These ultrasonic leak detectors are used to listen for the high pitched noise of air leaks which is beyond the range of human hearing. This high frequency sound is then converted into an audible frequency which can be heard through the ear phones by the operator. Unlike digital detectors the sounds heard through the headphones are a direct analogue of the original sounds i.e. they are simply shifted to the audible frequency. This means that false positive noises can be rejected by the operator. Genuine leaks have an unmistakable hissing noise which, whilst normally at too high a frequency, can easily be distinguished from other non-leak generated noises which might be picked up on a digital display.