Washington Administrative Code 

§ 173-62-030. Standards.

(1) No person shall operate any motor vehicle or any combination of such vehicles upon any public highway under any conditions of grade, load, acceleration or deceleration in such a manner as to exceed the maximum permissible sound levels for the category of vehicle in Table I, as measured at a distance of 50 feet (15.2 meters) from the center of the lane of travel within the speed limits specified, under procedures established by the state commission on equipment in chapter 204-56 WAC, 'procedures for measuring motor vehicle sound levels.'

Table I
IN-USE MOTOR VEHICLE NOISE PERFORMANCE STANDARDS
Measured @ 50 feet (15.2 meters)
Vehicle Category (type):
Automobiles, light trucks and all other motor vehicles 10,000 pounds GVWR or less: 45 mph or less – 72 dBA. Over 45 mph – 78 dBA.

(2) Every motor vehicle operated upon the public highways shall at all times be equipped with an exhaust system and a muffler in good working order and constant operation to prevent excessive or unusual noise.
(3) No person shall operate a motor vehicle in such a manner as to cause or allow to be emitted squealing, screeching or other such noise from the tires in contact with the ground because of rapid acceleration or excessive speed around corners or other such reason, except that noise resulting from emergency braking to avoid imminent danger shall be exempt from this provision.
(4) No person shall operate any motor vehicle upon any public highway if the vehicle exhaust system exceeds the maximum permissible sound levels of Table II for the category and year of vehicle, as measured at a distance of twenty inches (0.5 meter) from the exhaust outlet under procedures established by the state commission on equipment in chapter 204-56 WAC, 'procedures for measuring motor vehicle sound levels.'

Table II
IN-USE MOTOR VEHICLE EXHAUST SYSTEM NOISE PERFORMANCE STANDARDS
Measured @ 20 inches (0.5 meters)
Vehicle Category (type)
Automobiles, light trucks and all other motor vehicles 10,000 pounds GVWR or less: Before 1986 – 95 dBA. 1986 and after – see above.

173-62-060. Enforcement.

 
(1) Measurements shall be made with a sound level meter meeting Type 1, S1A, 2 or S2A requirements as specified in the American National Standards Specifications for Sound Level Meters S1.4-1971 as required under measurement procedures established in chapter 204-56 WAC, 'procedures for measuring motor vehicle sound levels.'
(2) Violation of any in-use motor vehicle noise standard set forth in this chapter shall be a traffic infraction, enforced by such authorities and in such manner as violations of chapter 46.37 RCW.
(3) Law enforcement personnel selected to measure vehicle sound levels shall have received training in the techniques of sound measurement and the operation of sound measuring instruments.
(4) Any enforcement officer who by use of the initial inspection procedures of chapter 204-56 WAC suspects that a motor vehicle may be in violation of the standards of this chapter may require the operator to have the vehicle presented for sound level measurement. Measurements of a motor vehicle may be performed at off-road sites to determine compliance with the in-use standards.
(5) Any operator who fails to comply with the directive to present the vehicle to a sound level measurement test shall be in violation of this chapter.

§ 46.37.390. Mufflers required -- Smoke and air contaminant standards -- Definitions -- Penalty, exception 

(1) Every motor vehicle shall at all times be equipped with a muffler in good working order and in constant operation to prevent excessive or unusual noise, and no person shall use a muffler cut-out, bypass, or similar device upon a motor vehicle on a highway.
(3) No person shall modify the exhaust system of a motor vehicle 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 motor vehicle not equipped as required by this subsection, or which has been amplified as prohibited by this subsection. A court may dismiss an infraction notice for a violation of this subsection if there is reasonable grounds to believe that the vehicle was not operated in violation of this subsection.
This subsection (3) does not apply to vehicles twenty-five or more years old or to passenger vehicles being operated off the highways in an organized racing or competitive event conducted by a recognized sanctioning body.