Asymptotic approximation for sound propagation in ducts carrying low subsonic mean flow

This article presents an extension to a previous approach used to describe the two-dimensional problem of an incident plane sound wave traveling along a rigid circular duct. Here the effects produced by a low subsonic mean flow on the values of acoustic impedance are studied using the WKB approximation. The asymptotic approximations for low Mach numbers are applied to a semi-infinite exponential horn. The results show that an increase in the flow velocity reduces the sound power transmitted through the horn.