Experimental demonstration of a spatiotemporally modulated sound diffusing metasurfaces

Abstract

Spatiotemporal modulation of material properties has been studied across physics in an effort to control propagating waves. Using a semi-analytical approach, we have recently shown that the performance of a conventional sound diffuser can be significantly improved through spatiotemporal modulation of its surface admittance [Kang et al., Appl. Phys. Lett. 121, 181703 (2022)]. The improved performance is a result of energy in scattered sound at the incident wave frequency plus and minus harmonics of the modulation frequency of the temporal modulation of the input admittance. The additional frequencies of reflected sound increase the diversity of propagating diffraction modes and therefore increased angular distribution of scattered energy. This work presents the design of a sound diffuser element to implement the spatiotemporal modulation. The rigid bottom of a straight-tube diffuser well is replaced by a piezo disc shunted by a negative impedance converter (NIC), in which a capacitance can be controlled by varying a resistor value in the NIC. We present impedance tube measurements that show the presence of modulation harmonics in the reflected wave for single-frequency incident wave caused by a modulation of an acoustic admittance, and the diffusion performance measured in the anechoic chamber is compared with the non-modulated case.