Round 2" metal horn operates from 6 to 9VDC. It produces a really loud sound when power is applied to the red and black lead (no need to observe polarity). It is designed to emit one loud "click" sound each time power is applied between the black and white leads. These were commonly found in use on bikes years ago. Size 2" dia. x 1.13" deep with threaded hole for mounting on back bracket. New!