Search

Spinning Mode Synthesizer is an effective engine noise test rig for aero-engine noise propagation and radiation research. Spinning mode generation is realized through amplitude modulation and phase shift between individual loudspeakers. The target spinning mode is not the primary mode. It is even submerged in the undesired modes due to the system error of artificial source generators, especially the magnitude and phase mismatch of loudspeakers. In this paper, a global calibration factor is used to minimize the effect of loudspeaker nonuniformity in mode generation. The mode generation process of the calibrated system is mainly divided into two steps. The first step is to identify the calibration factors of individual loudspeakers with an optimized microphone array, which is used primarily for mode analysis. No particular error sensor or additional devices is used in the measurement. The theoretical complex source strengths are multiplied by the calibration factor to calibrate artificial source generators. The second step is to carry out mode generation experiments using the calibrated system, and Radial Mode Analysis is used to analyze the mode components in the duct. A spinning mode synthesizer with three axially spaced rows of loudspeakers is introduced, and each row consists of 12 excitation units. The maximum controllable circumferential mode order is m = 9, and the maximum controllable radial mode order is n = 2. It has been proven that the calibrated system can generate the target mode more precisely, and the mode coefficient signal-to-noise ratio ( SNR A) of any propagable mode is greater than 10 dB in the operating frequency range. [ABSTRACT FROM AUTHOR]

Acoustic imaging from non-synchronous measurements is a powerful method that can achieve excellent performance of a densely sampled large array by using an arbitrary prototype array. It has attracted significant interest recently since it is beyond the fundamental limitation of the working frequency determined by the size and microphone spacing of an array. One basic assumption of the non-synchronous measurements theory is the spatial continuity of the acoustic field, which is implemented by introducing the spatial basis function. Nevertheless, a comprehensive investigation of spatial basis function in the context of non-synchronous measurements is still missing. Two kinds of methods are proposed to construct spatial basis functions based on the homogeneous Helmholtz equation solution in this paper. One is the generalized harmonic polynomials (GHPs) and the other is the plane wave expansion (the quasi-uniform plane wave and the non-uniform plane wave). Comparisons of their spectral matrix completion errors show that the proposed construction methods of spatial bases can both yield low errors. Moreover, the construction of the proposed bases is simple to be implemented with only one dimension parameter to be chosen and suitable for the complex sound field. The effectiveness of the proposed bases in the non-synchronous measurements is also validated with simulations and an experiment. [ABSTRACT FROM AUTHOR]