Control and broadening of multiple noise frequencies using an assembly of sub-metamaterials connected by membranes for aircraft noise mitigation.

Simultaneously attenuation of multitonal and broadband noise at low frequencies is a challenge for the aerospace, ground transportation and building industries. The technologies proposed in the literature, using layered porous materials with embedded Helmholtz resonators (HR) with a structured neck, exhibited considerable potential when tuned at tonal, multi-tonal or narrow frequency bands. However, the resonance frequencies due to the structured necks of metamaterial are narrow. Our recent investigations have shown that parallel arrangements of several structured metamaterials separated by membranes can broaden the resonance frequencies and increase the number of resonance frequencies to be controlled. This paper presents a parallel assembly of four structured sub-metamaterials separated by membranes. Each of these sub-metamaterials is also a serial assembly of a periodic unit cell (PUC) with a half-neck + cavity + half-neck configuration. The metamaterial is embedded in a layer of glass wool. Coupled fluid-structure numerical calculations in the frequency domain were used to predict the sound absorption coefficient of the metamaterial. The results obtained show a broadening of the absorption peaks and the appearance of additional frequencies due to the membranes.