Extraordinary acoustic transmission （EAT） has been investigated in a tunable bull＇s eye structure. We demonstrate that the transmission coefficient of acoustic waves can be modulated by a grating structure. When the grating is located at a distance of 0.5 mm from the base plate, the acoustic transmission shows an 8.77-fold enhancement compared to that by using a traditional bull＇s eye structure. When the distance increases to 1.5 mm, the transmission approaches zero, indicating a total reflection. Thus, we can make an efficient modulation of acoustic transmission from 0 to 877%. The EAT effects have been ascribed to the coupling of structure-induced resonance with the diffractive wave and the waveguide modes, as well as the Fabry-Perot resonances. As a potential application, the modulation of far-field collimation is illustrated in the proposed bull＇s eye structure.
Unidirectional acoustic transmission has been investigated in an asymmetric bull's eye structure, which consistes of a subwavelength hole with concentric grooves on one side of a thin steel plate. When acoustic waves impinge normally on the groove side of the asymmetric structure, a strong acoustic transmitted energy flux is observed in the frequency range of 400–450 k Hz, while there is no obvious transmitted energy flux in the same frequency range if the acoustic waves impinge normally on the other side. Thus, a remarkable unidirectional acoustic transmission behavior is exhibited by the current structure. With changing the period of the grooves, it is found that the transmitted acoustic energy flux keeps unchanged while the frequency of the transmitted waves can be modified. The experiments are performed, which has confirmed the unidirectional acoustic transmission behavior in the asymmetric bull's eye structure. The asymmetric bull's eye structure may have potential application in ultrasound diagnosis and therapy.
