A granular acoustic diode

Diodes, which allow electricity to flow in only one direction, have been the key to the development of modern electronics. Creating acoustic analogues that restrict sound transmission to one direction has proven challenging. In no small part that’s because wave propagation is inherently symmetric: A system’s transmission is unchanged if the source and detector are reversed (see Physics Today, May 2016, page 14 ). There have been some successes in creating acoustic diodes, transistors, and logic elements, but most have relied on nonlinear media that distort the waveforms. Now Tianzhi Yang (Tianjin University) and Jian-Guo Cui and Li-Qun Chen (Shanghai University) demonstrate a simple, nondispersive system for nonreciprocal acoustic propagation: a one-dimensional chain of 23 beads. The flexibility to select the number, shape, stiffness, and arrangement of the component elements in such granular crystals facilitates the tailored engineering of their behavior. (See the article by Mason Porter, Panayotis Kevrekidis, and Chiara Daraio, Physics Today, November 2015, page 44 .) The central feature of the new work is an amplitude-dependent bandgap that arises from the weakly nonlinear contact forces between the 1.6-cm-diameter polypropylene beads. As theoretically predicted in 2015, a tapered rod (red in the schematic) placed at one end of the chain will amplify acoustic waves incident from that side and allow them to surmount the bandgap and pass through with little distortion. But waves traveling in the reverse direction don’t get that boost; instead, they get significantly attenuated. The acoustic diode’s operating frequency, about 1400 Hz, falls within the audible range. The readily realized geometry could lend itself to such practical applications as a “one-way sound wall,” the acoustic equivalent of a one-way mirror. (J.-G. Cui, T.-Z. Yang, L.-Q. Chen, Appl. Phys. Lett. 112, 181904, 2018, doi:10.1063/1.5009975 .)