Safely and effectively demonstrate the Doppler effect! Swing this battery-operated buzzer over your head from its tethered string. Students will quickly observe a significantly higher pitch when it moves towards them, and a lower tone when it moves away. The frequency changes will be indisputable. Sound source is loud enough to easily be heard at the back of a classroom. Or, add extra long string to super-size the demonstration and swing it in a large gymnasium. The longer the string, the more dramatic the effect! Instructions, buzzer and 3 feet of string are provided. Requires 9-volt battery, not included..