268 E.C. Stasiunas et al. 25.2 DFAT with MIMO Control Direct-field acoustic testing (DFAT) is an acoustic test method that consists of exciting a test item the direct acoustic field— the portion of the field in which sound waves have not undergone any reflection—of a sound source. In practice, the direct field is created by surrounding a test article with a wall of specially modified concert-style loudspeakers, and placing multiple control microphones in-between the test article and speakers. A closed-loop control system uses the control microphone measurements to adjust the speaker outputs in order to obtain the desired test specification. Multiple response microphones are placed around the test item to better characterize the acoustic field [1]. When applying multi-input multi-output (MIMO) control to DFAT, the multiple independent controls and corresponding drives of the control system allow for detailed tailoring of the resulting acoustic field. Since each control microphone has an independent test specification as well as a defined phase and coherence between them, it is possible to create both uniform acoustic fields and shaped acoustic fields, depending on the control parameters. The uncorrelated outputs used in MIMO control have also been shown, in analytical models and in experiments conducted in the industry, to reduce both standing waves and spatial variability in the resulting acoustic field, which is highly desirable for hardware safety and realistic acoustic environment simulation [2, 3]. 25.3 Test Design Sandia’s current acoustic test system consists of twelve MSI VT-99 speaker cabinets and six MSI VS-Q speaker cabinets. The VT-99 cabinets contain low, mid, and high frequency speakers and are driven by six acoustic amplifiers circuits; the VS-Q cabinets contain low frequency speakers and are driven by 3 acoustic amplifier circuits. In the large-scale Sandia reverberation chamber, this acoustic system is capable of achieving an overall sound pressure level (OASPL) of approximately 146 dB. Outside of the reverberation chamber in a DFAT configuration, levels of 136 dB OASPL have been achieved. While this DFAT level is much lower than the realistic acoustic loading environment, it was still a desirable test method due to the ability to obtain shaped acoustic fields, which is difficult to achieve in a reverberation chamber (due to acoustic reflections). With the given number of speakers and independent drives available, analytical simulations were performed to determine the optimal test configuration. Vertical and horizontal test item orientations were examined as well as various speaker stack configurations. Simulation output data in the form of acoustic sound pressures levels (SPL) throughout the stack volume, as shown in Fig. 25.1a, was analyzed to assess any spatial dependence of sound pressure level. The final design, a vertical test article surrounded by an octagon of speaker stacks with reflective panels between speakers as illustrated in Fig. 25.1b, was chosen because this allowed for a fairly even field in the angular direction while allowing for a length-wise gradient by Fig. 25.1 (a) Pre-test analysis SPL and (b) final test design
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