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Highly efficient anomalous refraction of airborne sound through ultrathin metasurfaces
Kun Tang, Chunyin Qiu, Manzhu Ke, Jiuyang Lu, Yangtao Ye, Zhengyou Liu
TL;DR
Audible airborne-sound components are often limited by the thickness required for practical wavefront control. This paper uses spatially varied coiling-slit subunits to build an ultrathin anomalous-refraction metasurface, achieving high conversion efficiency over broad frequency and angle ranges with measured wavefronts agreeing well with simulations.
Problem
Practical airborne-sound components, especially for audible wavelengths, are frequently hampered by device thickness.
Method
The paper designs an ultrathin metasurface from spatially varied coiling-slit subunits and optimizes their geometries to control transmitted sound wavefronts.
Results
The proposed flat metasurface exhibits high conversion efficiency over broad frequency and incident-angle ranges, with measured redirected wavefronts agreeing well with full-wave simulations.
Takeaways & Limitations
The design provides a route toward compact acoustic elements for steering transmitted airborne-sound wavefronts.
Takeaways & Limitations
The modeling ignores dissipation mainly caused by air viscosity within a thin layer near the channel surface.
Abstract
from arXiv · showhide
Similar to their optic counterparts, acoustic components are anticipated to flexibly tailor the propagation of sound. However, the practical applications, e.g. for audible sound with large wavelengths, are frequently hampered by the issue of device thickness. Here we present an effective design of metasurface structures that can deflect the transmitted airborne sound in an anomalous way. This flat lens, made of spatially varied coiling-slit subunits, has a thickness of deep subwavelength. By elaborately optimizing its microstructures, the proposed lens exhibits high performance in steering sound wavefronts. The experimental results are in excellent agreement with the theoretical predictions. This study may open new avenues for numerous daily life applications, such as controlling indoor sound effects by decorating rooms with light metasurface walls.
Discussion
The discussion attributes the metasurface’s performance to coiled-slit effective-medium behavior, impedance properties, and angularly invariant phase delay. It demonstrates ultrathin anomalous refraction with broad frequency and angle performance and outlines extensions to three-dimensional wavefront shaping.
- Physical mechanism: The coiling subunits act as a thin effective medium with high refractive index, producing large phase delays through elongated sound paths.The effective refractive index is estimated from the elongated slit length relative to the layer thickness.
- Physical mechanism: The effective medium combines moderate impedance with relatively wide resonances over a considerable transmission background, supporting high transmission.The discussion reports effective mass density approximately 75 times that of air and impedance approximately 10 times that of air.
- Angular response: Extreme anisotropy transports wave energy mainly along the sample thickness, maintaining phase delay across incident angles and enabling broad-angle anomalous refraction.This angularly invariant propagating distance supports anomalous-refraction designs based only on normal incidence.
- Comparison: Unlike airborne gratings requiring thickness comparable to the wavelength, the proposed metasurface is ultrathin and retains sound-wavefront manipulation capability.The discussion contrasts the proposed design with gratings whose high transmission is achieved using much greater thickness.
- Demonstrated performance: The demonstrated metasurface is approximately 1/6.7 of the operational wavelength and provides high conversion efficiency across broad frequency and incident-angle ranges.Measured redirected wavefronts agree well with full-wave simulations.
- Extensions: The design principle can extend to three-dimensional coiled hole-arrays for shaping acoustic vortices and non-diffracting Bessel beams.These extensions are presented as possible future applications of the design strategy.
Methods
The methods combine full-wave simulation, multiscale geometric design, and 3D-printed sample fabrication. Simulations calculate transmission and diffractive-branch weights, while experiments use geometries constrained by laboratory and manufacturing scales.
- Simulations: Full-wave simulations use actual microstructure dimensions, with radiation or periodic boundary conditions selected for the relevant cases.The simulations are performed using COMSOL Multiphysics, and the plastic frame is modeled as acoustically rigid.
- Assumptions: The model assumes a 340 m/s sound speed, acoustically rigid plastic, negligible plastic transmission through the thinnest plate, and ignored near-wall viscous dissipation.The dissipation assumption is identified as an author-supported limitation of the modeling approach.
- Simulations: Total power transmission is obtained by integrating Poynting vectors and normalizing to incidence, while diffractive-branch weights come from the sample’s scattering matrix.These calculations quantify transmitted power and the relative contributions of diffracted branches.
- Sample preparation: The design accounts for four descending length scales: sample length, wavelength, subunit size, and microstructure size.The largest feature is limited by the laboratory table, while the smallest is limited by manufacturing accuracy.
- Sample preparation: The experiment uses an approximately 13.3 cm wavelength and subunits with h = d = 2.0 cm, horizontal-bar thickness t = 0.8 mm, and spacing s = 1.0 mm.Each subunit has tunable bar number n and bar length l.
- Sample preparation: The sample is fabricated from thermoplastics by 3D printing, with each supercell completed in a single procedure.The sample is assembled by gluing many supercells together.