Practical flow duct acoustics

Abstract

Predictions of plane acoustic wave propagation through acoustically reactive flow duct systems are of practical relevance in design studies for industrial, transportation and environmental noise application. Acoustic conditions throughout the flow duct are described by the complex values of the incident and reflected wave amplitudes relative to that of the wave component incident at an open duct termination. Examples are given of the calculation of these relative component amplitudes with a comprehensive range of typical flow duct configurations. The practical consequences of wave attenuation and other losses are quantified, but consideration is restricted to plane wave propagation. In practical application the flux of acoustic energy from a prescribed source through some prescribed system will be governed by the acoustic characteristics of both. To maintain generality of application in this presentation, sources are represented simply by equivalent specified termination conditions. Thus modelling of source characteristics and its coupling with the system is not included here, neither is flow noise, nor is acoustic generation or amplification by flow processes acting as sources. Within these restrictions, effective methods are presented for quantifying and assessing acoustic performance, expressed in terms of realistic predictions for design application to practical situations. The emphasis throughout is based on fluid dynamic and acoustic principles and directed towards the development of physical insight with the identification of the relevant processes controlling wave propagation. Thus the methods described remain general enough for application to any relevant flow duct configuration, as well as those specifically considered here.