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

The dynamic response of thin-walled slender tubes 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 are sought. The Wave Finite Element Method (WFEM) is used to estimate the dynamic response of propagating waves in the tubes. Extrudable rings of thermoplastic material are added to the inside of the steel tubes as local resonators for changing the response. The rings are added in sections to create a periodic waveguide which introduces stop-bands for the flexural waves. Additionally local bending modes of the rings couple with the global axial, torsional and bending modes to reduce global vibrations at the resonant frequencies of the local modes. The results from the dispersion diagram obtained through WFEM is compared to the response of the structure in simple linear harmonic analyses conducted in the standard FE software ANSYS. This is done in different loading scenarios to determine the impact of adding the resonator rings to different excitations and frequencies.