A device for detecting ultrasonic noise, particularly such noise as generated by fluid leaks, includes a directional ultrasonic microphone, amplifiers and ultrasonic filters, an envelope detector, an audio preamplifier and filter, a level control, a meter, an audio power amplifier and speaker, and a...http://www.google.com.hk/patents/US4287581?utm_source=gb-gplus-share專利 US4287581 - Ultrasonic fluid leak detector
