Improved sound diffusion using spatio-temporally modulated acoustic metasurfaces




Kang, Janghoon

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Conventional acoustic diffuser designs are phase gratings on reflecting surfaces which are used to reduce specular reflections and the strength of room modes. Traditional designs have some requirements that limit implementation in practice, such as a thickness that is inversely proportional to the design frequency and directional scattering that results from the periodic arrangement of diffuser unit cells. This work investigates two different acoustic metasurface (AMS) designs as solutions to reduce thickness without compromising diffuser performance and to achieve improved sound diffusion for a given geometry. A coiled-space AMS design is proposed to reduce diffuser thickness by designing long and tortuous channels with minimal loss to enable tailored phase variation with minimally thick diffuser elements. Dimensions found from a parametric study using the finite element method (FEM), AMS diffuser elements with a quarter of the original depth were fabricated using additive manufacturing and experimentally shown to produce the desired reflected field phase while minimizing absorption. Impedance tube tests of individual unit cells and measurements of the scattered field from a two-dimensional diffuser in an anechoic chamber demonstrated comparable performance between the coiled-space AMS design and a conventional quadratic residue diffuser (QRD).

Spatiotemporal modulation of surface acoustic admittance in a wave-like manner was investigated theoretically and experimentally to improve the diffusivity of acoustic scattering from a reflective surface with a specified geometry. A semi-analytical model of the scattered sound field from spatiotemporally modulated diffuser surface was developed using Fourier series expansion of diffraction modes that includes harmonics of the admittance modulation frequency. The presence of modulation harmonics scatters sound in additional directions compared to those of the unmodulated diffuser, thereby improving diffusivity of the scattered sound field. The polar responses of the scattered field predicted by the semi-analytical model were verified by comparison with a transient numerical simulation using the finite element method (FEM). Performance improvements were experimentally verified by modifying the termination of the conventional QRD design through the addition of a compliant piezoelectric bender disk terminated with a negative capacitance shunting circuit. The electrical termination of the piezoelectric disk is then switched between open-circuit and negative capacitance circuit conditions using a computer-controlled solid-state relay switch to create a square-wave modulation function of the termination impedance. Modulation harmonics in the reflected field were observed using the impedance tube tests. Furthermore, measurements of a spatiotemporally modulated QRD in an anechoic environment lead to the observation of modulation harmonics with angle-dependent magnitudes that are functions of the modulation directions in good agreement with predictions of the semi-analytical model. These measurement results demonstrate the effectiveness of the spatiotemporal modulation of the surface acoustic admittance in both time and space.


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