Ultrathin Acoustic Metasurface-Based Schroeder Diffuser

Abstract

"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...

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