Ultra-broadband dual-mode nonlocal sound absorber based on phase gradient Fabry-Perot slit channels

Acoustic artificial structures have promoted the high-performance of sound absorbing devices. However, the local coupling and inherent structural losses limit the development of ultra-0broadband sound absorbing devices. Here, we propose a dual-mode nonlocal design method to improve the performance of the structure-based absorbers. By utilizing the nonlocal coupling effect caused by multiple reflections of the period phases gradient structures, the losses in the units can be amplified without extra resonance. A transverse nonlocal coupling is then excited by introducing pores in the wall of Fabry-Perot channels to enhance and transfer the internal loss. The loss power density distribution uncovers the physical mechanism of the coupling effect. A dual-mode nonlocal absorber incorporating two types of walls is designed and experimentally validated, achieving an ultra-broadband sound absorption performance from 970 Hz to 10,000 Hz with an average absorption coefficient exceeding 0.9. Our design provides a new route to improve the absorption performance of the phase gradient absorbers without considering the strong resonance of the units.