Investigating the Aeroacoustic Properties of Kevlar Fabrics

The aeroacoustic properties of porous fabrics are investigated experimentally in an effort to find a porous fabric as an ideal interface between wind tunnel flow and quiescent conditions. Currently, the commercially available Kevlar type 120 fabric is widely used for similar applications, such as side-walls in hybrid anechoic wind tunnels or as a cover of phased microphone arrays. A total number of 8 fabrics were investigated, namely, four glass fiber fabrics, two plain weave Kevlar fabrics, and two modified plain Kevlar fabrics with their pores clogged. Two, custom-made rigs were used to quantify the transmission loss and self-noise of all eight fabrics. It was found that the pores serve as a low-resistance gateway for sound waves to pass through, hence enabling a low transmission loss. The transmission loss was found to increase with decreasing open area ratio while other fabric properties had a minor impact on transmission loss. The self-noise of the fabrics has also been evaluated and it was found that the thread density (thread per inch) is a primary factor of determining the frequency range of self-noise with the open area ratio potentially playing a secondary role in the self-noise levels. For both metrics, the mass per unit area seemed to play a minor role in the aeroacoustic performances of the fabrics. Finally, surface pressure measurements revealed that the commercially available plain Kevlar (type 120) has no quantifiable effect on the hydrodynamic pressure field passing over the fabric, sug- gesting that Kevlar behaves as a no-slip wall from the flow's perspective when no pressure difference is present on the two sides of the fabric.