Abstract
In the process to design a fluid related system consisting of ducts, like a car muffler, an air conditioner or a wind tunnel, we need to analyze the noise propagating in the system in order to keep comfortable environment. For this analysis, we sometimes apply the scattering matrix method. In this method, we model the duct system as links of linear elements, e.g. a straight one, an expanding one or a branched one, whose theoretical scattering matrices have been already determined. However, a scattering matrix for an open part is not established yet. In the wind tunnel used to measure the sound generated by a car or a high speed vehicle, an open type measuring room is often adopted. The scattering matrix corresponding to the open part is necessary to analyze the sound propagation in such a kind of wind tunnels by use of the scattering matrix method. In this study, we propose a scattering matrix for an open part in a duct system in which two circular open ends face each other in coaxial arrangement. The sound popagating in a duct partly reflects at the open end and the rest is emitted to the open space. After propagating in the open space, part of the emitted sound reaches the other open end. The scattering matrix was expressed in terms of the reflection coefficient at the open end and the rate of sound collected by the other open end. The each element of the proposed scattering matrix was compared with experimentally obtained value. They agreed fairly well.
