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OCR for page 17
17 5 Approximate Combined Pattern: 0 Ellipse 1 Variation in SPL (dB) -5 Range - Limited Ellipse 2 -10 Ellipse 1 Circle -15 -80 -60 -40 -20 0 20 40 60 80 Cross-Range, ft Figure 7. Approximate profile of sound pressure level for simple source passing by Ellipse 1 array at 250 Hz. Table 1 summarizes these results for 250 and 465 Hz. The 3.2.4 Design Conclusions frequency of 465 Hz was used because it was one of the analy- sis bands used in the 2005 Caltrans study, and the study team The following conclusions were drawn from the design anticipated that some future design evaluations might need to analysis described previously for the frequency band of highest be compared to data obtained at this frequency. The numbers A-weighted sound levels of emissions (during cruise): in parentheses are the lobe widths for the array alone, without benefit of the spreading losses. These values are theoretical, A 70-element elliptical array provides adequate aperture and comparison with the experimental ones is provided in with acceptable side lobe suppression. Section 3.5.1 (see Figure 39). The lobe width for a hypotheti- The aspect ratio 1.7 of an elliptic aperture array provides cal point array (a single omni-directional microphone) is also beam patterns that are geometrically similar to the array shown in the table for comparison. shape at all frequencies of interest. 5 Approximate Combined Pattern: 0 Ellipse 2 Variation in SPL (dB) -5 Range - Limited Ellipse 2 -10 Ellipse 1 Circle -15 -80 -60 -40 -20 0 20 40 60 80 Cross-Range, ft Figure 8. Approximate profile of sound pressure level for simple source passing by Ellipse 2 array at 250 Hz.

OCR for page 17
18 Vertical directivity provides a beam focus spot 50 in. (1.3 m) With regard to low-frequency performance, the following wide (-6 dB-down) at 465 Hz. conclusions were drawn: The array provides excellent side lobe suppression of approximately -14 dB over the range of 250 to 2250 Hz, With a 12 ft (3.7 m) major (vertical) axis of the ellipse, the and approximately -11 dB at 8000 Hz. vertical beam half height (-6 dB) is about 4.5 ft (1.4 m) at A minimum spiral angle is needed to suppress side lobes, 250 Hz. This beam size will allow imaging resolution to the so element supports may be radial. upper half of a large truck cab. Side lobes at high frequencies seem well distributed (i.e., The horizontal effective beam width during passby, includ- appear amorphous), which minimizes the likelihood of ing both beam width and spherical source spreading loss, unwanted highlights that may lead to false source images. would be about 9 ft (2.7 m) for the minor axis [horizontal dimension of 4.1 ft (1.2 m)]. Table 1. Width of net focus spots for array alternatives including cross-range spreading. The elliptical array described herein represents a new result of this study. Based on the design study for the Ellipse Frequency Lobe Width (ft) for Array Design* Direction 2 array optimized for the current application, a 70-element (Hz) Circular Ellipse 1 Ellipse 2 Point 250 Vertical 19 14.5 10.8 30 array (schematically shown in Figure 9) was selected for Horizontal 18 (19) 21 (28) 25 (43) 30 465 Vertical 9 7.3 5.6 30 experimental engineering and implementation through the Horizontal 8 (9) 12 (13) 15 (18) 30 proof-of-concept testing, as described in the following * Numbers in parentheses represent values for the array alone sections. 80 60 40 20 ly, in 0 -80 -60 -40 -20 0 20 40 60 80 -20 -40 -60 -80 lx, in Figure 9. 70-element elliptical array.