Experimental Characterization of Noise Sources for Duct Acoustics

Many industrial applications involving the acoustics of ducting or piping systems require an improved description of noise sources, so that a better prediction and evaluation of system performance can be achieved. Some examples are (a) the computer simulation of intake and exhaust muffler performance, (b) the control of pressure pulsation in fluid piping systems due to control valve flow noise or reciprocating compressors, and (c) predicting pressure fluctuations in heating and air-conditioning system ductwork due to various types of fans or blowers. This paper describes two applications of a well-known linear electrical analogy for obtaining experimentally the internal acoustical source impedance and the strength of the source, both parameters which are independent of the acoustic system load impedance. Two methods are compared, one which utilizes direct measurement of source impedance with the source inactive, and a two-load method from which the source impedance is calculated from measured pressures, with the source in operation. Various applications are presented using a speaker, compressor, engine, and centrifugal fan as noise sources connected to different load impedances. Comparisons are made to highlight the relative merits of these two approaches and to demonstrate the degree of accuracy that can be obtained in predicting noise levels in any arbitrary linear acoustic system, using the measured source parameters. The methods are simple in concept and in application, and while they do not often describe the physical nature of noise sources, they do offer a way to bypass the much more difficult problem of modelling the source theoretically.