Evaluation of acoustical source impedance in a duct with low signal to noise ratio using a modified transfer function method

Acoustical source characterization in ducts has been an important topic because of its extensive applications in acoustical modeling of duct systems. Owing to the complexities of sound generation mechanism more often designers use experi¬mental methods for source characterization. Traditional empirical methods for the source characterization are either direct or indirect methods. Commonly used direct and indirect methods are the transfer function method as specified in American Society for Testing and Materials (ASTM) standards and the four-load method respectively. It is well known that the transfer function method fails in case of low signal-to-noise ratio (SNR). In this study, a new direct method called modified transfer function method is proposed to overcome the challenges due to low SNR. Relevant tests are carried with an electro-acoustic driver to verify the new method. The experimental results indicate that the proposed method is more effective than the present direct methods such as transfer function method. A modified coherency as a self-error estimator is also proposed to quantify reliability and confidence factor when the new modified transfer function method is used for the source characterization.