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34 Figure 39 allows assessment of the array's actual measured res- bels at steering angles approaching 45 degrees. In other words, olution performance compared with the predicted design per- when scanning the region of -6 to +6 m (-20 to +20 ft) in cross formance. The measured values for the -6 dB major and minor range and -6 to +6 m (-20 to +20 ft) in elevation at a stand-off axes coordinates of the spots at each frequency are shown as of 6 m (20 ft) from the track, the array will localize and accu- points, while the calculated values are shown as lines. This fig- rately define the magnitude of a stationary source. However, as ure illustrates that the acoustic focusing performance of the the following subsection will discuss, truck noise sources are array is substantially as predicted. more complex, and that complexity is apparent in the images. The array's ability to steer is illustrated in Figures 40 and 41. Finally, note that with the spherical source at a range of 20.2 m The steering at elevation of 1.24 m (4 ft) and an offset of 7.62 m (66 ft) the array still performs well, as shown in Figure 42. (25.2 ft) is shown in Figure 40 looking ahead and at 21.8 degrees to the right [xs = 3.05 m (10 ft)]. Similarly, Figure 41 shows, 3.5.2 Benchmark Measurements of for an elevation of 1.98 m (6.5 ft), the array's ability to steer Spherical Source on Moving Truck 45 degrees off axis [see lower images for xs = 5.8 m (19 ft) at an with Competing Truck Noise offset of 5.8 m (19 ft)]. These illustrations show that, for a spherical source, the array reliably images an omni-directional As a further test of the array steering, with the additional source at steering angles up to 45 degrees off axis, although at measurement complexity of the moving source and competing this angle, compared with 21.8 degrees (upper images), there truck sources, the omni-directional loudspeaker was strapped is a slight parallax. As seen in the lower image of Figure 41(a), to the bed of the 4400 truck on the low-speed track, as was the source is localized about 1.5 m (5.9 ft) closer than actually shown in Figure 15. Figure 43 mimics Figure 34 but with the located, with the maximum emission measured at a horizon- coordinate system and array overlaying an appropriately scaled tal cross range of xs = 4.3 m (14.1 ft) rather than the actual xs = photograph of a truck (in this case, the 9200i truck) stationed 5.8 m (19 ft). This discrepancy does not occur at 21.8 degrees. at the closest point of approach (CPA). As with the calibration Also, note that the measured spot sound levels (84 to 91 dB) are source, y = 0 corresponds to actual ground. within approximately ±3 dB of one another in all of these Figure 44 shows the image of the 4400 truck stationary and images at 922 Hz. idling with the spherical source mounted on the truck's bed Because the purpose of these measurements was to test and activated. The speaker spot and its ground reflection can localization, not to calibrate array levels, no attempt was made be seen clearly, as well as engine noise escaping through the to precisely set the source to the same sound level for each of wheel well and also by ground reflection. Note that in this case these measurements. This allowed the sound levels for all runs the engine noise reflection image appears about 0.5 m (1.6 ft) to be set to a nominal setting, which resulted in certain mea- below the ground plane. surement variability. Thus, in summary and within measure- Figure 45 shows the image of the truck moving to the right at ment repeatability, the array steering provided accurate 25 mph (40 km/h), still with the speaker activated. The spheri- localization and amplitude measurement to within a few deci- cal source and its reflected image are just as clearly represented, Frequency, Hz Figure 39. Measured and calculated total spot width (6 dB) at broad side for elliptical array at 6 m range.