Year of Publication

2011

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Engineering

Department

Mechanical Engineering

First Advisor

Dr. Tingwen Wu

Second Advisor

Dr. David W. Herrin

Abstract

Series impedances, including source and transfer impedances, are commonly used to model a variety of noise sources and noise treatment elements in duct systems. Particle velocity is assumed to be constant on the plane where the series impedances are defined. The research reported herein details investigations into measuring source and transfer impedance. Especially, the measurement and prediction of the transfer impedance of micro-perforated panel (MPP) absorbers is considered.

A wave decomposition method for measuring source impedance and source strength was developed that was purely based on acoustic concepts instead of the equivalent circuit analysis. The method developed is a two-load method. However, it is not necessary to know the impedances of either load a priori. The selection of proper loads was investigated via an error analysis, and the results suggested that it was best to choose one resistive and one reactive load.

In addition, a novel type of perforated element was investigated. MPP absorbers are metal or plastic panels with sub-millimeter size holes or slits. In the past, Maa's equation has been used to characterize their performance. However, Maa's equation is only valid for circular perforations. In this research, an inverse method using a nonlinear least square data fitting algorithm was developed to estimate effective parameters that could be used in Maa's theory.

This inverse approach was also used to aid in understanding the effect of dust and fluid contamination on the performance of MPP absorbers. In addition, an approach to enhance the attenuation of MPP absorbers by partitioning the backing cavity was investigated experimentally and numerically. Results indicated that partitioning improved the attenuating of grazing sound waves.

The effect of modifying both the source and transfer impedances on the system response was also studied using the Moebius transformation. It was demonstrated that the Moebius transformation is a mathematical tool that can be employed to aid in determining and understanding the impact of acoustic impedance modifications on a vibro-acoustic system.

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