Experimental evaluation of a new two-microphone method for the mode detection and radiation of broadband noise in ducts

摘要

This paper attempts to experimentally evaluate a new two-microphone measurement technique for determining the modal amplitude distribution and far-field radiation of the broadband noise from hard walled ducts. The new technique is based only on the complex coherence function of the acoustic pressure measured by two closely spaced microphones located at the duct wall. Particularly, the cuton ratio, rather than an index couple of azimuthal and radial mode orders (m, n), was introduced to represent the behaviour of individual propagating modes. The hard-walled circular duct was excited at one end by a diffuse sound field, and the other open end is terminated within an anechoic chamber with which to measure the radiated sound field. The measured complex coherence was used to infer the mode amplitude distribution and far field directivity, based on the underlying Fourier transfer relationship. The results verified the good agreement between direct measurements of sound power and directivity.