E. N. Economou
FORTH, P.O. Box 1527, 711 10 Heraklio, Crete, Greece
In periodic media, frequency regions may exist at which no bounded at infinity solution of the wave equation exists. In random systems frequency regions may appear at which all bounded solutions are localized, i.e., decay exponentially at infinity. Both regions give rise to stop bands. Such stop bands appear easily for acoustic waves in bubbly liquids. They are also predicted to exist for elastic waves in binary composites consisting of gold or lead spherical inclusions periodically placed in a Si, Be, or SiO[sub 2] (for gold inclusions only) matrix and occupying about 10% of the volume. The midfrequency (omega)[sub g] of the stop band was found to satisfy the relation (omega)[sub g]=yc[sub il]/R[sub i], where c[sub il] is the longitudinal velocity in the inclusion material, R[sub i] is the radius of each inclusion, and y is a number between 1 and 2.