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48 (a) (b) (c) Figure 59. Source distribution at 975 to 980 Hz for various models of the 9200i truck with muffler traveling at 31 to 35 mph. The main point of these figures, however, is the nature of the for images (a) and (b) in order to illustrate clearly the differ- exhaust noise. As noted previously, exhaust noise is marked by ences between these noise sources. As a result, the other sources high tonal content and the source is localized just forward of that remain nearly the same in both compared cases, such as the stack--above the cab. The sound is conjectured to actually the tirepavement noise in Figures 60 and 61, appear to be of be amplified by the aerodynamic fairing, which acts as a res- much different strength if judged simply by the image color onator of the noise emitted from the stack. The source is per- rather than by the actual values of the measured sound level. haps a series of volume resonances of the hollow fairing cavity excited by a primary exhaust efflux source. The fundamental 3.5.4.4 Evaluations of the Truck Acoustic half-wave resonance of a 6 ft (1.8 m) cavity would be at about Source Level During Passby as a Function 90 Hz, consistent with the observed approximate lowest peak of Vertical Elevation frequency of the exhaust noise autospectrum (Figure 57). The sound from the stationary truck was also noted to have these The acquired acoustic data together with the recording of features, as discussed previously for Figure 48. the exact truck position with time during the passby can be Note that the foregoing discussion for this truck is concerned used to generate a map of the sound level as a function of ver- primarily with evaluation of the engine compartment and tical elevation in the truck plane. This map provides a time his- exhaust noise. In Figures 59 through 62, the color bars indicat- tory of the vertical density of sources at each time increment ing the measured sound levels show a range of approximately during the passby. The time history can then be readily inter- 10 dB for each image; however, the ranges are of various values preted by relative motion as location along the truck. The cal-

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49 (a) (b) Figure 60. Source distribution at 665 Hz for the 9200i Eagle truck: (a) with muffler and (b) without muffler. (a) (b) Figure 61. Source distribution at 709 Hz for the 9200i Eagle truck: (a) with muffler and (b) without muffler.

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50 (a) (b) Figure 62. Source distribution at 975 Hz for the 9200i Eagle truck: (a) with muffler and (b) without muffler. culation can be done for each frequency giving a data set of beam widths plotted in Figure 39 [i.e., 3 m (10 ft) and 2 m sound pressure level as a function of time during passby (or (6.5 ft) for the frequencies of 937 Hz and 1406 Hz, respec- location along the truck), elevation in the truck plane, and fre- tively]. Also, noting the image plot for this data set in Figure 44, quency. This type of processing was fully developed for analy- when the array is trained to y = 0, the reflection from the sis of roadside measurements. During proof-of-concept testing, road surface is included in the aperture of the array, espe- preliminary examples were generated for the 4400 truck with cially at 900 to 1000 Hz. Thus in Figure 63(a) there is little and without the onboard spherical source activated. To per- variation between levels in the pixels at y = 0 and 0.5 m (0 and form the calculation the array beam was trained to x = 0 (the 1.6 ft), while there is noticeable difference in Figure 63(b) horizontal position dead ahead) and to a series of vertical posi- for which the total beam width is only 2 m (i.e., 1 m) [6.5 ft tions y = yindex above the road surface, and acoustic spectra was (i.e., 3.25 ft)]. Given that the truck is stationary, the source obtained as a function of time increment during the passby for level distribution is constant over time and appears as a each elevation. This procedure gives a timefrequency record stripe along the y = 1 m (3 ft) pixels. of all sound arriving at the array center from each of the tar- The frequencies discussed in these figures are slightly dif- geted locations on the truck. The process is repeated for the ferent than those discussed in Figures 44 through 46 because sequence of values of y = yindex up to a maximum that, in this a narrower bandwidth was used here for which the FFT gives case, was set to about 4 m (13 ft) above the surface of the road. narrower analysis bands. The vertical scans were used for the The increment for indexing is about 0.5 m (1.6 ft) and a total roadside measurements to provide vertical distributions of of nine vertical positions are used. A-weighted one-third octave band levels using summations Figure 63 shows a time record of the vertical scans for the of narrower bandwidth spectra, so the data presented here are stationary, idling truck with its spherical source activated. The preliminary to that application. By bracketing the 922 Hz source was 4 ft (1.2 m) above the road surface. Because the point of Figures 45 and 46, say, differences between 922 Hz truck is stationary, the source levels at the different elevations and 937 Hz for this truck were determined to be negligible. are essentially constant except for possible small propagation For the 4400 truck passing to the right at 25 mph (40 km/h) variability associated with the outdoor acoustic field. These with the onboard source activated, Figure 45 shows an image vertical scans show a concentration of source level in the y = that localizes the spherical source at a point about 4.6 m (15 ft) 1 m (3 ft) pixels with markedly reduced levels in the y = 2 m behind the bumper. Thus, when the truck is vertically scanned (6.5 ft) (and greater) pixels. The vertical discrimination is during passby in the manner described previously, the map- smaller for the levels at 1406 Hz than at 937 Hz, given the ping of levels that is shown in Figure 64 is obtained. In this case,

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51 f = 937 Hz f = 1406 Hz (a) (b) Figure 63. Vertical scan for the 4400 truck stationary, the engine at 2200 rpm, and the spherical source activated at frequency: (a) 937 Hz and (b) 1406 Hz. The time axis is in seconds relative to an arbitrary reference. Figure 64. Vertical scan at 937 Hz for the 4400 truck passing to the right with the engine at 2200 rpm and the spherical source activated. The time axis is converted to bumper position relative to the array center. Truck photo is reversed on the horizontal scale with its ~6 m (20 ft) wheel base indicated. Vertical axis of the acoustic scan is expanded with the 1 m mark on the truck indicated with the line.

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52 the time scale has been interpreted for the reader as position vicinity of the wheel well and reflection that does not appear along the truck. (The scale has also been reversed, so the image in Figure 45, probably due to effects of directivity at the time is presented as if the truck were passing to the left.) This is done instant of this image. However, Figure 64 shows a distribu- by taking advantage of the known translational velocity of the tion of sound sources in the forward extremity of the truck, truck by which the bumper position is known at all times dur- because this depiction of the data gives the continuous com- ing the event. Thus as the truck passes, the array scans the plete map of the received sound versus time, regardless of truck. The horizontal scale can then be plotted as a horizontal directivity. Note that the depiction of the type shown in Fig- position axis for which the reader has been oriented relative to ure 45 would show these other sources at another instant. the truck by the small photograph that has been sized to the When the onboard spherical source is deactivated, its con- horizontal scale of the color chart and positioned to reflect the tribution disappears from the vertical scan. This phenomenon position on the truck to which the array is "looking" for each is illustrated in Figure 65 for which the corresponding image is horizontal pixel. given in Figure 46. Figure 64 is a combination of the distribu- Consistent with Figure 45, Figure 64 shows higher levels tions that are shown in Figures 63 and 65. Also, note that the between 1 and 5 m (3 and 16 ft) behind the front bumper and sound source distribution for this low-speed passby is concen- 1 m (3 ft) above ground. Figure 44 also shows sources in the trated to the 1 m (3 ft) elevation from the surface of the road. Figure 65. Vertical scan at 937 Hz for the 4400 truck passing to the right with the engine at 2200 rpm and the spherical source deactivated. The time axis is converted to bumper position relative to the array center. The truck photo is reversed on the horizontal scale with its 6 m (20 ft) wheel base indicated. Vertical axis of the acoustic scan is expanded with the 1 m mark on the truck indicated with the line.