Amprobe ULD-400-R [5117508] Ultrasonic Leak Detector Receiver Only. When equipment begins to fail due to an air or gas leak or vibration, or electrical discharge, the leakage point emits an ultrasonic sound wave that is above the natural range of human hearing. The ULD-400 Series Ultrasonic Leak Detectors convert this ultrasonic sound into a signal that can be used to pinpoint the exact location of the equipment failure. 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 working in unpressurized systems, or the 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 which would otherwise hide the noise of the fail.