A detailed understanding of the sound propagation and transmission within the engine and adjacent ducts is mandatory for the development of efficient noise reduction techniques for the tonal sound field produced by the turbomachinery components of aircraft engines. For this purpose, experimental acoustic investigations are needed. In the first part of this paper, an acoustic excitation system for the generation of acoustic spinning modes with circumferential mode order one and varying radial mode order, as well as a microphone array optimized for a radial decomposition of the sound field have been systematically designed.
To verify the excitation method and the design of the excitation system, corresponding experimental measurements are carried out in an acoustic wind tunnel. Amongst others, the sound power of the specific excited acoustic modes of order (1,0) and (1,1) are compared with the respective powers achieved with a non-specific sound field excitation. To test the range of flexible use of the sound generator, measurements are carried out over a wide frequency range. It is shown that the intended modes can be controlled at the design frequency of the sound generator as well as off-design frequencies. However, the dominance of the excited modes strongly depends on the number of cut-on modes and the excitation frequency as non-linear resonance effects may interfere. Furthermore, the benefit of an increased number of loudspeaker rows for stable mode excitation is discussed. The experimental results are supported by numerical simulations.