Longitudinal wave localization using a one-dimensional phononic crystal with differently patterned double defects

Several studies relevant to double-defect-mode-induced energy localization of a phononic crystal (PnC) have progressed. However, most works have examined the typical setting in which the double defects and adjacent lattices in a PnC have the same configuration. This work thus focuses on investigating the energy-localized behaviors of a one-dimensional PnC with closely arranged but differently patterned double defects under lon-gitudinal waves for the first time. A quarter-wave stack is used as the unit-cell design. Three cases are considered, each with double defects applied to the same (Cases I-DD and II-DD) or different (Case III-DD) kinds of stacks in two unit cells. Defect-band analysis is performed on various combinations of the double-defect lengths. Two significant findings of this work are outlined as follows. First, although the imposition pattern of the double defects is different, defect-band splitting and double-defect-mode-enabled energy localization appear if the defect-band frequencies of each defect, independently imposed as a single defect, are the same. Second, unlike the conventional double-defect-mode shapes, the interactive energy-localized behaviors of the defects where each defect shows different symmetries are newly observed. In future works, it is expected that adaptively tuning defect properties via the incorporation of smart materials will open a new avenue in the field of PnC-based wave tailoring.