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Measured Equivalent Ls, OTO at F=922.8516Hz Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 1
6 6
87 87
86 86
4 4
85 85
2 2 84
84
83 83
sy/m
sy/m
0 0
82 82
-2 81 -2 81
80 80
-4 -4
79 79
78 78
-6 -6
-6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6
sx/m sx/m
xs=0
Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 1
Measured Equivalent Ls, OTO at F=922.8516Hz
6 84
6 84
83
83
4
4 82
82
81
81
2
2
80 80
sy/m
sy/m
0 0 79
79
78 78
-2 -2
77 77
-4 76 -4 76
75 75
-6 -6
-6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6
sx/m sx/m
xs=3.05 m
(b) Calculated
(a) Measured
Figure 40. Images of (a) measured and (b) calculated signals for spherical source emission at 922 Hz
for two cross-range source locations xs = 0 and xs = 3.05 m [source elevation 1.24 m (4 ft), offset
7.62 m (25 ft)].
but now with a little horizontal smearing as the result of the engine/fan operating condition and right-left asymmetry in the
truck motion over the measurement time. The engine noise source distribution and propagation paths.
seems to have shifted in its directionality, now being propagated
via ground reflection, for which the reflection image appears to
3.5.3 Benchmark Parallel Array-Based
be again at approximately -0.5 m (-1.6 ft). Details of the time
and Acoustic Intensity Measurements
resolution governances during passby will be discussed in Sec-
for Stationary Trucks
tion 3.5.4.1, as these can affect the noted smearing. In Figure 46,
the truck is now passing to the left at 25 mph (40 km/h) with the Three trucks with different engine equipment line-ups were
speaker deactivated. Now no source appears on the truck's bed, examined with both acoustic intensity measurements and
yet engine noise is still apparent, although with a new propaga- array-based measurements in order to develop a fundamental
tion path. The comparison of Figures 45 and 46 is a simple illus- recognition base for defining the truck sources using the array.
tration of a reoccurring facet of the truck sound: variability with In the following paragraphs, the relative intensity levels and
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Measured Equivalent Ls, OTO at F=922.8516Hz Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 1
6 86 6 86
85 85
4 4
84 84
2 83 2 83
82 82
sy/m
sy/m
0 0
81 81
80 80
-2 -2
79 79
-4 78 -4 78
77 77
-6 -6
-6 -4 -2 0 2 4 6 -6 -4 -2 0 2 4 6
sx/m sx/m
xs=3.05 m, zs=7.62 m (21.8 deg.)
Measured Equivalent Ls, OTO at F=922.8516Hz
Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 1
6 88 6 88
87
87
4
4
86
86
2 85
2 85
84
84
sy/m
sy/m
0
83 0
83
82
-2 82
-2
81
81
-4 80 -4 80
79
79
-6
-6 -4 -2 0 2 4 6 -6
-6 -4 -2 0 2 4 6
sx/m
sx/m
xs=5.8 m, zs=5.8 m (45 deg.)
(a) Measured (b) Calculated
Figure 41. Images of (a) measured and (b) calculated signals for spherical source emission at 922 Hz
for two cross-range source locations [source elevation 1.98 m (6.5 ft)].
Measured Equivalent Ls, OTO at F=922.8516Hz
78 Theoretical Equivalent Ls, OTO at F=922.8516Hz and GRC= 1
6 78
6
77
77
4 76 4 76
75
75
2
2
74
74
sy/m
sy/m
0 73
0 73
72 72
-2 -2
71 71
70 70
-4 -4
69 69
-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 42. Images of (a) measured and (b) calculated signals for spherical source emission at 922 Hz for
20.2 m (66 ft) array stand-off at road side [source elevation 1.98 m (6.5 ft)].
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Y urements are essentially different and yield identical results
only under the ideal conditions of a single-path plane incident
wave and a single source. As implemented here, the intensity
Array measurement is made over a plane and provides the average
acoustic power which passes across that plane. The planar
array captures sound pressures of all arriving ray bundles from
the distribution of sources in front of it and, by implementing
X inter-element phase delays, "localizes" the regions where the
rays appear to converge. These regions are called the "sources."
Actual directivity patterns of the sources and the existence of
multiple acoustic paths (e.g., ground reflection) could cause
discrepancies.
