Any authorized emergency vehicle may be equipped with a siren, whistle, or bell, capable of emitting sound audible under normal conditions from a distance of not less than five hundred feet (500?) and of a type approved by the registry. The siren shall not be used except when the vehicle is operated in response to an emergency call or in the immediate pursuit of an actual or suspected violator of the law, in which latter events, the driver of the vehicle shall sound the siren when reasonably necessary to warn pedestrians and other drivers of its approach.