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55 3.7 Results of Roadside level slightly less than the direct path image, especially as fre- Measurements quency increases. The reflected image of the source is below ground and is 1 to 2 dB lower in level than the direct image. For initial check-out of the array performance and calibra- This suggests a reflection coefficient of 0.8, which is compa- tion of the test site geometry, a short test was performed first rable to that apparent in the proof-of-concept measurements using a stationary loudspeaker. After this initial test, the actual earlier in the study. Overall, the measurement system per- truck passbys were measured using the microphone array and formed as expected and was adequate for conducting the data acquisition system. In the following subsections, roadside roadside truck measurements. test results are presented and discussed in three categories: As noted previously, one of the extensions made to the (1) calibration of the test site geometry using imaging tests processing was to provide vertical source distributions. Fig- performed with the loudspeaker source; (2) acoustic image ures 69 to 71 provide vertical profiles of the sound source dis- and noise source distribution results of the vehicle passbys; tributions at frequencies 250, 1000, and 2000 Hz, respectively. and (3) example of truck source modeling for simulating noise In these figures on the left-hand side is a level distribution of propagation. the stationary source over a 1.3 s time interval, which demon- strates the (expected) invariance of the sound profile with 3.7.1 Calibration of the Test Site Geometry time for each frequency. On the right-hand side is a line plot of the A-weighted sound level vertical profile at one of the A loudspeaker source, Mackie Model SRM450, was used to time increments. Comparison of Figures 68 through 71 illus- calibrate the positioning alignment of the array relative to the trates the duality of the source two-dimensional imaging and road surface in the path of the trucks in the near lane of the the vertical distributions in localizing the noise sources. highway. The source was mounted on a tripod and the assem- bly was located on the roadway with its center approximately 3.7.2 Image Results of the Vehicle Passbys at the expected wheel path of the closest side of the trucks at a height of 4.5 ft (1.4 m) above the road surface. This test was Of 100 truck passbys recorded in a single day of data acqui- performed during a short break in the traffic, just prior to the sition, passbys for 59 heavy trucks and 4 medium trucks were commencement of truck measurements. Noise from a pink analyzed and are discussed here for the definition of source lev- noise generator was played through the speaker at a high vol- els. Because the objective was to interpret source level images ume for the test. Figure 68 shows acoustic images of the and vertical distributions of the sources, the following require- speaker obtained at frequencies of approximately 270, 1000, ments were necessary for a passby to be analyzed: the vehicle and 2000 Hz. In these plots, a small offset to the vertical coor- in the curb lane, significant timespace separation between dinate [about 0.27 m (0.9 ft)] was provided to compensate for passby vehicles to allow for distinct one-to-one vehiclepassby misalignment of the array acoustic plane with the road sur- identification, a photograph of the vehicle, and nearly constant face and road crowning. This offset is not (and obviously vehicle speed. The remaining 37 passbys were also acoustically should not be) a function of time or of frequency. Note the viable as recorded passby data, but they could not be examined presence of the ground-reflected image source with the sound in detail because at least one of the required characteristics for (a) (b) (c) Figure 68. Images of the loudspeaker positioned on the roadway at the test site for frequencies of (a) 270 Hz, (b) 1000 Hz, and (c) 2000 Hz.

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56 (a) (b) Figure 69. Calibration source, vertical scan at 250 Hz: (a) timelevel distribution; (b) line plot at 0.62 s. complete analysis was missing or they were tractors with atyp- levels by vehicle, one-third octave frequency band, time, and ical trailers (boat or car haulers, wide-load modular houses, vertical position. These profiles can be used to define maxi- etc.). Acoustic source maps obtained during truck passbys were mum levels, mean levels, and sound exposure levels--all across then used to provide time histories and spatial distributions of the ensembles of trucks. The roadside setup geometry used for sources and source paths from the engine, exhaust, tires, and these measurements followed that used for the proof-of- certain body components. Individual vehicle speed during concept tests, with improvements in the timing of the truck passbys varied from 55 to 70 mph (88 to 112 km/h). position relative to the center of the array in order to reduce The total sets of 59 heavy truck passbys and 4 medium truck positioning error. For this, two photocells were positioned at passbys (to a much lesser extent) provide ensembles of sound distances 5 ft (1.5 m) before and 20 ft (6 m) after the array (a) (b) Figure 70. Calibration source, vertical scan at 1000 Hz: (a) timelevel distribution; (b) line plot at 0.62 s.

