By using the four load method, the one-port source characteristics can be determined in the duct system and a modification is made to avoid the instability problems occurring in the conventional four load method. In this method, the input data set is the same with the conventional four load method, i.e., the acoustic power spectra measured inside or outside the duct for several acoustic loads are used, while an error function based on the linear time-invariant source model is obtained. It is numerically shown that the method is less sensitive to input errors compared to the conventional four load method. The two methods are utilized to obtain the source parameters of a loudspeaker and a centrifugal fan using various types of loads including the open tube. The estimated impedance values are compared with those obtained from the direct method. As a direct measurement method, the three-microphone method is employed. It is observed that the conventional four load method results large errors, whereas the present modified four load method yields far better results than the conventional method using the same loads.
