Ultrasonic sound (20 kHz to 100 kHz) is generated by turbulence created by air or a gas forced through a small orifice. Ultrasonic sound is very directional in nature and can be used pinpoint the exact location of the leak point. Leaking air or gas is generally considered to be viscous flow and as the flow velocity increases, the frequency of the ultrasonic sound emitted will become higher. The ULD-300 amplifies and converts these sounds to frequencies and levels that the human ear can hear.