Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 9
9
Table 1. Correlation indicators for CPX and OBSI methods
to passby.
Sections S1, S5, Sections S1, S4, S5, W3 All Sites
Metric W3
CPX OBSI CPX OBSI CPX OBSI
Slope 0.94 0.96 0.87 0.94 0.80 0.87
r2 0.94 0.95 0.94 0.93 0.79 0.87
Offset, dB 21.9 23.7 21.7 24.0 22.4 24.6
Std Dev, dB 1.2 0.9 1.1 1.1 1.8 1.7
Avg. Dev, dB 1.0 0.7 0.8 0.9 1.4 1.3
data indicate that an at-the-source measurement cannot be measured by the CPX method are consistently reduced by 3 to
expected to account for an arbitrary range of site characteris- 4 dB relative to what would be expected from the passby spec-
tics in the prediction of wayside levels. tra or the OBSI spectra as illustrated in the example presented
Further insight can be gained by plotting the overall CPX in Figure 7. Although this spectral distortion has only subtle
levels against the corresponding OBSI levels. These data sug- influence on the correlation of overall levels, some evidence
gest that CPX or OBSI source levels could be predicted from of its effect was seen in the rank ordering of tires. The spectral
the other within a standard deviation of 1.1 dB when all of distortion is thought to be related to the enclosure surrounding
the test pavements are included (Figure 6). The standard the test tire on the CPX trailer. Using the techniques currently
deviation is reduced to 0.8 dB, however, when the porous AC under consideration by ISO Working Group 33, methods for
Section S4 is excluded. It was noted that the two methods determining corrections are being developed to account for the
handle porous pavements differently. The actual passby lev- effects of the enclosure on the CPX measurements (14 ). How-
els were consistently higher than what would be predicted ever, unless trailers were totally identical, correction spectra
from the CPX to passby correlation curve, and consistently would need to be determined for each CPX trailer.
lower than what would be predicted from the OBSI to passby
correlation. Thus, the CPX levels over-predict the effect of
Recommendation of an
porosity on the passby levels while the OBSI levels under pre-
At-the-Source Measurement Method
dict it. This is likely due to the CPX sound pressure measure-
ments being more affected by pavement sound absorption The selection of the OBSI method for further development
than the sound intensity measurement. was based on both the technical issues resulting from the
One of the largest drawbacks for the CPX method is spectral evaluation testing and from considerations dealing with
distortion which occurs in comparison to passby and OBSI the expense/practicality and training/expertise required for
results. In general, the 1/3 octave band levels below 1000 Hz implementing either of the two methods (detailed analysis is
108
AC S1
106 y = 1.04x - 6.52
AC S4 R2 = 0.93
AC S5
CPX Sound Pressure Level, dBA
104
AC W3
102
Waverly PCC
100 1-to-1 Line
Linear Regression
98
96
94
92
90
88
86
88 90 92 94 96 98 100 102 104 106 108
OBSI Level, dBA
Figure 6. CPX sound pressure level versus OBSI level for all sites and
both tires.
OCR for page 10
10
98
96 S5 CPX
94 S5 OBSI
92 S5 Pby +24 dB
90
Tire Noise Level, dBA
88
86
84
82
80
78
76
74
72
70
68
66
64
315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000
1/3 Octave Band Center Frequency, Hz
Figure 7. Comparison of spectra for CPX, OBSI, and passby levels with 24 dB
added for Dunlop tire on Section AC S5 at 45 mph.
presented in Appendix B). In regard to expense and practical- · Practical issues of acquiring, validating, operating, main-
ity, factors such as facilities expense, instrumentation costs, taining and storing a CPX trailer.
labor costs, and operational issues are associated with either
method. For training and expertise, trade-offs between the The first two reasons resulted from the evaluation testing;
two methods resulted in no net advantage for either method. the last two reasons deal with the use of a CPX trailer. The
The OBSI method was selected for the following reasons: option of exposed microphone CPX was considered desirable
from a cost and ease of implementation point of view, but
· Slightly better correlation between OBSI and passby data technical issues of wind noise, test vehicle reflections and
than for CPX data, noise, and operation in traffic would lead to inconsistency
· Lack of spectral distortion seen in comparing OBSI and from one user to another. On the other hand, the issues
passby data, against an OBSI approach do not appear to be significant
· Expense of an enclosed trailer for CPX measurements, and enough to preclude its use.
