On acoustic in-duct characterization of medium speed IC-engine exhaust system

Predicting and controlling noise from a fluid machine requires a model of the noise sources themselves, i.e., acoustic source data. If the source connects weakly to the surroundings, the standardized free field methods can be used for the source sound power determination, i.e., source characterization. In the duct systems connected to the fluid machines, the acoustic source interacts strongly with the system boundaries and the in-duct source characteristics cannot be described using only the emitted sound power. The following general problems exists: The source characteristics are dependent on the acoustic load and the sound power is reflected from the duct discontinuities as well as from the duct terminations, the sound pressure measurements might be contaminated by the turbulence in the flow, and above a certain frequency, i.e., first non-plane wave mode cut-on frequency, the sound pressure varies over the duct cross-section and non-plane waves are introduced. For a number of applications the plane wave range dominates and the non-plane waves can be neglected. But for machines connected to large ducts the nonplane wave range is also important. In this paper, methods to determine the in-duct acoustic source data including also the non-plane waves are discussed. The low and high frequency in-duct characteristics of a test engine exhaust system are obtained and combined. The combined source data is useful when designing the components of the exhaust system, for example