Plate-type Acoustic Valve Resonator for, Low-frequency Sound Absorption

Abstract

Plate-type acoustic metamaterials with deep-subwavelength thickness have generated considerable enthusiasm for addressing low-frequency noise. These metamaterials can be tailored to obtain high absorption efficiency by manipulating the vibro-acoustics behavior of the flexible structures. A plate-type acoustic valve resonator is developed and optimised to maximise absorption by enhancing the Helmholtz resonance with coincident structural vibrations of the plate-type acoustic valve. The current research initially examines the concept experimentally with a 3D printed valve, then with the use of analytical and numerical modelling, a structural analysis is performed which allows the eigenmodes and eigenfrequencies of the plate to be determined. When the resonator properties are modified, either by changing the depth of the backing cavity, or the thickness of the plate, for example, the system can be designed such that the Helmholtz resonance can be coincident with a particular eigenfrequency leading to higher absorption than can be achieved in the absence of such a flexible plate. In addition, absorption also occurs at frequencies other than the Helmholtz frequency due to the vibration of the plate at additional eigenfrequencies. Both of these aspects of the technology advance the state-of-the-art in Helmholtz resonator design. A good agreement has been found between the modelling and experimental results. Near-perfect absorption was achieved experimentally, e.g. up to a = 0:995 below 1 kHz, and given that the thickness of the technology can be a very small percentage of the acoustic wavelength that it is absorbing, deep sub-wavelength ratio absorbers can be designed, e.g. a ratio of up to 58 was achieved in this study with a 5 mm deep technology at 1.18 kHz. The plate-type acoustic valve resonator was further characterized using thin plate theory and 2D- 3D numerical simulation. Experimental modal analysis was performed to support the theoretical approximation, using the laser Doppler vibrometry (LDV) method. The theoretical results showed good agreement with the experimental measurements, validating the accurate identification of the structural resonance modes. It was also demonstrated that the structural resonance of the thin plate can be tailored to achieve high amplitude and broadband sound absorption at low frequencies by incorporating the Helmholtz resonance.