor vehicle is increased above that emitted by the vehicle as originally manufactured.

Incidentally, “no person shall sound any horn… while not in motion.” Yep, honking just because the guy in front ain’t moving (and thus, by definition, you are not moving) is illegal.