Launch Vehicle Acoustics : Paper ICA 2016-684 Projection algorithms for target spectrum matrix definition in MIMO direct field acoustic control

Reducing the cost and risk of spacecraft acoustic qualification tests make many of the space contractors prefer to conduct Direct-Field-Acoustic-eXcitation (DFAX) tests instead of Reverberant-Field-Acoustic-Tests (RFAT). The first consists in setting a loudspeaker array around the test specimen. DFAX test does not demand the construction of a large reverberant test facility dedicated exclusively for acoustic qualification as RFAT. However, reliably reproducing the characteristic diffuse field and the uniform sound pressure spatial distribution of RFAT still represents a challenge for DFAX. Close loop control methodologies were proposed to achieve this goal. Multiple-Input-Multiple-Output (MIMO) strategies seem to give more flexibility than Single-Input-Single-Output (SISO) to handle the acoustic wave interferences. Hence, they have enhanced the diffusivity and uniformity of the acoustic field for DFAX. However, MIMO demands a more complex test configuration, e.g. the test requirement must be defined as a target spectral density matrix (SDM) instead of a single power spectrum density (PSD) profile. Besides, a centralized MIMO controller is computationally more expensive than SISO. Regarding the target SDM definition, a recent publication has introduced the energy-sink phenomenon. It was shown, theoretically and experimentally, how different target definitions lead to different control performances. In this research new target definition approaches are described. They are based on the projection of the test requirement into the subspace of achievable pressure responses. Experimental results show how the projection approaches improve the control performance of rectangular systems with MIMO control strategies.