Measurement of the reflection matrix of a discontinuity in a duct

An original technique of measuring reflection (or transmission) matrices of duct discontinuities is described. The technique is based on measuring the acoustic pressure by microphone pairs in two closed cross sections of a constant area circular duct, which separates incident and reflected modes after a modal decomposition by a Fourier-Lommel transform. Theory, experimental setup; and data treatment are presented. Then experimental reflection and conversion coefficients at an open duct end for plane wave and higher order modes are presented as an application and compared with analytical results. (C) 1998 Acoustical Society of America.