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Non-Hermitian scattering systems respecting parity-time symmetry exhibit unidirectional invisibility or reflectionlessness at an exceptional point of the scattering matrix. In this study, we investigate the scattering properties of a one-dimensional acoustic parity-time-symmetric multilayered medium. Parity-time symmetry is defined by a spatial small-amplitude square-wave modulation of the medium parameters in the complex plane. Such a multilayered medium has been demonstrated to reach an exceptional point and support unidirectional invisibility for balanced real and imaginary part modulations of the refractive index. Through the analysis of interference within the medium, we show that the complex modulation of acoustic impedance instead of the refractive index governs the exceptional point of the scattering matrix and determines the occurrence of the unidirectional invisibility. This unidirectional invisibility is further investigated for more general cases of complex parameter distribution including unequal modulation amplitudes and rectangular wave modulation. Our study provides an intuitive physical picture of the occurrence of unidirectional invisibility induced by an exceptional point in acoustic wave systems. Moreover, this study could facilitate the design and practical realization of acoustic parity-time-symmetric structures. [ABSTRACT FROM AUTHOR]

The pursuit of tunable asymmetric sound transmission has been a long-term topic since it could contribute to providing more flexibilities in many areas of acoustic engineering. The interference effect can be a feasible approach in which two waves with the same frequency superposed to form the resultant wave with manipulated amplitude according to the relative phase difference between them. However, strictly speaking, restricted by the spatial variance of phase, the manipulated domain created by the specific phase difference is always limited to a spot with dimensions much smaller than the wavelength. Here, we proposed a design to break this barrier that can realize the tunable asymmetric transmission via the combination of zero-index metamaterials and the binary metasurface. The zero-index metamaterial can provide the effective extremely large speed to shrink the infinite domain into a spot acoustically and the binary metasurface can be used to tune the specific phase difference. Numerical simulations and experimental measurements have good agreement and show that the acoustic waves impinged from the side of metasurface will be manipulated to have controllable transmission, while the acoustic waves impinged from the side of zero-index metamaterials will keep a high transmission. We think the proposed design is full of physical significance, which may find potential applications in many fields, like noise cancelation, acoustic imaging, and ultrasound therapy. [ABSTRACT FROM AUTHOR]

Zero index metamaterials have shown the ability to achieve total transmission or reflection by embedding particular defects with various effective parameters. Here, we present that tunable sound transmission can be realized by configuring a subwavelength-sized dopant inside zero index acoustic metamaterials. Despite its small spatial signature, the dopant is able to strongly interact with the acoustic waves inside the whole zero index metamaterials. It is due to the essence of the zero effective index that can homogenize the pressure field within the metamaterials. Sound transmission can thus be fully switched on and off by adjusting the dopant's surface impedance. A simple rectangular cavity with varied lengths is proposed to provide the required impedance boundary. Our model of correlating the dopant design with sound transmission performance is validated theoretically and numerically. We further demonstrate the utilization of the proposed design to effectively modulate the sound focusing effect. Such a dopant-modulated sound transmission scheme, with its simplicity and capability, has potential applications in fields like noise control and ultrasonography. [ABSTRACT FROM AUTHOR]