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Broadband absorption of low-frequency sound waves via a deep subwavelength structure is of great and ongoing interest in research and engineering. Here, we numerically and experimentally present a design of a broadband low-frequency absorber based on an acoustic metaporous composite (AMC). The AMC absorber is constructed by embedding a single metamaterial resonator into a porous layer. The finite element simulations show that a high absorption (absorptance A > 0.8) can be achieved within a broad frequency range (from 290 Hz to 1074 Hz), while the thickness of AMC is 1/13 of the corresponding wavelength at 290 Hz. The broadband and high-efficiency performances of the absorber are attributed to the coupling between the two resonant absorptions and the trapped mode. The numerical simulations and experimental results are obtained to be in good agreement with each other. Moreover, the high broadband absorption can be maintained under random incident acoustic waves. The proposed absorber provides potential applications in low-frequency noise reduction especially when limited space is demanded.

The passive lossless acoustic metasurfaces (AMs) usually use nonlocal behaviors to realize high-efficiency acoustic field manipulation. However, nonlocal behaviors always need complicated unit-cell designs or structure optimizations. In this work, a simple passive lossless AM without nonlocal responses is proposed to perfectly manipulate multi-order diffraction beams. We demonstrate theoretically that the proposed AM can efficiently split an incident acoustic wave into desired reflected directions with arbitrary power flow ratio. Then two examples of two-beam and three-beam lossless AM splitters are constructed with well-designed rigidly ended tubes. Simulation results verify that nearly 100% reflection efficiency can be achieved in the two AM splitters and the power flow distributions are in accord with the designed values. Finally, we fabricate the two-beam AM splitter and confirm experimentally that it can achieve high-efficiency reflection splitting with arbitrary power distribution. Our proposed AM may contribute to designing devices for different applications in energy harvesting and communication.

Acoustic ventilation barriers (AVBs) constructed from metamaterials can effectively insulate sound waves while maintaining airflow, but previous schemes suffer from complex geometric structures. Here, we propose a reconfigurable AVB with subwavelength thickness (0.15λ), which is composed of simple zigzag structures with regular intervals and can be assembled into a full enclosure with an arbitrary shape. It is the interaction between the scattered wave of the zigzag structure and the un-scattered wave through the air channel that leads to a transmission dip and hence a sound insulation. Numerical simulations and experimental demonstrations consistently verify that the AVB possesses excellent omnidirectional sound insulation, while the width of the air channel can reach three times the unit width. As the number of cavity increases, the acoustic insulation bandwidth is broadened from 0.875/λ to 2.75/λ. As an example, a circle AVB is further demonstrated to shield acoustic waves emitting from either the interior or exterior. Experimental measurements indicate that the average sound transmission losses can reach roughly 19 dB and 15 dB, respectively. We believe that the proposed AVB may find potential applications in architectural acoustics, room acoustics, and duct noise control.

Acoustic needle focusing, which features elongated depth of focusing and narrow lateral size, is of great importance in applications requiring acoustic convergence in a large region, such as acoustic fast imaging. Here, a method is proposed to design an acoustic metasurface lens (AML) composed of space-coiling subunits for shaping a needle focusing. By manipulating the transmitted phase delay, the designed AML can generate two symmetrical parabolic accelerating beams along the designed convex trajectories, finally forming the acoustic needle focusing. Numerical simulations demonstrate that the focal intensity can reach roughly 17 times that of the incident wave while keeping the depth of focusing 18.2 times the wavelength. The working bandwidth of the highly efficient needle focusing is from 3.042 to 3.842 kHz. In addition, the focal intensity, depth and position of needle focusing can be freely modulated by simply changing the incident direction or tailoring the trajectory of two accelerating beams. The proposed acoustic needle focusing may have potential applications in acoustic imaging, acoustic energy delivering and particles trapping and manipulation.

We design an asymmetric transmission system (ATS) with two flat acoustic metasurfaces (AMs) to yield bidirectional asymmetric acoustic focusing (BAAF). The acoustic waves could be focused on both sides of the ATS with different focal lengths and intensities. To achieve high intensity energy concentration, the accelerating acoustic beams are selected to realize the BAAF. The working bandwidth of the BAAF based on our ATS could reach ~ 0.4 kHz. It is found that by adjusting the distance between two flat AMs, the focal length and intensity of the bidirectional focusing could easily be modulated. Because the distance between two flat AMs is large enough, the BAAF even could be converted into a unidirectional acoustic focusing. The proposed BAAF may find applications in non-destructive evaluation, biomedical imaging and medical diagnosis.

An acoustic metasurface (AM) composed of space-coiling subunits is proposed to generate acoustic Airy-like beams (ALBs) by manipulating the transmitted acoustic phase. The self-accelerating, self-healing, and non-diffracting features of ALBs are demonstrated using finite element simulations. We further employ two symmetrical AMs to realize two symmetrical ALBs, resulting in highly efficient acoustic focusing. At the working frequency, the focal intensity can reach roughly 20 times that of the incident wave. It is found that the highly efficient acoustic focusing can circumvent obstacles in the propagating path and can be maintained in a broad frequency bandwidth. In addition, simply changing the separation between the two AMs can modulate the focal length of the proposed AM lens. ALBs generated by AMs and the corresponding AM lens may benefit applications in medical ultrasound imaging, biomedical therapy, and particle trapping and manipulation.