Laboratory for Dynamics of Machines and Structures 
Improved estimation of FRFs from NAH using dynamic substructuring methodology

Diploma thesis, LADISK, Faculty of Mech Eng, Univ of Ljubljana, March 2020
Measurement of two-dimensional sound pressure field can be used to determine three-dimensional wave field using Nearfield Acoustic Holography. Reconstructed displacement field can be estimated by knowing the relation that connects source sound pressure field and particle velocity. The method serves as alternative to conventional measuring methods to obtain the vibration response of the structure. Method offers high spatial resolution however it is highly sensitive to background noise and inaccuracies in the microphone position. In addition, ill-posed inverse acoustic problems introduce additional numerical noise into the system. A novel methodology for improved estimation of full-field frequency response functions from acoustic holography is proposed in this thesis by using dynamic substructuring methodology. Recently developed System Equivalent Model Mixing method is used to enable mixing of dynamic 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. Nearfield Acoustic Holography introduces a high spatial resolution to the hybrid model, while discrete laser measurements improve the accuracy of the reconstructed displacement field. Finally, improved robustness and applicability of the method is presented using experimental validation.