Controlling sound waves

A 2-D array of microspheres adhering to a substrate.

IN some ways, granular materials, such as a pile of sand, can behave much like a crystal, with its close-packed grains mimicking the precise, orderly arrangement of crystalline atoms. Researchers at the Massachusetts Institute of Technology have pushed that similarity to a new limit, creating two-dimensional arrays of micrograins that can funnel acoustic waves, much as specially designed crystals can control the passage of light or other waves.

The research work has been published in the journal Physical Review Letters.

Research on the properties of granular materials is a rapidly developing field, but most of it has focussed on the properties of sand-sized particles, about a millimetre across, says Nicholas Fang, the lead author of the work. Although the properties of particles that are about one-thousandth that size are expected to be “qualitatively different”, this is the first work to actually examine them.

In its experiments, the team used a single layer of microspheres to guide and slow down sound waves travelling across a surface. The researchers used ideas they had previously applied in research on controlling light waves. Such Surface Acoustic Waves are widely used in electronic devices such as cell phones. But according to Fang, the use of a 2-D granular material to guide and slow the waves could allow such devices to be one-sixth their present size. The 2-D nature of this system could allow it to be fabricated right on a chip, along with the necessary control circuits and other components. The system could potentially also be used to develop new kinds of sensors, such as microbalances capable of measuring tiny changes in weight, Fang says.

On a larger scale, the same principle could lead to a new kind of blast-shielding material for use in combat or by public-safety personnel or equipment. If acoustic waves, such as an explosion’s shock waves,hit the 2-D material at a right angle, much of their energy can be converted to surface waves that travel sideways out of the material. A sandwich of many layers of such material might provide protection from a blast. Such applications will, however, require further research.