Sound diffusion with spatiotemporally modulated acoustic metasurfaces

Abstract

Traditional sound diffusers are quasi-random phase gratings attached to reflecting surfaces to introduce spatiotemporal incoherence into the backscattered acoustic field. Early designs consisted of periodically tiled diffuser grating unit cells to cover large surfaces. However, spatial periodicity leads to coherent constructive and destructive interference, which is undesirable for achieving acoustic diffusivity. This problem was partially addressed by using aperiodic tiling of unit cells based on pseudorandom sequences. While an aperiodic diffuser spacing can overcome the problems introduced by spatial periodicity, the improvements in performance come at the expense of increased thickness. In this work, we investigate spatiotemporal modulation of the surface acoustic admittance of a metasurface diffuser to improve sound diffusion. Using semi-analytical and finite element models, we demonstrate that the effects of the spatial periodicity can be mitigated without introducing an aperiodic spatial spacing, thus simultaneously minimizing diffuser thickness and improving diffusivity of the backscattered field. We develop a semi-analytical model that employs Fourier series expansion to determine the scattered sound field from a surface admittance consisting of a quadratic residue diffuser whose individual well admittances are modulated in a traveling wave fashion with modulation frequency, [Formula: see text], amplitude, [Formula: see text], and a wavenumber that matches the unit cell length, Λ. We observe significant improvement in diffusion due to the fact that the spatiotemporal modulation scatters sound into additional frequency-wavenumber pairs associated with harmonics of [Formula: see text] and their diffraction orders. The semi-analytical model results are verified using a time-domain finite element model and compared with periodic and aperiodic diffuser designs.

Multilateral Sound Manipulation Using Magnetically Tunable Apertures

“…The permanent magnet and magnetic properties of the rod ensured that the rod would not slip or fall off the substrate. The use of magnetic actuation with magnetorheological elastomers [41,42] showed how our design could be integrated with soft robotics techniques. In addition, magnetorheological elastomers could be fabricated using the alignment of magnetic particles, which was not implemented in this work.…”

“…The proposed acoustic device is easy to replicate, uses easily accessible and low-cost materials for fabrication, and is applicable for implementation on a larger scale. The transmissive design of our device differs from reflective designs [2,42] which require the input source waves to be on the same side as the output for operation. This transmissive characteristic could simplify integration with commercial devices, where it could potentially be part of a thin partition or window.…”

Reflections of High-Frequency Pulsed Ultrasound by Underwater Acoustic Metasurfaces Composed of Subwavelength Phase-Gradient Slits

“…Numerous types of units (also referred to as meta-atoms), such as Helmholtz resonators, labyrinthine structures, helical structures, coiling-up spatial structures, and membrane structures, have been designed to build functional acoustic metasurfaces [19][20][21][22][23]. With these engineered meta-atoms, the associated metasurfaces can passively reconstruct fascinating wavefront profiles, including acoustic beam focusing, beam self-bending, acoustic vortex, sound diffusing, asymmetric transmission, and acoustic carpet cloaking [24][25][26][27][28][29].…”