Experimental demonstration of a two-dimensional sound diffusing acoustic metasurface with subwavelength thickness

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.