Abstract
Diffuse reflection of sound is desirable in many practical scenarios, such as architectural acoustics, but most existing designs of acoustic diffusers have bulky size, corrugated profile, or limited spatial resolution. We design an ultrathin planar acoustic diffuser for producing narrowband diffuse reflection with high uniformity via precise modulation of reflected wavefront and propose a metasurface-based implementation comprising a monolayer of Helmholtz-like resonators much smaller than the working wavelength in all three dimensions. Our design is benchmarked with a commercialized Schroeder diffuser and is numerically proven to be capable of scattering the illuminating wave more uniformly than the conventional mechanisms based on the quadratic residue sequence over wide incident angles. We anticipate our design with simplicity and capability to find important applications in diverse scenarios.
Highlights
Due to the reduction in the width of the basic building blocks, our designed high-resolution metasurface-based diffusers (HRMDs) of the same total length is composed of 28 Helmholtz resonator (HR)-like unit cells, consistent with the sequence given by Eq (3)
Typical numerical results of the comparison of the scattered acoustic waves produced by the HRMD and SD illuminated by a plane wave are shown in Fig. 3, in which two particular cases are considered: normal incidence and 45○ incidence
As evidenced by the much less pronounced side lobes in the spatial pattern of the reflected wave produced by HRMD, the results manifest that the designed device scatters the acoustic energy normally incident on its surface into all directions more uniformly than the conventional method while downscaling the thickness to the deep-subwavelength scale
Summary
As an important problem in acoustics, the production of diffuse reflection is highly desired in a variety of practical scenarios in need of elimination of standing waves such as for improving the acoustic quality of rooms.[1,2,3,4] As a classical design proposed almost half a century ago, Schroeder diffusers[5,6] (SDs) capable of producing diffuse reflection more efficiently than a simple randomized configuration have wide applications ranging from architectural acoustics[7,8] to medical ultrasound imaging[9] to microparticle separation[10] and have inspired other disciplines.[11]. The QRS design of phase responses in SDs and the ultrathin Schroeder diffuser in Ref. 15 comprises of half-wavelength wide unit cells can help to produce better diffuse reflection than a totally randomized phase distribution, but they possess fundamental limitations on the spatial resolution of the generated phase profile. Our proposed mechanism is based on the extraordinary capability of the metasurface to produce the full control of the reflected phase within the physical distance much smaller than the working wavelength This allows us to downsize the building blocks of our designed diffuser to subwavelength scale in all three dimensions. Due to the subwavelength transverse width of each unit cell, the generated spatial distribution of the reflected phase has a high resolution and well mimics a continuous phase profile on the diffuser, which allows precise manipulation of the reflected wavefront. According to generalized Snell’s law, the angle of reflection on a metasurface with phase shift Φ(r) as normally illuminated by an incident plane wave is[29] sin θr
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