7. On the Estimation of Structural Admittances from Near-Field Acoustic Holography Using a Dynamic Substructuring Approach

Near-field acoustic holography (NAH) provides a reconstruction of the normal particle velocity radiated by the source of interest on a nearby surface. Reconstructing structural admittance based on normal velocity establishes NAH as alternative to the conventional methods such as accelerometers or laser vibrometers. Measuring vibrating object using microphone array offers high-resolution measurements and with this the possibility to obtain spatially dense displacement field of the structure. Although useful, the method’s ill posed nature needs to be considered which introduces numerical noise to the reconstructed result. In this paper an alternative approach for improved estimation of FRFs from NAH is introduced by using dynamic substructuring methodology. Recently developed System Equivalent Model Mixing (SEMM) method is proposed that enables mixing of response models of the same structure into one hybrid model. In the hybrid formulation, each model contributes its own advantages to yield best combination of the two. NAH introduces a high spatial resolution to the hybrid model whereas discrete laser vibrometer measurements improve the accuracy of the reconstructed displacement field. Experimental validation of methodology is performed on a freely supported structure that is excited using electrodynamic shaker. Noisy acoustical environment introduces spurious peaks in FRFs acquired using NAH. The laser vibrometer measurement is not affected by a noisy environment, as only the structural response is measured. It is shown that hybrid model provides a more consistent FRFs of the structural response, especially with regard to the amplitude in the resonance regions.