Turn signal. A lattice of mercury pillars can redirect high-frequency sound waves 45 degrees.

Sound Waves Take a Detour

A precise pattern of obstacles can bend a sound wave 45 degrees, researchers report. The technique might lead to acoustic filters that trap some frequencies in never-ending loops and allow others to pass. It may not keep the sound of your neighbor's stereo out of your bedroom, but the same principle might one day be put to work in light guides for tiny optical circuits.

The intrinsic regularity of waves, be they light or sound, makes them especially easy to manipulate. Confront a wave with an equally regular line of pillars placed sentinel-like across its path, and the wave diffracts, creating crests and troughs of intensity. With many regular rows of pillars, it's possible to tweak the system so that no part of the wave survives the successive stages of overlapping, and the wave cannot get through.

Now a team of researchers led by Manuel Torres of the Spanish Council for Scientific Research in Madrid has demonstrated a variant on this theme using sound waves. The basic element of their device is a square lattice of 2-millimeter-diameter mercury pillars, spaced with their centers 2.8 mm apart. When a high-frequency sound wave meets the pillar array head-on, it can pass though freely, but is blocked at an angle of 45 degrees.

The trick to bending these sound waves is to attach a second square lattice at 45 degrees to the first. Sound waves traverse the first lattice until they encounter pillars belonging to the second lattice. Unable to continue in their current direction, the waves take the easiest route through the second lattice, which turns them 45 degrees from their original path, the researchers report in the 7 May issue of Physical Review Letters.

It's a "clever and novel way to achieve bending of ultrasonic waves," according to University of Crete physicist Eleftherios Economou, who says that arrays of lattices could form acoustic filters by trapping some frequencies in never-ending loops while allowing others to pass freely. Team member Jose Aragon says the technique should work just as well for light, and offers a more easily engineered alternative than current photonic crystal light guides.

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The Spanish Council for Scientific Research's Institute of Applied Physics