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In this paper, an acoustic system with broadband asymmetric transmission is designed and fabricated, which consists of a water-immersed aluminum plate engraved with quasi-periodically-patterned ridges on single surface. It demonstrates that when the acoustic waves are launched into the system from the structured side, they can couple into the Lamb modes in the plate efficiently and attain a high transmission; on the contrary, when the waves are incident from the opposite flat side, the coupling is weak, and the transmission is low. Superior to systems with periodic patterning, this quasi-periodically-patterned system has a broad working frequency range due to the collective contributions from the multiple diffractions specific to the structure.

In this paper, an easily fabricated acoustic system has been demonstrated to gain unidirectional transmission with good performance. The unidirectional acoustic transmission originates from the asymmetric excitation of zero-order anti-symmetric Lamb modes. The results show that the single-sided patterned ridges on the steel plate enhance the scattering of the incident waves from the structured side and improve the coupling of the Lamb modes in the plate with them, which leads to high transmission ratio and great transmission difference value of acoustic waves along the opposite directions. All the experimental results show good agreement with the numerical simulations. The proposed acoustic system has potential applications in underwater acoustic and medical ultrasonic devices.

We study the transmission response of acoustic waves through a water-immersed thin hard plate embedded with elastic inclusions. The transmission spectra show a striking peak at the subwavelength region, whose position is almost irrelevant with the incident angle. Our study states that the transmission enhancement stems from the resonant excitation of the localized bending mode in the inclusion. Different from the well-known Fabry-Perot effect, here the resonant frequency reduces rapidly as the sample is thinned down. The theory is further validated by experiment. Potential applications of the abnormal transmission effect produced by such thin plate structures can be anticipated.