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57 (a) (b) Figure 71. Calibration source, vertical scan at 2000 Hz: (a) timelevel distribution; (b) line plot at 0.62 s. center. The positions of the photocells and array relative to the above the road grade at the time of the maximum overall A- test track were the same as illustrated in Figures 50 and 51. In weighted sound level (OASPL) for each truck. The dotted black the dimensions used at the highway site, the length of the truck lines indicate the statistical maximum and minimum levels, as it intermittently cuts the photocell beam is shown by the total and the bold solid black line indicates the mean level across the length of its signal. Figure 72 shows an example from roadside selected truck population. Truck 60 is also shown for reference, case 60, which is a heavy truck. The front bumper is again taken as it is a typical truck with sound levels near the mean of the for all runs as the reference point on the truck, so that the max- population. All of the individual trucks are not identified here, imum sound occurs slightly later (typically about 0.06 s for this but their levels are all shown to indicate the distribution of lev- highway speed limit) than the bumper passage by the array els within the population. More discussion of the distribution center. The data processing, including the distance correction will be presented below. Note that a commonly occurring spec- for equalizing the passby levels and the Doppler shift for fre- tral feature is the pair of 500 and 1000 Hz peaks, likely due to quency correction, is described in Sections 3.5.4.1 and 3.6 and tire noise (see also Figure 72). requires these time-accurate truck coordinates. Components Figure 74 shows the spectra for the four medium trucks in of the signal alignment and truncation as relevant to time- the data set. Again, the prominent 500 and 1000 Hz peaks are synchronizing the timefrequency spectrogram and the pho- present. These spectra are all measured with the single refer- tograph of the truck 60 are shown in the figure. The upper ence microphone stationed at the array distance [20 ft (6 m)] traces include the first photocell signal (red), the second and at height of 1.9 m (6 ft) above the road grade at the time photocell signal (green), and the signal from the reference of maximum level for each truck. Although the population is microphone in the array (blue). The truck passing at 58 mph too small for generalization, the levels are slightly lower than (93 km/h) is positioned so that its bumper is at the first photo- for the heavy truck mean levels. cell. The size of the truck photograph is determined by the That the sound from the typical truck is due to tire noise is speed and the known spacing between the red cones [7.3 m emphasized in Figure 75, which shows a full set of images for (24 ft)]. The spectrogram is taken approximately every 0.05 s the one-third octave frequency bands from 315 to 2000 Hz with the 87 Hz frequency intervals. Clearly seen is the corre- obtained for truck 60. Each point spread function is shown in spondence between instances of high sound levels at multiples the upper left corner with the same scale as in the main image. of approximately 450 Hz that are due to the tires, although high Though the number legends for these are unreadable, they act background levels are also seen below 200 Hz. as tic marks that correlate with the main figure. The color bar Figure 73 shows the distribution of the one-third octave provides a relative 10 dB scale for which the reference value is band A-weighted spectra for all 59 heavy trucks. Each spectrum approximately the red dot level for truck 60 in Figure 73. is measured with the single reference microphone stationed at Note that the tractor drive-axle tires and the trailer tires are the array distance [20 ft (6 m)] and at height of 1.9 m (6 ft) both contributing, but prevail in different frequency bands.

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58 Figure 72. Signal alignment and acoustic spectrogram with the photograph of truck 60. Figure 73. One-third octave band sound spectra (in dBA) for 59 heavy trucks.

