Abstract
This paper presents a resonator with a variable helical structure that induces a change in the internal impedance of the resonator, which can improve the performance of the resonator. This resonator structure achieves an asymmetric absorption mechanism of acoustic waves by coupling a pair of Helmholtz resonators with the same coupling mode in parallel in a two-port acoustic duct. Theoretical analysis, numerical simulations, and experimental measurements confirm that the acoustic energy is almost completely absorbed (97.4%) at 287.5 Hz. By designing the acoustic “soft” boundary, the system absorbs acoustic energy when the acoustic wave is incident from one side and mostly reflects when the acoustic wave is incident from the other side. These two different absorption patterns are caused by the impedance matching/mismatch between the two sides of the system, respectively. Consequently, the asymmetric absorption system at low frequencies achieves a near-perfect absorption of acoustic energy. Asymmetric absorption at broadband and multi-band is obtained for different cases. The asymmetric design in this paper improves the active control of noise cancellation in a two-port system and allows for a more diverse use of absorption devices.
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