On articulated plates with micro-slits to tackle low-frequency noise

In recent years, new concepts of acoustic absorbers dedicated to the reduction of low-frequency noise have been developed. Among them, liners with moving parts, such as membrane-based liners, have been an object of particular interest. In the present paper, we propose a liner concept based on a cantilever beam made of articulated plates with micro-slits. Compared to membrane technologies, these micro-slits introduce a small leakage from the backing cavity that reduces the high compressibility effects occurring at very low frequencies in a small cavity. An acoustic liner including an ensemble of such articulated plates has been fabricated and characterized for grazing acoustic incidence in absence and in presence of flow. Measurements in an impedance tube at normal incidence have also been performed, and perfect absorption is obtained at a frequency where the liner thickness corresponds to 1/16th of the acoustic wavelength. A new and simple model is proposed to predict the attenuation of this type of acoustic treatment. The results are in good agreement with the measurements, indicating a correct identification of the physical phenomena here at stake.