Observations of characteristic transmission and reflection coefficients of a splitter duct attenuator

Abstract

The characteristic transmission coefficients (including the reflection coefficients) of a splitter duct attenuator with two air channels containing higher-order wave modes were investigated. These were evaluated at interfaces taken in the straight duct sections in both sides of the attenuator and were determined experimentally and numerically (by BEM). The determination method introduced here requires the solution of two sets of the simultaneous equations given by the superposition principle of traveling waves. One is that the sound pressure at a point is the sum of the pressures of all the traveling waves that could possibly exist there, and it was applied to decompose the incoming and outgoing waves of all modes at each interface. The other is that each outgoing wave pressure of each mode at each interface equals the sum of the products of the incoming wave pressures of all modes at all interfaces and the corresponding transmission factors. It was applied to determine, without using an echoic termination, each characteristic transmission coefficient by which each incoming wave contributes to each outgoing wave pressure. The transmission coefficients of the attenuator were obtained for the fundamental and the first-order modes. Agreement of those by experiment and numerical results is substantially good.