Four-pole parameters of a duct with low Mach number flow

Abstract

The relationship between the four-pole parameters of a duct with and without low Mach number flow is established. A transformation of the variable is used to enable the sound propagation problem with low Mach number flow to be reformulated as the solution of an ordinary Helmholtz equation for the transformed variable. Thereby, the expression for the four-pole parameters of a duct with flow is derived which can be determined by evaluation of the four-pole parameters for the case of the stationary medium. The results show there is a noticeable variation in the four-pole parameters, but there is not any variation in the transmission loss with mean flow.