Localization of broadband acoustical sources in the cylindrical duct via measurements outside the duct end

Abstract

Identification of acoustical sources inside the duct could significantly guide the low-noise design of turbine engines. Current methods of inner duct source identification are executed based on measurements by microphones mounted at the duct wall, which would lead to a poor spatial resolution for broadband source identification and even failure to locate the source approaching cut-on frequencies. In comparison to conventional methods using the duct wall-mounted microphone array, a methodology based on an out-duct placed microphone array is proposed to identify in-duct broadband sources, which could locate sources with better localization accuracy and spatial resolution in a broadband range containing cut-off frequencies. The forward model between a monopole source inside the duct and an observation outside the duct is established, including acoustical propagation in the cylindrical duct and radiation from the duct termination. Numerical simulations and experiments are implemented to validate the method. Both a duct wall-mounted array with multiple rings and an annular array placed exterior to the duct are considered for measurements, after which identification methods including beamforming, DAMAS, and the equivalent source method are adopted to locate the in-duct broadband source. Broadband azimuthal results indicate that the proposed method is with remarkably fewer side lobes and better accuracy of localization in the circumference direction, and broadband radial results indicate that the proposed method is with fewer side lobes in the radial direction.