Manipulation of acoustic localizations based on defect mode coupling in a corrugated waveguide

Defects in periodic structures can always result in localization of various waves, such as Anderson localization, line defect photonic crystal waveguides, and locally resonant sonic metamaterials. Due to the unique physical properties of localization, it has found applications in many functional devices including optical/acoustical tweezers, laser cavities, spatial modulators, and detectors. Here, we introduce two defects into an acoustic waveguide with perfect periodicities and experimentally observe the related two defect modes and their coupling effects. The double defects can cause the two extraordinary transmissions in the forbidden band of periodic structures, and the induced acoustic localization is quite different from that in a single defect that the measured sound pressure along the axis of the waveguide shows the maximum in each defect. The measured sound pressure also indicates that the localization of defect modes has in-phase or out-of-phase characteristics, which change alternately with increasing number of periods between the defects. When the distance between the two defects increases, the defect coupling turns to be weaker and the transmitted peaks fuse together as one. Finally, the defect mode disappears for a larger distance. The experiments also reveal that the localization of double defects is very sensitive to the defect geometry. The maximum sound pressure of defect modes with a longer wavelength always appears in the longer defect. Based on our findings, the acoustic localization can be efficiently manipulated by the regulation of structures with defects, which may pave the way for various functional devices in wave control engineering.