From Wikipedia, the free encyclopedia Phononic stop bands are
formed for materials with periodic elastic properties. The
material does not need to be a crystal. The basis of phononic
crystals dates back to Newton who imagined that sound waves
propagated through air in the same way that an elastic wave
would propagate along a lattice of point masses connected by
springs with an elastic force constant E. This force constant is
identical to the modulus of the material. Of course with
phononic crystals of materials with differing modulus the
calculations are a little more complicated than this simple
model. Based on Newton’s observation we can conclude that a key
factor for acoustic band-gap engineering is impedance mismatch
between periodic elements comprising the crystal and the
surrounding medium. When an advancing wave-front meets a
material with very high impedance it will tend to increase its
phase velocity through that medium. Likewise, when the advancing
wave-front meets a low impedance medium it will slo