Consisting of a self contained transmitter and receiver operating in the 40KHz region. The transmitter wave is modulated at a variable frequency of about 2KHz and is detected by the receiver which amplifies the signal and drives a miniature integral loudspeaker.Output signal can also be visualized on a CRO for more quantitative measurements. The system is ideal for demonstrating sound waves above the hearing threshold and reflection properties associated with sonar etc. Complete with batteries.