No abstract
In this study, we show that robust and tunable acoustic asymmetric transmission can be achieved through gradient-index metasurfaces by harnessing judiciously tailored losses. We theoretically prove that the asymmetric wave behavior stems from loss-induced suppression of high order diffraction. We further experimentally demonstrate this novel phenomenon. Our findings could provide new routes to broaden applications for lossy acoustic metamaterials and metasurfaces.
"Schroeder diffuser" is a classical design, proposed over 40 years ago, for artificially creating optimal and predictable sound diffuse reflection. It has been widely adopted in architectural acoustics, and it has also shown substantial potential in noise control, ultrasound imaging, microparticle manipulation et al. The conventional Schroeder diffuser, however, has a considerable thickness on the order of one wavelength, severely impeding its applications for low-frequency sound. In this paper, a new class of ultrathin and planar Schroeder diffusers are proposed based on the concept of an acoustic metasurface. Both numerical and experimental results demonstrate satisfactory sound diffuse reflection produced from the metasurfacebased Schroeder diffuser despite it being approximately 1 order of magnitude thinner than the conventional one. The proposed design not only offers promising building blocks with great potential to profoundly impact architectural acoustics and related fields, but it also constitutes a major step towards real-world applications of acoustic metasurfaces. DOI: 10.1103/PhysRevX.7.021034 Subject Areas: Acoustics, MetamaterialsIn the 1970s, Schroeder published two seminal papers on sound scattering from maximum-length-sequence and quadratic-residue-sequence diffusers [1,2]. For the first time, a simple recipe was proposed to design sound-phase grating diffusers with defined acoustic performance. These two papers opened a brand-new field of sound diffusers with applications in architectural acoustics [3][4][5], noise control [6][7][8], ultrasound imaging [9], and microparticle separation [10] and have inspired other disciplines such as energyharvesting photodiodes [11]. D'Antonio and Konnert [12] presented one of the most accessible review papers examining the theory behind Schroeder's diffusers (SDs). Most importantly, they commercialized SDs and promoted them to be widely adopted in architectural acoustics, where the diffusers can be used to spread the reflections into all directions, reducing the strength of the undesired specular reflection and echo, as well as preserving the sound energy in space [3]. In contrast to diffusers, sound absorbers reduce the energy in the room, which can be problematic for unamplified performances in concert halls, opera houses, and auditoria. Sound diffusers are also used to promote desired reflections in order to enhance spaciousness in auditoria, to improve speech intelligibility, and to reduce the noise on urban streets [3,13,14]. Instead of using a surface with random or geometric reflectors, Schroeder innovatively designed a family of diffusers based on numbertheory sequences, with the ultimate goal to produce predicable and optimal scattering (i.e., the sound is scattered evenly in all directions regardless of the angle of incidence). In spite of the great success that SDs have achieved, they are conventionally designed to have a grating structure with a thickness that can be as large as half of the wavelength at the design frequency in order to achieve...
This paper reports on the experimental observation of topologically protected edge state and exceptional point in an open and Non-Hermitian system. While the theoretical underpinning is generic to wave physics, the simulations and experiments are performed for an acoustic system whose structure has non-trivial topological properties that can be characterized by the Chern number provided that a synthetic dimension is introduced. Unidirectional reflectionless propagation, a hallmark of exceptional point, is unambiguously observed in both simulations and experiments. arXiv:1803.04110v1 [cond-mat.mes-hall]
In this paper, we investigate a type of anisotropic, acoustic complementary metamaterial (CMM) and its application in restoring acoustic fields distorted by aberrating layers. The proposed quasi two-dimensional (2D), nonresonant CMM consists of unit cells formed by membranes and side branches with open ends. Simultaneously, anisotropic and negative density is achieved by assigning membranes facing each direction (x and y directions) different thicknesses, while the compressibility is tuned by the side branches. Numerical examples demonstrate that the CMM, when placed adjacent to a strongly aberrating layer, could acoustically cancel out that aberrating layer. This leads to dramatically reduced acoustic field distortion and enhanced sound transmission, therefore virtually removing the layer in a noninvasive manner. In the example where a focused beam is studied, using the CMM, the acoustic intensity at the focus is increased from 28% to 88% of the intensity in the control case (in the absence of the aberrating layer and the CMM). The proposed acoustic CMM has a wide realm of potential applications, such as cloaking, all-angle antireflection layers, ultrasound imaging, detection, and treatment through aberrating layers. In many medical ultrasound or nondestructive evaluation (NDE) applications, ultrasound needs to be transmitted through an aberrating layer [1][2][3][4][5][6][7], where either the transmission is desired to be maximized or the reflection needs to be minimized. One of the most representative examples is transcranial ultrasound beam focusing, which could find usage in both brain imaging and treatment [6,7]. However, transcranial beam focusing is extremely challenging because of the presence of the skull. A common approach to achieve transcranial beam focusing is based on the timereversal or phase-conjugate technique and ultrasound phased arrays [8,9]. Although the focal position can be corrected, one significant shortcoming of this strategy is that it does not compensate for the large acoustic energy loss due to the impedance mismatch between the skull and the background medium (water). Recent development of acoustic metamaterials [10][11][12] could open up the possibility for noninvasive ultrasound transmission through aberrating layers. For example, an acoustic metamaterial could be used to cancel out or cloak the aberrating layer, allowing the acoustic wave to pass through the layer without energy loss (Fig. 1). Conventional cloaking strategies [11,13,14], however, compress the space and hide the object inside an enclosure in which there is no interaction with the outside world; therefore, it is not suited to the problem of interest in this study. Lai et al. demonstrated that cloaking or illusion based on electromagnetic wave (EM) complementary metamaterials (CMM) [15] can open up a virtual hole in a wall without distortion [16,17]. In addition, this type of approach does not require the cloaked object to be inside an enclosure or cloaking shell, and it is valid in free space [18]. Because of the s...
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