Abstract: An ultrasonic leak detector which discriminates against leak signals originating from outside a desired detection zone. Incoming ultrasonic waves are divided into mutually exclusive low and high frequency signals, which signals are then processed to form an intensity ratio. The intensity ratio is compared to a pre-selected ratio corresponding to an ultrasonic source at the boundary of the detection. Ultrasonic leak signals originating from outside the desired detection zone are ignored. The presence of a leak within the detection zone is indicated by audio and/or visual devices.