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59 Figure 74. One-third octave band sound spectra (in dBA) for four medium trucks. This phenomenon suggests that different tire patterns were Figure 77 shows the measured vertical source distributions used on the tractor and trailer of this truck. All nine of these for three individual trucks. These distributions compare rea- images were made for the same computer settings of the sonably well with those obtained in the Caltrans study (30, array's spatial scanning, so any biases in sound level equaliza- 31). To illustrate this comparison, Figure 78 reproduces Fig- tion for distance correction are the same for all frequencies. ure 8 from Donavan et al. (30). The level distributions are Thus, the frequency-to-frequency relative comparisons in similar in shape, although those in Figure 78 show the maxi- this figure are valid. mum levels at or slightly below the road surface, while in Fig- The vertical distribution of sound sources at the nearest ure 77 the maximum levels occur at or slightly above the road truck's side plane can now be assessed in various ways. One surface. This discrepancy is caused by an uncertainty in cali- way is to examine the vertical profiles of source sound levels brating the position of the array's acoustic axis relative to the at 6 m (20 ft) reference distance for the mean values of all ground plane. It is expected that the certainty with which the heavy trucks. Figure 76 shows the source height distributions ground plane can be accurately established from either set of of the mean A-weighted one-third octave band levels for each data is within about 0.2 m (0.6 ft). Some contributors to this band from 100 to 2500 Hz and for the mean OASPL. As dis- uncertainty include the distance to the actual sources versus cussed in Section 3.6.2, these distributions are obtained by the beamforming analysis plane, the size and directivity of the mathematically focusing the array at a sequence of vertical calibration loudspeaker, the frequency-dependent reflection positions above and below the road surface directly in front coefficient for the path of reflected sound between the sources of the array. The sound levels are determined at the instant of and the array, and the precision with which the array can be the maximum OASPL passby level at the reference micro- aligned because the relative tilt angle of the array and the incli- phone for each truck, and are evaluated by the microphone nation of the road crown could not be measured. This 0.2 m array at that instant for each truck. The mean levels for each uncertainty is equivalent to effective acoustic inclination frequency band and steering elevation over all trucks are then error of less than 2 degrees. The resolution of this discrepancy evaluated and plotted in the figure. may become the subject of a separate follow-up study. It is clear from Figure 76 that the vertical distribution for The statistical distributions of the sound levels measured the average truck sound level is dominated by sources between across the population of heavy trucks are provided in the -1 and +1.5 m (-3 and +5 ft). This distribution holds for the form of histograms in Figure 79 for the OASPL at the reference frequencies above 315 Hz. Below 200 Hz, although the verti- microphone and at the 0.4 m (1.3 ft) height for the one-third cal beam width is larger for the array, the source distribution octave frequency bands centered at 500, 1000, and 2000 Hz. seems to drift upwards. Comparison with the images of Fig- All of these levels were recorded at the time of maximum ure 75 confirms that the tire sound sources are positioned just OASPL for each truck. The data for these histograms and for above the road surface. Figure 73 discussed previously are the same. The breadth of

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60 Figure 75. Source image maps of truck 60 for one-third octave frequency bands from 315 through 2000 Hz.

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61 Figure 76. Vertical distributions for mean one-third octave band and overall A-weighted sound levels (in dBA) for 59 heavy trucks. Figure 77. Vertical distributions for one-third octave band and overall A-weighted sound levels (in dBA) for individual heavy trucks (truck images are to the same scale): (a) truck 18, (b) truck 31, and (c) truck 54.

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62 Figure 78. Vertical distributions for one-third octave band and overall A-weighted sound levels for an example truck at the Lakeview site, CA [from Donavan et al. (30, Fig. 8)]. (a) (b) (c) (d) Figure 79. Statistical distributions of the (a) overall sound levels and one-third octave band sound pressure levels (in dBA) at (b) 500 Hz, (c) 1000 Hz, and (d) 2000 Hz for 59 heavy trucks.

