Tunable and broadband asymmetric sound absorptions with coupling of acoustic bright and dark modes

We propose a strategy for constructing an asymmetric sound absorber by coupling two mutually tuned dark and bright modes. The subsequent requirement is that the dark mode must be strongly dissipative to consume sound energy, whereas the bright mode exhibit weak damping and be characterized as an effective soft boundary to block the sound energy into the exit port. Thus, the system provides an absorbing response to sound incident from the port near the dark mode but a reflecting response to sound from the opposite port. We theoretically and experimentally demonstrate the asymmetric absorber in a straight waveguide shunted by Helmholtz resonators (HRs), in which the bright-dark modes are served by the HRs with different short neck radii. The asymmetry is further enhanced by coupling additional modes with a moderate loss factor set in between. Moreover, the operation frequency of the asymmetric absorptions can be reconfigured by the synchronously varied HR specimens along with the cavity depth, which guarantees the construction of an asymmetric broadband absorber by means of the parallel coupling of four enhanced asymmetric absorptive systems. Our investigations offer an efficient means of asymmetrically manipulating sound absorption/reflection and provide a sound absorber with superior characteristics to the traditional single-port absorber. (C) 2020 Elsevier Ltd. All rights reserved.