Effects of flow on sound insulation of phononic crystals and metaliners

Abstract

In this study, the effects of flow on sound insulation of phononic crystals and metaliners are identified. Phononic crystal is made of periodic scatterers embedded in a matrix with high impedance contrast, preventing sound transmission in a bandgap frequency range through Bragg scattering. Metaliner is a metastructure-based acoustic liner with a subwavelength scale capable of high sound insulation for a duct in the low-frequency range while allowing fluid passage without causing flow resistance. However, most previous studies have been done within a framework of the stationary medium, and only a few researchers have studied the change in sound insulation for phononic crystals or metaliners by considering the effect of flow. Firstly, this study identifies the effects of flow on bandgap characteristics of phononic crystals for steady and turbulent flows. For steady flows such as uniform and compressible potential flows, the effective speed of propagation can be slowed inside the phononic crystals, and the reflectance spectra are shifted to lower frequencies by the factor 1-M_eff^(2 ) (M_eff: effective Mach number) due to fluid convection. For turbulent flows, the bandgap characteristics of phononic crystals for various combinations of the inflow speed, filling ratio, and the number of layers are investigated. A bandgap quenching phenomenon of phononic crystal in a moving medium is identified, accounting for the effects of aerodynamic noise and convection. Secondly, the effects of flow on sound insulation and absorption of metaliners are investigated. An effective impedance model of the metaliner is established by considering the flow effect, and transmission loss and absorption coefficient of the metaliner are numerically predicted in a duct with the flow. Then, this study presents a design procedure for a metaliner with high transmission loss for various flow speeds. Experiments show that the designed metaliners exhibit transmission loss above 45 dB/m within ±5% of the target frequencies of 500 Hz or 1000 Hz for different flow speeds of 0, 17, and 34 m/s.