Number 190 (Story #3), August 16, 1994 by Phillip F. Schewe and Ben Stein TIME REVERSED SOUND , a phase-conjugate mirror version of an acoustic signal, has been demonstrated by Mathias Fink at the University of Paris. In Fink's experiment sound waves fall on an array of rodlike piezoelectric elements (a sort of "acoustic retina") which transduces the sound into electric signals, where they are computer processed, and re-composed as time- reversed signals and re-broadcast in the opposite direction. Phase-conjugate mirrors for light waves have been known for many years but this is the first acoustic analog. Fink has been using his device in sharpening ultrasound medical images and in the search for defects in metal alloys. (Science, 22 July.)