We study the effect of planar defects in phononic crystals of spherical sca
tterers. It is shown that a plane of impurity spheres introduces modes of v
ibration of the elastic field localized on this plane at frequencies within
a frequency gag of a purl phononic crystal: these show up as sharp resonan
ces in the transmittance of elastic waves Incident on a slab of the crystal
. A periodic arrangement of impurity planes along a given direction creates
narrow impurity bands with a width which depends on the position of these
bands within the frequency eap of the pure crystal and on the separation be
tween the impurity planes. We show how a slight deviation from periodicity
(one impurity plane is different from the rest) reduces dramatically the tr
ansmittance of elastic waves incident on a slab of the crystal.