Wisconsin Statutes Vehicles (Ch. 340 to 351) § 347.38. Horns and warning devices

(1) No person shall operate a motor vehicle upon a highway unless such motor vehicle is equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than 200 feet, but no person shall at any time use a horn otherwise than as a reasonable warning or make any unnecessary or unreasonably loud or harsh sound by means of a horn or other warning device.

(2) Except as otherwise provided in this section, no vehicle shall be equipped with nor shall any person use upon a vehicle any siren or compression or exhaust whistle.

(3) Any vehicle may be equipped with a theft alarm signal device if such device is so arranged that it cannot be used by the driver as an ordinary warning signal.

(4) An authorized emergency vehicle shall be equipped with a siren, but such siren shall not be used except when such vehicle is operated in response to an emergency call or in the immediate pursuit of an actual or suspected violator of the law, when responding to but not upon returning from a fire alarm, when transporting an organ for human transplantation, or when transporting medical personnel for the purpose of performing human organ harvesting or transplantation immediately after the transportation, in which events the driver of such vehicle shall sound the siren when reasonably necessary to warn pedestrians and other drivers.