ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

2aNS4. A modified transfer function method for source impedance in ducts.

M. G. Prasad

W. S. Kim

Dept. of Mech. Eng., Stevens Inst. of Technol., Castle Point on the Hudson, Hoboken, NJ 07030

Experimental characterization studies of source impedance have resulted in several direct and indirect methods. The transfer function (TF) method, which is a well-known direct method, has been quite successful in measuring source impedance. However, one important drawback of the TF method is that it fails for the case of low signal-to-noise ratio in the system. The noise in the system could be due either to the operating source (to be characterized) or to the flow in the system. A novel method to overcome the problem due to low signal-to-noise ratio is presented. A modified transfer function (MTF) is determined by using an additional measurement of transfer function of the system with a very low level of signal of the secondary source used in the method. Then the two transfer functions are used to estimate the MTF. The source impedance is then calculated using the reflection coefficient which is obtained using the MTF. This new MTF method has given very good results on a model electroacoustic speaker-duct system. A modified coherency function is used for error analysis. This MTF method can be used to measure the impedance of sources such as fans and blowers used in cooling systems.