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20 The preliminary testing results indicated that the array per- formed generally as expected for a truncated assembly. Lacking vertical symmetry, its directivity in both vertical and horizon- tal axes was found reasonable for an array of this size. The study team also concluded that the array setting in both vertical and horizontal directions (level and square) is important for image localization and reducing distortion of the images obtained. Overall, the preliminary testing served as a first practical veri- fication of the experimental beamforming array performance. 3.4 Proof-of-Concept Testing The proof-of-concept tests were conducted during two Figure 11. Preliminary testing of experimental 1-week periods in July 2006 at IT's Truck Development and microphone array (lower section). Technology Center (TDTC) in Ft. Wayne, Indiana. The TDTC is a controlled environment with test tracks and shop facilities locally in Virginia. The overall goal was to ensure that the for working on truck setups. IT provided access to its outdoor microphone array, data acquisition system, and software testing facilities, coordination with its ongoing testing activi- worked properly prior to the actual proof-of-concept test- ties, and a representative sample of trucks selected from the ing. The test was performed in a residential outdoor area, on vehicle test matrix. IT also provided a professional driver for a grassy lawn free of obstructions. Only the lower section of the tests and technicians to perform all work on the trucks. the array with 25 microphones was tested using a Mackie The first week of testing was conducted at the low-speed Model SRM450 loudspeaker with a pink noise generator, as passby sound pad. After a 1-week interval of preliminary data well as a lawnmower and a gas-powered string trimmer as processing and analysis, the second week of tests was con- noise sources in static positions in front of the array. These ducted at the high-speed proving grounds/endurance track. noise sources were placed alone and in combinations at Both test phases were documented using digital videotape varying distances from the array, both at on-axis and off- recording. Weather at the test sites was monitored to avoid axis locations relative to the array. The data were then testing during periods of rain or high winds. processed with the beamforming software, to make sure that the different sources could be correctly identified. One of 3.4.1 Low-Speed Tests the test settings is shown in Figure 11. The resultant images of the loudspeaker at two different frequencies are shown in The microphone array, data acquisition system, and other Figure 12. equipment were transported to and assembled at IT's low- OTO Hz Band levels Re 20muPa, SegNo = 7 OTO Hz Band levels Re 20muPa, SegNo = 7 6 6 31 32 4 4 30 31 29 30 2 2 28 29 27 sy/m sy/m 0 28 0 26 27 -2 -2 25 26 24 25 -4 -4 23 24 22 -6 23 -6 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 sx/m sx/m (a) 315 Hz (b) 740 Hz Figure 12. Loudspeaker image at frequency of (a) 315 Hz and (b) 740 Hz.

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21 Figure 14. Omni-directional speaker in front of microphone array at low-speed track. The speaker was secured to the rear frame of an Inter- national 4400 medium utility truck (with no flatbed; 2002 model year). The truck was powered with an International DT466 250 hp engine and was equipped with an automatic transmission, a single horizontal under-frame muffler and a horizontal tailpipe. The truck was equipped with four Goodyear G124 low-profile tires on the single drive (rear) axle and two Goodyear G159 tires on the steer (front) axle. The truck with mounted omni-directional loudspeaker is shown in Figure 15. To evaluate the array/beamforming system performance, a number of tests were performed with the truck stationary and moving both with and without the speaker (pink noise signal) Figure 13. Microphone array at low-speed track. turned on. These tests were intended to determine the array's ability to identify the truck noise sources (e.g., engine, tires, and exhaust) in comparison with the loudspeaker, as the speed sound pad designed for conducting standard truck passby noise-emission measurements. At this site, the trucks truck passed by the array at a constant speed. Several speeds, drive on a single lane with sealed, dense-graded asphalt pave- ment and a maximum speed limit of 35 mph (56 km/h). The microphone array was placed on the asphalt diamond mea- surement pad, parallel to the direction of truck travel, at a dis- tance of 20 ft (6 m) from the edge of the driving lane, as shown in Figure 13. Each measurement channel of the system was calibrated using a Brel & Kjr Type 4231 acoustic calibrator. Initially a series of stationary loudspeaker tests were per- formed, using a Mackie Model SRM450 loudspeaker and a CESVA Model BP012 omni-directional dodecahedron loud- speaker. Sound from a pink noise generator was played through the speakers at a high volume. The speakers were used for initial evaluation of the fully assembled system as known "point" sources placed at several on- and off-axis locations with different distances and heights in front of the array. One example of such a test setup is shown in Figure 14. After the stationary on-the-ground loudspeaker tests were Figure 15. The 4400 truck with omni-directional completed, the CESVA loudspeaker was placed on a truck. speaker.