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41 modification made the exhaust noise prevalent at low frequen- 3.4.3, shows that the wheel well, gas tank area (below the cab cies. At these frequencies, the location of the source is clearly door), and the region between the gas tank and the wheel well directly above the cab, not at the exhaust pipe opening. The all contribute as pathways for engine sound. The array was aerodynamic fairing was open at the back facing the exhaust unable to discriminate among these paths except at 1799 Hz, pipe, which likely excited acoustic volume resonances in the because these pathway "sources" are generally rather distrib- fairing cavity thus amplifying the sound and shifting the source uted and the array horizontal beam is too broad (the beam to the fairing. This will be discussed further when the results width is greater than 6 ft (1.8 m) at -6 dB below 1000 Hz, see of the passby for this truck are examined. Noise by the vari- Figure 39) except at high frequencies. Table 14 quantifies these ous paths from the engine compartment appears dominant comparisons with reasonable agreement in the image sound at 923 Hz. Direct radiation from the exhaust, rather than level and nearly equal relative ranking of "sources" by the from the exhaust-excited hollow wind fairing appears equally sound intensity measurements. important to the engine noise through ground reflection at Further studies in this area might suggest some potential 1523 Hz. Note that in the images, the ground-reflected engine source- or path-targeting treatments. noise sources again appear about 0.5 m (1.6 ft) below ground, similar to the location seen with the 4400 truck. The sound 3.5.4 Example Results from Low- power levels presented for this truck in Figure 32 generally cor- and High-Speed Track Passbys roborate these results by showing dominant exhaust noise at frequencies below 800 Hz. Above this frequency, exhaust and Analysis Technique for Low- wheel-well noises are generally of comparable order. Table 13 and High-Speed Track Passbys compares the sound levels discussed for this truck, with the notation "(ref)" in the cases where the levels at the array are by For passby measurements, the arrangements such as the ground reflection. example photographed in Figure 22 were used, with photo- Finally, the 5900 truck with the engine set to 1,400 rpm and cells placed on either side or both sides of the array to mark the cooling fan running provides a relatively demanding com- the instant of the front bumper passing a known point in the parison because no single source is dominant. Rather, sound passby track. Figure 50 illustrates the geometric details, emanates from the engine compartment via multiple paths where L1 and L2 denote optional locations of the photocells that are all of relatively similar importance. Figure 49 shows relative to the array for determining truck position. Differ- these resolved direct and reflected-path sources. Here again, the ent locations were experimentally examined and used on locations of ground-reflected sources appear 0.5 to 1 m (1.6 to the low- and high-speed tracks, although positions were 3.3 ft) below ground. This location indicates that, in general, fixed in each case. The cell that indicated approach (i.e., to engine compartment sources via ground reflection appear at the left with a rightward approach), used at the high-speed positions -1 m < y < -0.5 m (-3.3 ft < y < -1.6 ft) depending track, was found to give the best timing. The length L1 for on truck and frequency: the higher the frequency, the closer the the first cell was 5 ft (1.5 m) and the length L2 for the second reflected sources to the ground surface. This general observa- cell was about 20 ft (6 m), as used at the high-speed track. tion may provide an important distinction between engine and The first cell was always used to mark bumper position in tire noise sources because the latter should occur at the road the approaching truck, which was desired as close to the surface and appear in the zone within approximately -0.5 m array center as possible without interfering with the micro- < y < +0.5 m (-1.6 ft < y < +1.6 ft). Figure 30, which resulted phones. The second cell was used primarily to infer the aver- from the sound intensity measurements described in Section age speed of truck passby during maneuvers such as braking Table 13. Image sound pressure levels and intensity levels (dB) for the stationary 9200i truck with engine at 1500 rpm, without muffler. Frequency (Hz) Exhaust Wheel Well Lower Cab Image Intensity Image Intensity Image Intensity Image Intensity 415 400 76 81 <66 73 <66 73 645 630 76 84 72 (ref) 80 67 77 922 1000 72 85 80 85 76 81 1523 1600 73 84 72 (ref) 84 69 (ref) 77 1938 2000 69 84 71 83 62 77

