Michigan Compiled Laws, Chapter 257. Motor Vehicles § 257.707a

(a) “Decibel” means a unit of sound level on a logarithmic scale measured relative to the threshold of audible sound by the human ear, in compliance with American national standards institute standard S 1.1-1960.

(b) “Decibels on the a-weighted network” or “DBA” means decibels measured on the a-weighted network of a sound level meter, as specified in American national standards institute standard S 1.4-1971.

(c) “Fast meter response” means the meter ballistics of meter dynamic characteristics as specified by American national standard S 1.4-1971.

(d) “Maximum noise” means the noise emitted from a vehicle during that manner of operation which causes the highest DBA level possible from that vehicle.

(e) “Muffler” means a device for abating the sound of escaping gases of an internal combustion engine.

(f) “Exhaust system” means the system comprised of a combination of components which provides for enclosed flow of exhaust gas from engine parts to the atmosphere.

(g) “Noise” means any sound.

(h) “Total noise” means noises radiating from a vehicle but does not include noises emitted from a horn, siren, bell or other similar device of an authorized emergency vehicle.

(i) “Gross vehicle weight rating” means the value specified by the manufacturer as the loaded weight of a vehicle.

(j) “Combination vehicle” means any combination of truck, truck tractor, trailer, semi-trailer or pole trailer used upon the highways or streets in the transportation of passengers or property.