On reduction of noise emissions from thin-walled slender tubes by means of acoustic metamaterials

The dynamic response of a thin-walled slender tube is dominated by noisy flexural modes. One topic of interest in research today is the use of acoustic metamaterials to reduce noise emissions from structures. These aspects are combined in the present work in which a solution for reducing the noise emission from thin-walled tubes is sought. The Wave Finite Element Method (WFEM) is used to estimate the dispersion relation of the free propagating waves in the tube. Extrudable rings of thermoplastic material are added to the inside of the steel tube as local resonators for changing the response. The rings are added in sections to create a periodic waveguide which introduces interference stop-bands for the flexural waves. Additionally, local bending modes of the rings couple with the global modes of the tube to reduce global vibrations at the resonant frequencies of the rings. The results of free wave propagation analyses obtained through WFEM are compared to the forced response of the structure in standard linear harmonic analyses conducted in the commercial FE software Ansys. This is done in different loading scenarios to determine the impact of adding the resonator rings to different excitations and frequencies.