IIT Set of Two Ultrasonic Deer Whistle Alerts - Wind Powered Vehicle & Passenger Collision Protection Alert deer & other animals of approaching vehicle up to 1/4 mile away Protects you & your passengers by emitting ultrasonic sound heard by animals but not by humans Powered by wind as vehicle travels above 35 mph - no batteries or wiring needed Easy peel-&-stick adhesive backing for car, truck or motorcycle. Mounting instructions included Small size (1-1/2 x 2") virtually unnoticeable, Black. Set of two whistles