Abstract: A flaw detection system for a prestressed concrete cylinder pipe uses hydrophones inserted into valves in the pipe while the pipe is in use. The hydrophones listen for sounds that are consistent with the breaking of the pre-tensioned reinforcements and the sliding and re-anchoring of broken ends. Detecting the sound at two or more locations and establishing precise time differentiation allows the precise calculation of the location of the defect. Global positioning system (GPS) satellite receivers provide precise timing of reception of the targeted sound events in milliseconds. The location of each of the hydrophones is established using GPS. Each hydrophone is connected to a box, which includes a pre-amplifier, a signal processor, a memory, an output and a GPS receiver. The hydrophone, or box, or both, are tunes to recognize a significant sound that emanates from a breaking, moving or re-anchoring reinforcement. The precise time of arrival of that sound is recorded in the boxes using the GPS time signals.