Lasers, Lego, time reversal & sound waves

Have you ever been in a whispering gallery? These rooms – usually circular or elliptical, often with domed ceilings – focus sounds so that a person whispering on one side of the room may be inaudible to their neighbour but clearly heard on the far side of the room.

Using lasers, microphones and computers, the technique of “time-reversal focusing” allows similar targeting of soundwaves at any point in any kind of room.

How does it work? By precisely measuring sound vibrations at a target location – which include reflections and echos from the environment – then playing the signal backwards. 

In this video produced for a paper in the Journal of the Acoustical Society of America, physicist Brian Anderson demonstrates the technique by targeting specific Lego mini-figures on a metal plate. The targets are knocked over while their neighbours remain standing.

Targeted sound vibrations have a wide range of applications, from medicine to the military to communications. Read more here.

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