25 Performing Direct-Field Acoustic Test Environments on a Sandia Flight System to Provide Data for Finite Element Simulation 269 Fig. 25.2 Sandia Flight System and speaker configuration controlling the top speaker row to a different test specification than the bottom two speaker rows. Reflective panels on the sides of the octagonal stack increased the overall level inside the stack and provided some additional smoothing of the field in contrast to a design with open walls. 25.4 Test Setup As discussed in the test design analysis results, the DFAT configuration consisted of suspending the Flight System vertically in the center of a 10-foot diameter octagon of speakers, 1 foot above the floor. Four towers of the VT-99 cabinets (lowmid-high frequencies) were stacked three high and placed 90-degrees apart, as shown in the simplified diagram of Fig. 25.2. In order to provide more speaker coverage along the length of the Flight System, 2-foot long extension bars were installed in-between the short VT-99 cabinets, resulting in an 11-foot high stack. The six VS-Q subs (low frequency speakers) were stacked alternating one and two high, and were placed on the floor at the 45-degree locations (not shown in diagram). Analysis determined that the low frequency VS-Qs were not as location-dependent as the VT-99s. The reflective panels required by the test design were provided by ¾-inch thick plywood, installed in the open spaces of the speaker configuration, such as on the extension bars between the VT-99s, and the space above the VT-Qs. Adding panels between loudspeaker cabinets was predicted to increase the overall sound pressure, though the increase in reflection may affect the ability of the control system to achieve high spatial pressure gradients. The Flight System was suspended using a specially-designed test fixture that incorporated a pivot joint near the center of gravity of the test item, when suspended by a strongback (shown in as purple in Fig. 25.2). With this fixture, instrumentation could be attached to the Flight System in a horizontal configuration at a comfortable working height. Once ready for testing,
RkJQdWJsaXNoZXIy MTMzNzEzMQ==