The propagation of classical waves in composite media has been studied extensively in the past decades [1–14]. Recently, a significant effort has been devoted to the study of phononic crystals with a periodic structure because of their novel physical properties and many potential applications [ –11]. Interest in phononic crystals comes from the rich physics of elastic systems, where wave scattering and the strong coupling of their longitudinal wave modes and transverse wave modes exist [2–10]. Because of the strong coupling, the eigen modes is complex. While, the band structure of pure longitudinal waves and pure transverse waves and the corresponding band gaps, inspired our intensive interest.
Early work about three dimensional solid phononic crystals has mostly focused on the absolute band gaps decided by the mixed eigen modes (consisting of longitudinal modes and transverse modes) [6,7]. It has been shown that in a finite phononic crystal system, when the incident waves