Abstract
A lined duct is often used to reduce noise generated from machinery. The performance of the duct can be predicted by means of numerical analysis such as BEM or FEM. In order to predict the performance by using numerical analysis, sound absorbing materials ling the duct are usually modeled as a surface impedance. However, in the case when the cross sectional area is smaller than the wavelength, the accuracy of the prediction deteriorates because the locally-reacting assumption is not effective. When the absorbing materials are modeled correctly by Biot-Allard theory, the performance of the duct can be predicted more accurately even in that case. However, it is difficult to apply to a practical issue since a lot of parameters are needed to obtain and the simulation model is too complex. In this paper, a new method of identifying the surface impedance of the absorbing material is proposed which is especially effective in the narrow duct case and the validity is verified in some cases.