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42 f = 461 Hz f = 830 Hz f = 1245 Hz f = 1799 Hz Figure 49. A series of images for source distribution of the 5900i truck stationary opposite the array with engine at 1400 rpm. and acceleration. It was also used to confirm timing and truck interrupted the light ray, which provided an immedi- position. To ensure capturing a complete cab plus trailer ate projection of the truck length onto the time record as length between cells, the second cell was placed further shown in Figures 50 and 51. This feature is useful in inter- down track relative to the array center than the first. The preting the images. photocell signal provided a step function as the passing The collected data runs were longer than needed and, to conserve computer memory, the samples were truncated once the passby photocell signal was examined. Typical truncation Table 14. Image sound pressure levels and limits are shown in Figure 51, in which the first and last sec- intensity levels (dB) for the stationary onds of data are discarded (shaded areas). In this case, the time 5900i truck with engine at 1400 rpm. to the CPA is 1.25 s after eliminating the first second of the Gas Tank Region/ record as indicated by the instant the bumper crosses the Frequency (Hz) Wheel Well Lower Cab photocell signal. The retained data window shown in this exam- Image Intensity Image Intensity Image Intensity ple is 3 s; typically, 2 to 3 s were retained, depending on the 461 500 72 76 * 76 speed and length of the truck. With the retained data, the truck 830 800 78 81 * 82 1245 1250 81 83 74 83 can be tracked as it approaches and recedes. The increasing 1799 2000 74 80 69 81 and decreasing sound level is due to range (distance) change, * Included in wheel well as illustrated in Figure 52 for the same run. In this case, for the

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43 Vehicle Path Front 6 m offset to Bumper closest wheel Rear track Bumper Photo Photo Array Center Cell Cell L1 L2 L=Vs t Figure 50. Diagram of a typical passby geometry. 3 s retained, the range started at about 15 m (49 ft), closed to 1.65 s into the run. At any frequency, the sound level varies by 6 m (20 ft) at the CPA, and then opened to 19 m (62 ft). As about 15 dB through the run. When range-corrected in the part of the data analysis, the range from the array center to the manner described previously, the resulting corrected spectrum, front bumper is calculated as reference at each sample time. shown at the bottom of the figure, demonstrates less variabil- Because the acoustic sources on the truck are distributed ity. There is over-correction at the beginning and end of the along the wheel base and otherwise are not known a priori, the record, however, owing to the uncertainty in the actual range range correction approximated to the bumper as a reference. and to the possibility that the tails of the record near 0 and 3 s Thus, for any actual source position the actual range correc- are contaminated with some background noise. There is a hint tion could be larger or smaller than calculated. An example of of contamination in approximately the first 1.2 s of the overall corrected and uncorrected sound pressure autospectra for one run, where the rise with time is less steep than later in the run. microphone in the array is shown in Figure 53. The two- Of course, once the sources have been localized, it is straightfor- dimensional displays of autospectra for a representative sensor ward to recalculate the range correction to obtain a more pre- in the array are shown in color bar in 1 Hz bands relative to 1 Pa cise pressure level history. This iterative step has not been taken for the passby of Figures 51 and 52. The record is broken into because this demonstration was made in conjunction with stan- 14 segments of time (the last 13 are plotted) with the range cal- dard passby measurement procedure that provides absolute A- culated for each. The autospectrum at the top of the figure is weighted sound level and the array provides a relative breakout uncorrected for range and shows a maximum level at about of the contributions with only approximate overall level. 0.14 tr,R 20 0.12 0.1 18 0.08 16 0.06 Rear Bumper Range, m 0.04 14 0.02 12 0 -0.02 10 -0.04 8 -0.06 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 6 0 0.5 1 1.5 2 2.5 3 Truck transit time Time, s Retained across array center Figure 52. Range from array center to front Passby data bumper reference as a function of time Figure 51. Example of a passby signal record. during passby.