Measurement of radial and circumferential modes in annular and circular fan ducts

The distribution of acoustic energy among the modes in a duct is important in determining the source distribution, the radiation characteristics and the effect of any acoustic linings. This distribution can be determined by processing the microphone signals obtained over planes perpendicular to the duct axis. In an annular or circular duct, the circumferential analysis is simply a Fourier transform, but the radial analysis is in terms of Bessel functions. Methods of determining the radial mode distribution are discussed and results of such analyses on idealized distributions are presented. The use of the most suitable methods of analysis is then demonstrated in measuring the modal distribution of the distortion generated noise of an isolated fan.