Thermo-acoustic Attenuation Behaviour of Rib-Knitted Fibrous Screen for Exhaust Applications

Glass nonwovens are among the most commercially utilised sound-absorbing materials in automotive and industrial silencers. However, the inherent brittleness of glass fibres often results in fibre blow-out through exhaust ducts, exacerbated by vibration and air pressure. Encapsulating the glass nonwoven between continuous filament-based knitted fabrics effectively controls fibre blow-out, enhancing the durability and reliability of the acoustic insulation. The structure of knitted fabrics and the selection of roving substantially influence the thermal and acoustic performance of sandwich-absorbing materials. To investigate the effects of these structural parameters, a full factorial design experiment (DOE) was conducted. This DOE incorporated roving linear density (ranging from 200 to 600 Tex) and knit stitch length (8–12 mm) as continuous variables, along with roving type (flat and texturized) as a categorical variable. The structure of texturized roving was characterised using optical imaging. Rib-knitted fabrics were developed based on the DOE, and their structural characteristics, such as total porosity, straight porosity, and straight pore radius, were analysed through optical microscopy image processing. The sound absorption properties and flow resistivity of the knitted specimens were evaluated. Additionally, high-temperature contact heat transmission and sound absorption behaviour of the sandwich structure were assessed. A significant relationship between the structural parameters of the knitted fabric and the thermo-acoustic performance of the sandwich structure was established.