Number 249 (Story #3), November 21, 1995 by Phillip F. Schewe and Ben Stein ACOUSTIC TIME-REVERSAL MIRRORS (TRMs) are devices that record a sound wave emanating from a source and generate a new one that behaves as if the original were travelling backwards in time. Previously TRMs had been rigorously tested for sound propagating through fluids such as water or air. For example, shouting "too" at the device would yield a reversed acoustic wave (sounding something like "oot") that converges backward towards the speaker's mouth. The principle has now been shown to be valid in solids by a team at the University of Paris (Didier Cassereau, Didier.Cassereau@loa.espci. fr). Demonstrating TRMs in solid objects has been more difficult because there are two types of sound waves that propagate through solids: longitudinal and transverse. Therefore, sound produced in a solid object will result not in a single wavefront but in at least two that travel at different speeds. The Paris researchers first use the TRM to send an ultrasonic wave into the solid sample. Then, the echo reflected back (say, from a defect in the sample) is detected by the TRM, which utilizes a network of rodlike transducers that both record the incoming echo and then broadcast a time-reversed version. This signal, in turn, reflects from the defect, an echo returns to the detector, and so forth, in an iterative process leading to a clearer location of the defect. TRMs have potential applications for detecting tiny metallic defects in airplanes and for locating and destroying kidney stones. For example, shining ultrasound waves through a patient with a kidney stone produces distinctive echoes from the stone. The TRM would record the echoes, and then generate a reversed wave, sending back sound energy that would travel back to the stone. (Paper 1pPA5, Acoustical Society of America Meeting, St. Louis, Monday, November 27)