Figure 47 shows images at four frequencies for the sta-
tionary 4400 truck with the engine set to 2000 rpm, cooling
Z fan running and the spherical source activated. At 922 Hz,
as noted previously, the engine compartment sound trans-
Figure 43. Illustration of the array and truck
coordinate orientations during passby event
mitted through the wheel well, lower cab, and by ground
at the instant of CPA. reflection is nearly of equal magnitude to that generated by
the spherical source. The spherical source dominates all
truck sources at 231 Hz and 600 Hz. At 1937 Hz, the contri-
relative array-based acoustic levels are compared to establish butions to the sound are from the spherical source and from
the credibility of the array in rank-ordering the truck sound beneath the cab, but the side lobe effects start to contami-
sources. Note that in all cases to be discussed, intensity levels nate the source map. These observations agree with Figure
are in one-third octave frequency bands, while the source 27 in showing complete dominance of the sound by the
images are generally in substantially narrower bandwidths: spherical source below 630 Hz and by the engine noise con-
about 64 Hz constant bandwidth is used with the levels tributions within 10 dB of the spherical source levels at
adjusted to provide approximate one-third octave band frequencies above 1000 Hz. Table 12 summarizes these
equivalent sound levels. The array-based and intensity meas- results. The sound power levels (PWL) relative to 10-12 W
Figure 44. Image at 922 Hz of the 4400 truck idling stationary
with engine at 2000 rpm and spherical source activated.
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6 83
82
4
81
80
2
79
sy/m
0
78
77
2
76
4 75
74
6
6 4 2 0 2 4 6
sx/m
Figure 45. Image at 922 Hz of the 4400 truck traveling to the right
at 25 mph with engine at 2000 rpm and spherical source activated.
6 72
71
4
70
2 69
68
sy/m
0
67
66
2
65
4 64
63
6
6 4 2 0 2 4 6
sx/m
Figure 46. Image at 982 Hz of the 4400 truck traveling to
the left at 25 mph (40 km/h) with engine at 2000 rpm and
deactivated spherical source.
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f = 231 Hz f = 600 Hz
f = 922 Hz f = 1937 Hz
Figure 47. A series of images for source distribution of the 4400 truck stationary opposite the array with engine
at 2000 rpm and spherical source activated.
Table 12. Image sound pressure levels and intensity levels
(dB) for the stationary 4400 truck with engine at 2000 rpm
and spherical source.
Frequency (Hz) Spherical Source Wheel Well Lower Cab
Image Intensity Image Intensity Image Intensity Image Intensity
231 250 95 96 <86 87 <86 86
600 630 89 91 <80 87 <80 85
922 1000 80 84 81 85 76 83
1430 1251 to 1600 79 85 77 85 76 83
1938 2000 84 85 77 85 82 83
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are presented in the table at the one-third octave band cen- the PWL and Ls are within 4 dB in their absolute values for the
ter frequencies. spherical source. The relative rankings of the levels by both
The array-measured image sound levels are presented as measurement techniques, from the spherical source to truck
equivalent one-third octave band levels using the relationship sources, are generally consistent. Note that in these images, the
color scale range is 10 dB in order to avoid the appearance of
Ls = 10 log10 (G ( f ) · 0.233 f p0
2
) (1) array side lobes which would contaminate the image. The array
side lobes (normalized to a main response of unity) are calcu-
where G(f) is the 1 Hz spectrum level, p0 is the reference pres- lated to be approximately a fraction (0.2 to 0.25) of the main
sure of 20 µPa, 0.233f is the bandwidth of a one-third octave lobe (roughly -14 to -12 dB), as shown in Figure 3(c).
band at frequency f, and the frequencies are selected to be within For the stationary 9200 truck, the images show significant
the one-third octave frequency bands of the compared intensity low-frequency energy being emitted from above the cab, as
levels. The relative levels Ls and PWL should be equivalent seen in Figure 48. No spherical source was installed on this
under the ideal conditions noted above, and for the spherical truck. Its vertical exhaust muffler was replaced with a pipe
source they essentially are very close. With only one exception, ("straight-through" exhaust) before the measurement, and this
f = 415 Hz f = 645 Hz
f = 923 Hz f = 1523 Hz
Figure 48. A series of images for source distribution of the 9200i Eagle truck with "straight-through"
exhaust (no muffler) and engine at 1500 rpm, stationary opposite the array.
