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31 3.5 Proof-of-Concept Test Results Y This section discusses the proof-of-concept test results in four categories: Array Spherical Source · Beamformer calibrations with the omni-directional (spher- Elevation of Source's Center ical) source, in which in-situ frequency response and steer- ing characteristics of the array are established; X · Benchmark localization measurements with a moving Artifact truck with and without the spherical source on board, in which the array's ability to discriminate among multiple Principal Image of sources is verified and the rudiments of developing tempo- Source ral histories during passby are established; · Comparisons with the intensity measurements made on Z stationary trucks, in which the array's ability to localize the principal (i.e., largest magnitude) source on the truck and Figure 34. Array and typical image orientation for identify the lesser sources is established; and spherical source emission. · Example results from low- and high-speed track passbys, in which localization and characteristics of individual noise sources for the tested moving trucks are evaluated. characteristics to be found in the images that will be discussed in this section. 3.5.1 Beamformer Calibrations Without going into details yet, note that the ground plane with Spherical Source is at y = 0 and the array is positioned above it. The physical sources are all also above ground, with all reflected image The beamformer characteristics of the array were calculated sources below the ground plane. These characteristics are for comparison to the test results using a theoretical model of shown in Figure 34. All images of a single acoustic source will the received signals at each array element with a simple source therefore show a single "hot spot" above ground with its mir- above ground with ground reflection, for which the ground ror image below ground, as illustrated. Grating lobe effects in reflection coefficient was assumed to be between 0.7 and 1.0. the array, a coherent (localized) background source, or a non- Figure 33 shows a side-view diagram of the array, spherical specular reflection may contribute an artifact at low level, also sound source and its image in the ground half space, direct as illustrated. acoustic propagation path, and two example ray traces for Figures 35 through 38 show corresponding calculated and the reflected path that were used in the analytical model. Fig- measured images of the spherical source at a series of frequen- ure 34 is an illustration of the coordinate axis orientations of cies that serve to define the spatial resolution of the measure- the source and array planes. The latter figure also labels the ment array. In these figures and all other source images presented in this section, numbers in the color bar legend indi- cate approximately equivalent one-third octave band sound levels in decibels. At 922 Hz, for example, Figure 35 shows Array Plane elliptical spots whose major and minor axes are complemen- tary to those of the array. The vertical (-6 dB) width of the spot Direct Acoustic Path Spherical (Point) is about 0.38 m (1.2 ft), while the horizontal width is about Source 0.67 m (2.2 ft) at a range of 5.8 m (19 ft) which corresponds approximately to the side of the truck or the closer wheel track Reflected Path Ys and an elevation of 1.98 m (6.5 ft). Figures 36 and 37 show examples at lower and higher frequencies, respectively, for the same source at this range and elevation. The two low frequen- Ground Plane (GRC) cies define the lower limit of the array performance. The two high frequencies define the upper limit of the array perfor- Image mance. These figures show that the array performs adequately between approximately 250 and 2000 Hz. Figure 33. Side view of spherical source and its Figure 38 shows good focusing at a longer range of 7.62 m image in the ground half space. (25.2 ft), which corresponds to the center of the passby lane.

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32 Measured Equivalent Ls, OTO at F=922.8516Hz Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 0.7 6 91 6 91 90 90 4 4 89 89 2 88 2 88 87 87 sy/m sy/m 0 0 86 86 85 85 -2 -2 84 84 -4 83 -4 83 82 82 -6 -6 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 sx/m sx/m (a) Measured (b) Calculated Figure 35. Images of (a) measured and (b) calculated signals for spherical source emission at 922 Hz [source elevation 1.98 m (6.5 ft), array stand-off at road side 5.8 m (19 ft)]. Measured Equivalent Ls, OTO at F=230.7129Hz Theoretical Equivalent Ls, OTO at F=230.7129Hz and GRC= 1 6 100 6 100 99 99 4 4 98 98 97 97 2 2 96 96 sy/m sy/m 0 95 0 95 94 94 -2 -2 93 93 -4 92 -4 92 91 91 -6 -6 -4 -2 0 2 4 6 231 Hz -6 -6 -4 -2 0 2 4 6 sx/m sx/m Measured Equivalent Ls, OTO at F=276.8555Hz Theoretical Equivalent Ls, OTO at F=276.8555Hz and GRC= 0.65 6 6 97 97 96 96 4 4 95 95 2 94 2 94 93 93 sy/m sy/m 0 0 92 92 -2 91 -2 91 90 90 -4 -4 89 89 88 88 -6 277 Hz -6 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 sx/m sx/m (a) Measured (b) Calculated Figure 36. Images of (a) measured and (b) calculated signals at two low frequencies.

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33 Measured Equivalent Ls, OTO at F=1707.2754Hz Theoretical Equivalent Ls, OTO at F=1707.2754Hz and GRC= 0.7 6 6 90 90 4 89 4 89 88 88 2 2 87 87 sy/m sy/m 86 86 0 0 85 85 -2 84 -2 84 83 83 -4 -4 82 82 -6 81 -6 81 -6 -4 -2 0 2 4 6 1707 Hz -6 -4 -2 0 2 4 6 sx/m sx/m Theoretical Equivalent Ls, OTO at F=2122.5586Hz and GRC= 0.7 Measured Equivalent Ls, OTO at F=2122.5586Hz 6 6 85 85 4 84 4 84 83 83 2 2 82 82 sy/m 81 sy/m 81 0 0 80 80 -2 -2 79 79 78 78 -4 -4 77 77 -6 76 -6 76 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 sx/m sx/m 2123 Hz (a) Measured (b) Calculated Figure 37. Images of (a) measured and (b) calculated signals at two high frequencies. Measured Equivalent Ls, OTO at F=922.8516Hz Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 0.7 6 90 6 90 4 89 4 89 88 88 2 87 2 87 86 sy/m 86 sy/m 0 0 85 85 -2 84 -2 84 83 83 -4 -4 82 82 -6 81 -6 81 -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6 sx/m sx/m (a) Measured (b) Calculated Figure 38. Images of (a) measured and (b) calculated signals for spherical source emission at 922 Hz [source elevation 1.98 m (6.5 ft), array stand-off at road side 7.62 m (25 ft)]. 1ST REVISE