Acoustic metaliners for insulating noise in a duct with varying cross-sectional area

Abstract

Ducts are used as guides for fluid transport in many mechanical systems. However, the duct serves as a guide for the fluid as well as a guide for sound waves, noise propagates through the duct when a noise source exists. Many studies are conducted to insulate the noise propagating through duct systems. However, previous studies have only addressed cases where the cross-sectional area of the duct is constant. In this thesis, an acoustic metaliner that can insulate noise propagating in a duct with linearly increasing cross-sectional area and the curvature of the center line of the cross-sectional area is 0 is designed with consideration of flow. First, through optimization and numerical simulation, optimal impedance of a metaliner that minimizes the transmitted wave through a duct with linearly increasing cross-sectional area is derived. In a duct with varying cross-sectional area, the flow speed varies according to the change in cross-sectional area. To consider this, the average flow speed of each section is calculated by dividing the duct into sections corresponding to the unit cells of the metaliner. Next, considering the effect of flow on the acoustic characteristic of the metaliner, a metaliner that satisfies the derived optimal impedance is designed. Considering the effect of the calculated average flow speed on the effective impedance of each unit cell, all unit cells are designed to satisfy the optimal impedance. It is examined that the metaliner, designed considering the change in cross-sectional area and flow, is suitable for the target frequency in the duct with linearly increasing cross-sectional area and the degradation of sound insulation performance due to flow is minimized.