Movie, based on data from a laser interferometer, depicts a "hyperfocusing"
of a sound wave using time-reversal acoustics, in which a device produces
a reversed version of a sound wave and sends it back to its original
location. Usually, a wave cannot be focused to a spot smaller than half
of its wavelength. "Hyperfocusing" means to focus the sound
wave to a size smaller than this traditional limit. In the above graphic
and animation, the sharp peak is an ultrasound wave focused to 1/14
of its initial wavelength. The point at which the wave focuses acts
as a "sink" which absorbs the energy of the sound wave.
To achieve hyperfocusing, it is crucial to time-reverse all of the
initial sound wave, including its "near field" components,
the fields that exist within a wavelength around the source of the sound.
Previous time-reversal devices did not emit this important component
of the sound field. Without this component, the time-reversed wave does
not focus very well (this is what happens in the animation below).
Non-hyperfocused sound wave-
click picture for animation (requires Realplayer)
Pictures and animations courtesy of Julien de Rosny, Laboratoire Ondes
et Acoustique, CNRS/ESPCI/University of Paris.
Reported by: de
Rosny and Fink, Physical Review Letters, 16 September 2002.
Physics
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