202 / 2021-03-31 21:57:01

An ultrathin asymmetric sound absorber with ventilation

acoustic metamaterials; asymmetric absorptions; deep-subwavelength; high degree of absorption asymmetry;

Acoustic metamaterials and absorbing materials

Asymmetric sound absorptive platforms enable high-efficiency absorption and excellent ventilation have demonstrated great scientific significance and extensive applicability. Common ventilated asymmetric absorbers composed of two or more resonant meta-atoms, however, present complicated frameworks and poor flexibility in modulating operating frequency, let alone for high degree of absorptive asymmetry. Here, we propose a paradigm to realize deep-subwavelength asymmetric absorptions with the coupling of a lossy meta-atom and a non-resonant impedance boundary, served by a folded Fabry-Perot resonator (FFP) and a narrow slit channel, respectively. As a demonstration, we have constructed an asymmetric absorber showing 97.4% (2.4%) absorptance at 100 Hz (wavelength λ being 100.8 times of absorber thickness) for sound incident from the left (right) port. The degree of absorption asymmetry approaches to 40.8, enable excellent absorbed and reverberated fields in opposite sides of the system . Further investigation demonstrates that the asymmetric absorption is robust on the lengths of slit channels. Moreover, due to the broadband non-resonance characteristics of the narrow slit channel, operating frequency of the designed absorber can be modulated only by FFP, which illustrates larger flexibility than ones relying on multiple resonators. Further, the reported recipe can be extended to the design of broadband asymmetric absorbers. As a concept proof, we demonstrate an absorber showing > 85% absorptance ranging from 308 Hz to 352 Hz (with wavelength λ from 33.2 to 29.1 times of thickness). Our results open promising possibilities for developing versatile functional devices with excellent ventilation to asymmetrically manipulate sound