No 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.

No abstract

Acoustic diffusers are surfaces that minimize the strength of room modes in small- and medium-sized rooms and reduce specular reflections in large spaces. Since diffusers are surface treatments that are mounted to walls of interior spaces, it is desirable to reduce their thickness in order to maximize usable space. Traditional diffuser designs, such as the quadratic residue diffuser (QRD) make use of variable depth wells to control the local phase of the reflected field. In these designs, well depth is directly proportional to the reflected phase, rendering the overall thickness dependent on the design frequency. Coiled-space (CS) structures are good candidates to minimize diffuser thickness while maintaining performance. However, CS structures have tortuous channels which are susceptible to viscous losses which can degrade diffuser performance. This work presents a CS design for a 2D QRD (N = 7) diffuser with minimal viscous losses and λ/8 thickness, a 4x reduction compared to traditional QRD. Six unit-cell designs were optimized, fabricated, and measured in an impedance tube. Measured reflection coefficients are in a good agreement with predictions. The 3D diffusion performance of the CS and traditional diffusers were measured on multiple planes orthogonal to the diffuser surface with CS and traditional designs performing comparably.