A layer of tiny grains can slow sound waves

Layer of microscopic spheres offers new approach to controlling acoustic waves. In some ways, granular material - 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. Now researchers at MIT 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 researchers say the findings could lead to a new way of controlling frequencies in electronic devices such as cellphones, but with components that are only a fraction the size of those currently used for that function. On a larger scale, it could lead to new types of blast-shielding material for use in combat or by public-safety personnel or equipment. A paper on the research appears in the journal Physical Review Letters , written by Nicholas Fang, the Brit and Alex d'Arbeloff Career Development Associate Professor in Engineering Design; Nicholas Boechler, a former MIT postdoc now at the University of Washington; and four co-authors. Research on the properties of granular materials - collections of small grains, such as sand or tiny glass beads - has become "a rich and rapidly developing field," the researchers write.