A layer of tiny grains can slow sound 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...