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"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...

The fine manipulation of sound fields is critical in acoustics yet is restricted by the coupled amplitude and phase modulations in existing wave-steering metamaterials. Commonly, unavoidable losses make it difficult to control coupling, thereby limiting device performance. Here we show the possibility of tailoring the loss in metamaterials to realize fine control of sound in three-dimensional (3D) space. Quantitative studies on the parameter dependence of reflection amplitude and phase identify quasi-decoupled points in the structural parameter space, allowing arbitrary amplitude-phase combinations for reflected sound. We further demonstrate the significance of our approach for sound manipulation by producing self-bending beams, multifocal focusing, and a single-plane two-dimensional hologram, as well as a multi-plane 3D hologram with quality better than the previous phase-controlled approach. Our work provides a route for harnessing sound via engineering the loss, enabling promising device applications in acoustics and related fields.

Invisibility or unhearability cloaks have been made possible by using metamaterials enabling light or sound to flow around obstacle without the trace of reflections or shadows. Metamaterials are known for being flexible building units that can mimic a host of unusual and extreme material responses, which are essential when engineering artificial material properties to realize a coordinate transforming cloak. Bending and stretching the coordinate grid in space require stringent material parameters; therefore, small inaccuracies and inevitable material losses become sources for unwanted scattering that are decremental to the desired effect. These obstacles further limit the possibility of achieving a robust concealment of sizeable objects from either radar or sonar detection. By using an elaborate arrangement of gain and lossy acoustic media respecting parity-time symmetry, we built a one-way unhearability cloak able to hide objects seven times larger than the acoustic wavelength. Generally speaking, our approach has no limits in terms of working frequency, shape, or size, specifically though we demonstrate how, in principle, an object of the size of a human can be hidden from audible sound.

We propose to produce efficient three-dimensional sound converging in broadband with binary reflected phases on a planar surface with unit cells consisting of only two kinds of elements. The mechanism is experimentally demonstrated by focusing airborne sound and by forming an “acoustic needle,” with handmade arrays of commercial test tubes with/without lids. Both the simulated and measured results show the precise control of converging acoustic energy despite misalignment errors obvious even to naked eyes. Our approach with extreme simplicity yet good robustness may apply in various scenarios that conventionally need complicated elements and continuous variation of parameters for focusing sound.

We theoretically and numerically present the design of multi-frequency acoustic metasurfaces (MFAMs) with simple structure that can work not only at fundamental frequency, but also at their harmonic frequencies, which breaks the single frequency limitation in conventional resonance-based acoustic metasurfaces. The phase matched condition for achromatic manipulation is discussed. We demonstrate achromatic extraordinary reflection and sound focusing at 1700Hz, 3400Hz, and 5100Hz, that is, they have the same reflection direction and the same focusing position. This significant feature may pave the way to new type of acoustic metasurface, and will also extend acoustic metasurface applications to strongly nonlinear source cases.

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We observe a series of unstable and dynamic behaviors of acoustic vortices propagating in stratified inhomogeneous media. These behaviors include bending, stretching, distorting, focusing, and untwisting of the vortices, migration of singular points, and reversal of energy flux and angular momentum. We gain insight into how these behaviors result from the stratification. The results will be useful for applications of acoustic vortices in communication and particle manipulations in inhomogeneous media such as oceans and biomedical tissues. Our work opens up the study of the propagation of acoustic vortices in complex media.