Engineering Applications of Plane Wave Duct Acoustics

The work in this thesis elucidates the physical and mathematical concepts involved in understanding the propagation and measurement of acoustic waves in a duct in the plane wave mode and its engineering applications. A discussion of the two-microphone technique using random stationary sound to measure in-duct properties is given. Source characteristics are studied and a method of source identification is given. The importance of a measurement bandwidth, the length of duct and the significance of the forward intensity emanating from the source is shown. From the insights gained, the source-muffler interaction and its effect on insertion loss is explained. This is useful for the complete modeling of engine exhaust systems. Measurement of end reflection coefficients and in-duct intensity leads to experimental evidence of the existence of a low frequency flow induced absorption. This is applicable for studies in fan noise and its measurement methods.