Watch Dog detects moving objects by means of sound waves, making a strange sound like a woof or growl as the visitor passes or moves closer to the exhibit. A transmitter at the front of the exhibit sends out an ultrasonic tone (40 KHz) that is much too high to be heard by human ears. These sound waves strike an object in front of the exhibit and are reflected back to a receiver. The audible sound from the exhibit is the amplified difference in frequency between the transmitted and reflected waves. Thus, a slow-moving object makes a lower tone than a fast-moving one. If the object moves slowly enough, the difference in frequency is too low to hear, but the motion of the speaker becomes slow enough to be seen.