A four load method for evaluation of acoustical source impedance in a duct

A new and simple method to evaluate the acoustical source impedance in a duct is presented. In the method (termed the Four Load Method) the sound pressure level data measured downstream for four different length ducts, as acoustic loads, is used. The simplicity of the method is due to the fact that neither cross-spectra nor complex pressure measurements are required. Also, a second signal generating source is not needed. In addition, the sound pressure level measurements can be made either inside or outside the finite length duct. In order to verify the method, the insertion loss of a simple expansion chamber is predicted by using the source impedance evaluated by the four load method. Excellent agreement is observed between the predicted and measured insertion loss. Some applications of the four load method to duct acoustic studies, including the mean flow case, are discussed.

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