An acoustic detector assembly for detecting a leak of a pressurized mixture of liquids and gases in a pipeline or pressurized vessel that includes an ultrasonic acoustic transducer generating an output in response to receiving an acoustical signal associated with the leaking mixture of liquids and gases, a detector connected with the acoustic transducer. The detector may include a band pass filter reducing the non-ultrasonic component of the acoustic transducer output to produce a filtered output, a loudness level detector receiving the filtered output and transmitting a “start” signal if the filtered acoustic signal reaches a specified loudness level, a time bridging module receiving the “start” signal, the time bridging module transmitting a continuous “count” signal while the “start” signal is received and for a predetermined time after the “start” signal is no longer received, and a time delay module transmitting an alarm signal if the time bridging module transmits the “count” signal for a predetermined duration.