Defect mode properties of an acoustic structure made up of periodic expansion chambers containing defects

Abstract

Noise pollution is an environmental menace in agricultural, industrial, commercial, and residential facilities of all countries around the globe particularly developing countries. This paper examines noise reduction of an acoustic band gap structure made from expansion chambers. The interface response predictions of the transmission loss are presented and compared with the experiment, and the boundary element method (BEM) data obtained from the literature showing good agreement at low frequencies. A wider band gap with and stronger muffling effects at a lower frequencies is shown, which depends on the geometry of the expansion chamber and the periodicity. Furthermore, the periodicity is broken by the introduction of defects to analyse the narrow frequency transmission bands within the band gaps. In this work, the defect is artificially produced and controlled by the length and/or the cross-section of the central waveguide segment. The influence of dimension parameters on the wave suppression band gaps is analyzed. We show that the defect mode transmission moves within band gaps. It location can be controlled by the dimension of the defective expansion chamber. The closest to the middle of the band gap, the narrowest becomes. The results presented in this work can help to guide the achievement of broader acoustic band gaps in waveguide systems.