Metadiffusers for quasi-perfect and broadband sound diffusion

Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omnidirectional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means.
This is due to the considerable size required by, and the lack of tunability, of typical quarter-wavelength scattering
strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a
challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions.
In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using
deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation between the geometry of the metasurface, the
bandwidth and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around
1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves a normalized
diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The
ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work
demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffusers