  A full band gap of the longitudinal mode of elastic waves centered near 2.8
MHz with a width of ~ 1 MHz has been observed in the phononic crystals made of
body centered tetragonal tungsten carbide spheres imbedded in aluminum matrix.
Two defects states in the band gap due to a 7-sphere defect cluster with
silicon nitride spheres have also been observed. Transmitted pressure field
pattern clearly shows that at the defect state frequencies the ultrasonic waves
transmitted through the doped crystal are emitted from the defect cluster.
