Rod arrays focus sound

Researchers from the Polytechnic University of Valencia in Spain have produced a pair of flat lenses that control soundwaves.

The lenses could eventually be used in acoustic microscopes that use sound waves to examine the internal composition of objects and materials. They could also be used in non-invasive surgical tools like lithotripsy apparatus, according to the researchers. Lithotripsy is a medical procedure that uses ultrasound to pulverize kidney stones.

The flat acoustic lenses are made of irregularly spaced arrays of aluminum cylinders. The design was produced by a tool that combines multiple scattering theory and a genetic algorithm, and is optimized to concentrate sound waves at a focal point.

Multiple scattering theory is the mathematical framework for modeling the way waves moved through inhomogenous substances. Genetic algorithms take a design through many random iterations by randomly combining the best designs of a set to produce a new design.

The researchers have a pair of prototype lens designs. Both use aluminum cylinders that are one, two and three centimeters in diameter and two meters long. One design has five layers and the other nine layers. The cylinders occupy some of the points in a hexagonal lattice, creating a pattern determined by the genetic algorithm. The lenses are considered flat because the cylinders are confined to a rectangular area. Both amplify sound up to 6.4 decibels at a focal point.

The researchers are also working on an electromagnetic counterpart to the acoustic lens, dubbed Scattering Optical Elements.

The acoustic method could find practical application in two to five years, according to the researchers. The work appeared in the January 1, 2005 issue of Applied Physics Letters.