When equipment begins to fail due to an air or gas leak, it emits an ultrasonic sound wave that is above the natural range of human hearing. The ULD-405 Ultrasonic Leak Detector convert this ultrasonic sound into a signal that can be used to pinpoint the exact location of a leak, vibration or discharge. See the strength of the leak clearly on the large LCD display bargraph and identify the source of the leak by listening to the converted audible sound emitted via the headphones. When pressure is not sufficient enough to detect or verify a leak with the Receiver alone, use the Transmitter to generate the ultrasonic signal (included with the ULD-420 kit). In extremely noisy environments where there is strong ultrasonic noise generated by running machinery or equipment, the Receiver's filter function can filter out up to three main noise frequencies