Frequency-preserved non-reciprocal acoustic propagation in a granular chain

Experimental evidence is presented for demonstrating frequency-preserved non-reciprocal acoustic propagation in a lower frequency range. A 23-bead granular chain and a conical rod are combined to realize this phenomenon. The conical rod is used to amplify the amplitude of the incident wave. Because of the inherent nonlinearity, this granular system offers an ideal testing platform that can provide an amplitude-dependent bandgap. By carefully tuning the taper ratio of the conical rod, a strong non-reciprocal acoustic propagation is observed. Moreover, such a phenomenon does not change the frequency of the incident wave due to the weak nonlinearity, which is different from previous studies. The result represents an advance in acoustic metamaterial systems using simple materials and paves the way for practical applications.