Abstract: The governing equation of three-dimensional sound propagation in an uniform duct with mean flow and linear temperataure gradient is derived,based on the foundamental equations of aerodynamics.By using the interation method,the sound propagation equation is expressed as the case which can be computed numerically with the boundary element method.The four-pole parametes of a muffler can be obtained using BEM,therefore the acoustic per.formance such as transmission loss can be predicted.The transmission loss of an expansion chamber has been calculated and compared with the result obtained by means of the one-dimensional theory.

Key words: Sound propagation, Mean flow, Temperature gradient, Boundary element method