Acoustic cloister.

Acoustic black hole (ABH) structures are widely used for vibration and acoustic waves control due to their ability to guide the zero reflection of elastic waves and the concentration of wave energy. However, ABH can hardly suppress the ultra-low-frequency waves. We propose the acoustic cloister to break the low-frequency limit of the cutoff frequency and realize the perfect ABH effect while suppressing the ultra-low frequency waves. Thus, the waves can be localized within this structure and realize the ultra-low frequency ultra-broadband bandgap. We theoretically elaborate the bandgap mechanism of the acoustic cloister and demonstrate the good robustness of the acoustic cloister, which is beneficial for generating stable ultra-low frequency nonlinear bandgaps. Nonlinear buckling theory has been applied to explain the ultra-low frequency nonlinear bandgaps of 3–22 and 24–28 Hz that appear in the experiments, which reduces the wave transmission by 20–40 dB, and it has been demonstrated that the bending stresses appeared in the experiments can generate and greatly extend ultra-low frequency bandgaps. In torsional excitation experiments, the acoustic cloister structure attenuates wave transmission in the 3–100 Hz range by 10–80 dB. Our work makes a significant contribution to advances in vibration and acoustic wave control. [ABSTRACT FROM AUTHOR]