The theory of airborne ultrasonic detection is relatively simple. Ultrasonic instruments are sensitive to sounds beyond the limits of normal human hearing.  An ultrasonic detector translates ultrasonic signals to the range of human hearing.  Frequency, the number of times a sound wave cycles from trough to crest, is expressed in Cycles per seconds and measured in "hertz." One kilohertz (kHz), for example, is one thousand cycles per second. The best human ears can hear noises in the range of 20 hertz to about 20,000 hertz (20kHz).  Many ultrasonic detectors start at approximately 20 kHz and work upward to sounds as high as 100 kHz. Thus mechanics using the ultrasound instrument can tune-in to and "hear" faults in electric transmission and distribution systems end in operating machinery, as well as leaks in vacuum or pressurized systems.

Electrical systems, fluid and gas systems and working machinery all produce constant ultrasound patterns.  Changes in the sonic signatures can be readily recognized as loose connections, faulty equipment or wear in components.  When an electrical disturbance occurs, the electricity ionizes air molecules which produce a distinct, detectable ultrasound signal.

An ultrasonic detector senses subtle changes in the ultrasonic signature of a component and pinpoints potential sources of failure before they can cause costly damage.  The longer wavelengths of lower pitched sounds can easily penetrate solid materials and can be heard without special equipment.   But higher frequency sounds that cannot penetrate most solids will slip through the tiniest of openings. Ultrasound detectors are ideal for isolating leaks.

Many of today’s lightweight, pistol-shaped ultrasonic tools are battery-powered so technicians can easily move about while operating them.  Some instruments feature a frequency adjust dial to provide tuning capability, enabling users to hear the ultrasounds through headphones and gauge their intensity by the definitions registered on an analog meter.  An ultrasonic detector equipped with a parabolic reflector can pinpoint problem areas such as electrical disturbances at distances of up to 300 feet.  Ultrasonic signals generated by electrical leaks, for example, hit the dish at various angles, and are reflected back toward the focal point due to the shape of the parabola.  The signals are received by a highly sensitive transducer assembly, an extremely directional microphone.  The narrow focal point minimizes a wide area of background noise as it focuses in on the corona sound or the leak.