Chapter 22 Evaluation of Microphone Density for Finite Element Source Inversion Simulation of a Laboratory Acoustic Test Ryan Schultz and Tim Walsh Abstract Simulation of the response of a system to an acoustic environment is desirable in the assessment of aerospace structures in flight-like environments. In simulating a laboratory acoustic test a large challenge is modeling the as-tested acoustic field. Acoustic source inversion capabilities in Sandia’s Sierra/SD structural dynamics code have allowed for the determination of an acoustic field based on measured microphone responses—given measured pressures, source inversion optimization algorithms determine the input parameters of a set of acoustic sources defined in an acoustic finite element model. Inherently, the resulting acoustic field is dependent on the target microphone data. If there are insufficient target points, then the as-tested field may not be recreated properly. Here, the question of number of microphones is studied using synthetic data, that is, target data taken from a previous simulation which allows for comparison of the full pressure field—an important benefit not available with test data. By exploring a range of target points distributed throughout the domain, a rate of convergence to the true field can be observed. Results will be compared with the goal of developing guidelines for the number of sensors required to aid in the design of future laboratory acoustic tests to be used for model assessment. Keywords Acoustics • Direct field acoustic test • Source inversion • Acoustic finite element • Modal assurance criterion Nomenclature DFAT Direct Field Acoustic Test FE Finite Element MAC Modal Assurance Criterion PSD Power Spectral Density ROL Rapid Optimization Library SPL Sound Pressure Level, ref. 20 Pa 22.1 Introduction System structural response in acoustic environments is important to understand for a variety of aerospace structures because these can be significant environments. Therefore, modeling system response to acoustic environments is required, as well as an assessment of model performance to capture the response to that distributed pressure-type loading. When performing model calibration, validation or assessment simulations against some laboratory test data, understanding the loading is critical. Traditionally, a structural dynamics finite element (FE) model will be calibrated using modal test data. In that Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy National Nuclear Security Administration under Contract DE-AC04-94AL85000. R. Schultz ( ) Analytical Structural Dynamics Department, Sandia National Laboratories, Albuquerque, NM 87185, USA e-mail: rschult@sandia.gov T.Walsh Computational Solid Mechanics and Structural Dynamics Department, Sandia National Laboratories, P.O. Box 5800 – MS0840, Albuquerque, NM 87185, USA © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_22 231
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