Design of Fresnel acoustic reflector for sub-wavelength broadband sound diffusion

Sound diffusion performance is considered as an important issue in building acoustics. A well-diffused sound field can effectively enhance the human listening experience. In this context, inspired by Fresnel lenses in optics, we design a sub-wavelength acoustic metasurface (termed “Fresnel acoustic reflector”) to achieve broadband uniform diffuse reflections. To this end, an optimization strategy that combines Monte-Carlo search and other derivative-free algorithms is proposed to ensure, as far as possible, a global optimum for the objective function when dealing with the multi-peak problem encountered in complex sound fields. Numerical simulations show that the average normalized diffusion coefficient of the designed Fresnel reflector can reach up to 0.6 in the sub-wavelength frequency range 500–3400 Hz, which exhibits a better broadband diffusion performance compared to the traditional Schroeder diffusers. The simulated sound pressure field is also verified by experimental measurements. From a general perspective, the proposed optimization strategy and simulation framework can benefit the design of interior components in building acoustics.