Insight Into the Sound Field During a Direct Field Acoustic Test (DFAT®)

by Alex Carrella

Comparisons of the Structural Response of a Test Article Excited by DFAT™
Diffuse and Non-Diffuse Acoustic Fields

by Marcos A. Underwood, PhD
Tu’tuli Enterprises and MSI DFAT Chief Scientist

This paper discusses the results from Direct Field Acoustic Tests (DFAT™) performed at MSI DFAT to evaluate how effectively nearly diffuse (low coherence) and non-diffuse (high coherence) acoustic fields, using MIMO andMISO control, excite the structural resonances of a test article. The criteria used is to compare the frequency of resonances excited by the resulting acoustic fields to what Modal Analysis predicts the modes of vibration, their natural frequencies, and damping to be. Test results from using the various acoustic fields created by DFAT™ to excite the test article (test panel), in the form of PSDs obtained from accelerometers mounted on the test panel are analyzed. This analysis is used to determine the relative ability of the various acoustic fields to properly excite test articles by examining how well the resonances shown by the PSDs match the damped resonant frequencies predicted by Modal Analysis.

New Method Determines an Optimal Reference SDM for MIMO Testing

by Marcos A. Underwood, PhD
Tu’tuli Enterprises and MSI DFAT Chief Scientist

The paper presents a new method for vibration testing of articles such as satellites, aerospace subsystems, transportation subsystems, civil structures, or articles whose reliability in operation may be evaluated using either mechanical or acoustic vibration testing. The method can be used for direct field or reverberant acoustic test facility testing (acoustic) systems or multiple-exciter (mechanical) testing systems to perform vibration testing. The method improves the ability of Multiple-Input-Multiple- Output (MIMO) acoustic testing systems to create diffuse or other types of acoustic fields and MIMO mechanical testing systems to produce vibration responses conforming to an initial reference Spectral Density Matrix (SDM) specification in the least mean-square error (LMSE) sense. It provides an updated positive definite or semi-definite reference SDM that enables such tests to run with less error, using minimum required drive power, as a function of the initial definition of the reference SDM, by modifying its coherence and phase off-diagonal terms, to approximate initially defined off-diagonal terms in the LMSE sense, but maintaining its initially defined diagonal terms exactly, while accounting for physical limitations existing in the overall MIMO testing system.

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