Broadband dissipation through Functionally-Graded Micro Perforated Metamaterials

The problem of lightweight and space-constrained sound absorbers in the low frequency range constitutes an area of continuous research. For instance, aeronautics has an important role to play in reducing noise to support sustainable traffic growth: one of the main challenges of air transport is to attenuate the environmental impact of air traffic despite its continuing expansion. Passive methods can provide significant noise reduction, but also additional weight and may obstruct fast routine maintenance inspection. They are also accompanied by some degradation in high-lift performance or are not practical for real-life applications when low frequency components are predominant. Although potential of active noise control in airframe noise reduction is promising, at present stage these active methods are applied in laboratory conditions and to downscaled models of airframe components. In this project we have improved low-frequency absorption of flow-induced noise through the development of acoustic meta-materials at sub-wavelength scales. We have combined thin micro-perforated structures periodically assembled across multiple length scales to obtain a lightweight sound-proof device. A comparison of the absorption coefficient for the proposed absorber (blue) and for a classical aircraft liner (red) is presented in the figure below for a total partition thickness of 10 cm.