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63 the sound level distribution shown in Figure 79 is identical to frequency band and steering elevation for all of the trucks. that shown in Figure 73 at the corresponding frequencies. These distributions also show the expected dominance by The larger spread at 500 Hz is due to the variation of levels in sound sources located near the road surface. this band apparent in both figures. Figures 82 through 84 show the same types of vertical dis- Other examinations of the vertical distributions of sound tributions for the four medium trucks measured during the sources are also possible. Figure 80 shows the vertical distri- roadside testing. Although this small sample does not form a bution of the highest sound levels among the 59 heavy truck statistically significant population, these results are included passbys in each one-third octave frequency band and for each here for completeness and to indicate the similar general dis- elevation. These distributions identify the levels for the nois- tributions as obtained for the heavy trucks. For each figure, iest truck at each vertical position. The levels are determined the sound levels are determined at the instant of the maxi- at the instant of the maximum OASPL at the reference micro- mum OASPL at the reference microphone for each medium phone for each truck and evaluated by the microphone array truck and are evaluated by the microphone array at that at that instant for each truck; then the maximum levels at instant for each truck; then the mean levels, the highest lev- each frequency and steering elevation for all trucks are eval- els, and the maximum 1 s SELs, respectively, are determined uated. Note that these distributions are for statistical maxima for each one-third octave frequency band and steering eleva- and do not represent levels from any individual truck sample. tion for all four medium trucks. The absence of sound A modest increase of approximately 8 dBA can be seen in the sources at the 2 to 4 m (6.5 to 13 ft) elevations may be due to near-road sources at 500 Hz due to tire variations. A signifi- a lack of samples in the population, but it also is likely due to cant increase in sound levels at elevations of 3 to 4 m (10 to the absence of vertical exhaust stacks on these trucks. 13 ft) is also noticeable. It is shown later in this section that As previously noted, the vertical distribution of the sound the latter increase correlates well with the truck source images sources has its highest levels near the surface of the road, which localized at the vertical exhaust position. dominate the levels for the average truck. The maximum lev- In a similar manner, Figure 81 shows the vertical distribu- els during heavy truck passbys also appear to have significant tion of the 1 s sound exposure levels (SELs). The levels are components at elevations between 2 and 4 m (6.5 to 13 ft). determined at the instant of the maximum OASPL at the ref- Figure 85 shows the vertical distributions of OASPL for the erence microphone for each truck and evaluated by the micro- 59 heavy trucks. In the following paragraphs, a few representa- phone array at that instant for each truck. Then the maximum tive truck passbys are discussed further in more detail. Truck A-weighted SELs are determined for each one-third octave 60 is a typical truck with the sound spectrum near the mean of Figure 80. Vertical distributions of highest A-weighted sound levels in one-third octave bands and of overall sound level (in dBA) for 59 heavy trucks.

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64 Figure 81. Vertical distributions of maximum 1 s sound exposure levels (in dBA) in one-third octave bands for 59 heavy trucks. the heavy truck population. Earlier in this section (see discus- for truck 38 in Figure 73, which dominates the level maxima sion of Figure 75), the tires on truck 60 were identified as the for the population. Truck 38 is a three-axle dump truck that primary sound originators. Truck 15 is representative of the produced both exhaust tones and tire tones over the fre- maximum sound levels, and the image in Figure 86 for this quency range. The bold red line in Figure 85 shows two max- truck also clearly identifies the tires on the tandem trailers as ima in the vertical OASPL distribution at the elevations of the source originators. about 0.4 and 3.6 m for this truck. Figure 87 shows the Trucks 38 and 50 are uniquely relevant to the low-frequency acoustic images of this truck in various frequency bands. As sound levels. Note the identification of the sound spectrum can be seen in the figure, near the road surface the tire noise Figure 82. Vertical distributions for mean one-third octave band and overall A-weighted sound levels (in dBA) for four medium trucks.

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65 Figure 83. Vertical distributions of highest A-weighted sound levels in one-third octave bands and of overall sound level (in dBA) for four medium trucks. dominated at higher frequencies (630 and 1250 Hz in this fig- appears to be prominent at frequencies below 500 Hz, and the ure), but at lower frequencies (250 Hz) the noise source at the tire noise near the road surface appears to be prominent at vertical exhaust was predominant for truck 38. Similarly, higher frequencies. In all these cases, the exhaust noise deter- truck 50 also produced exhaust noise at an elevation of nearly mines the source levels at the elevation between 3.5 and 4 m 4 m, as illustrated by the bold magenta line in Figure 85 and above the road surface. The mean sound levels for the total the acoustic images of the truck in Figure 88. Similar data heavy truck population appear to be minimally influenced by analysis for trucks 13 and 57 indicated that the exhaust noise the exhaust noise sources; however, the maximum sound Figure 84. Vertical distributions of maximum 1 s sound exposure levels (in dBA) in one-third octave bands for four medium trucks.

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66 Figure 85. Vertical distributions of overall A-weighted sound levels (in dBA) for heavy trucks. The legend on the left identifies the truck ID numbers. (a) (b) Figure 86. (a) Typical source image and (b) vertical profile of OASPL for truck 15.

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67 (a) (b) (c) (d) (e) (f) Figure 87. (a, c, e) Source images and (b, d, f) vertical profiles of OASPL for truck 38 in one-third octave bands centered at 250, 630, and 1250 Hz.