RCW 46.37.537: Motorcycles—Exhaust system.

No person shall modify the exhaust system of a motorcycle in a manner which will amplify or increase the noise emitted by the engine of such vehicle above that emitted by the muffler originally installed on the vehicle, and it shall be unlawful for any person to operate a motorcycle not equipped as required by this section, or which has been amplified as prohibited by this section.