In-Duct Acoustic Source Detection Using Acoustic Imaging Techniques

Noise source localization in rigid ducts is addressed in this paper. As a first step towards source localization in actual turbofans, source detection techniques are tested in an academic test-bench using a 17 cm diameter aluminum duct and a small broadband loudspeaker without flow. Two circular arrays of 24 microphones are used to monitor the acoustic pressure on the duct surface. Acoustic imaging techniques are applied to recover the loudspeaker location. Analytical duct propagation functions are included in the model so that the modal behavior is fully taken into account. Two localization methods are used : the standard beamforming algorithm and a global transfer matrix inversion method (Equivalent Source Method). The source localization techniques are efficient in the higher frequency range, as there are enough cut-on modes for the spatial source description. In the lower frequency range, the physical loss of information due to cut-off modes make the localization resolution poorer. It is seen that the Singular Value Decomposition of the transfer matrix is equal to the number of cut-on modes, making ESM a suited method for duct source localization.