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CHAPTER 3
Evaluation of Alternative Test Methods
Based on the results of the literature search, testing was (Waverly PCC) had an old PCC surface with transverse slab
conducted to evaluate the two candidate methods for on- joints, no roadway shoulder, and propagation over an acousti-
board, tire-pavement noise source measurement. This testing cally softer ground, providing some degree of sound absorp-
consisted of measuring CPX and OBSI noise levels on the tion. At the NCAT track, test speeds of 35, 45, and 60 mph were
same tires and then conducting controlled passby measure- measured at all four sites, except for AC W5 where 55 mph was
ments using the test tires along with three other tires of the substituted for 60 mph due to track banking. At Waverly, only
same design mounted on the test vehicle. The findings from 35 and 45 mph were tested due to posted speed restrictions. An
this testing along with other considerations leading to the example passby measurement setup is shown in Figure 3 for
selection of the OBSI method of tire-pavement noise mea- the Waverly test site. Photographs showing typical CPX and
surement at the source are summarized in this section (details OBSI measurement setups were shown in Chapter 2. Specific
of the testing and analyses are discussed in Appendix B). information regarding the test sites, test matrix, and test meth-
ods is given in Appendix B.
Three sets of tires were used for the testing (see Figure 4,
Overview of Evaluation Testing
details of the test tires are documented in Appendix B). One of
Passby measurements were made under both cruise and these tires is the ASTM Standard Reference Test Tire (SRTT)
coast conditions. Sound pressure levels of tire-pavement noise (13), which is currently under study by the ISO Working
at the source were measured in a manner following the ISO Group 33 as a possible new standard test tire for the ISO CPX
CPX test procedure (2). Sound intensity levels were measured procedure. Another tire was a Dunlop SP Winter Sport M3.
using the OBSI methodology employed in previous California This tire has been used in round-robin testing conducted by
Department of Transportation (Caltrans) studies (6). OBSI tire and vehicle manufacturers and was chosen by that group
levels were also measured on the CPX trailer for comparison to as a replacement for a light truck tire due to its more aggressive
those measured on the test car. In addition to these measure- tread pattern. The size of both tire types was P225/60R16. The
ments, testing was done to examine potential propagation dif- third tire design was the Goodyear Aquatred 3 in a P205/70R15
ferences between sites. Measurements were made at five sites: size. This tire design has been extensively used by a number of
four asphalt concrete (AC) pavements at the National Cen- researchers since 2000. Due to tire and wheel size incompati-
ter for Asphalt Technology (NCAT) test track in Auburn, bility, comparable passby measurements for the Aquatred tire
Alabama, and one portland cement concrete (PCC) pavement could not be made. However, CPX and OBSI measurements
in the nearby town of Waverly, Alabama (specific information were conducted at all of the test sites for all three tire designs to
on these sites and pavements is provided in Appendix B). provide a linkage to the historical Aquatred data.
Three of the pavements at NCAT were acoustically hard, pro-
ducing no sound absorption. Surfaces for these sections were
Summary of the CPX and OBSI
fine texture Superpave (Section AC S5), medium texture stone
Test Results
mastic asphalt (Section AC S1), and Superpave with added
transverse texture (Section AC W5). The fourth pavement had For the test evaluation of the CPX and OBSI methods, the
a porous, open-graded asphalt concrete (OGAC) pavement of overriding issue was how well the at-the-source measures cor-
coarse texture in the travel lane with propagation over an adja- relate to passby data. The simplest way to compare the CPX
cent non-porous AC lane (Section AC S4). The Waverly site and OBSI measurements to the passby data is to consider the
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8
Microphone
Position
25 ft
Direction
of Travel
Figure 4. Photograph of the Aquatred (left), Dunlop
(middle), and SRTT (right) test tires.
that of the CPX measurements. When the porous pavement
Section S4 is included, the standard deviations become identi-
Figure 3. Passby measurement setup for the Waverly cal (center columns). For this grouping, the only detractor for
PCC test site.
the CPX data is that the slope of linear regression deviates more
from an ideal slope of 1 than do the OBSI results. For this
cross-plots of overall A-weighted level and metrics generated grouping, both source measures correlate well to passby (even
by these plots. A typical cross-plot is shown in Figure 5 for CPX with the porous pavement included) as indicated by the r2 val-
versus passby for all pavements and test speeds. From such ues and standard and average deviations. With the scatter of
plots, the slope and r2 of a linear regression provide some indi- the passby data being on the order of 1 to 2 dB, it is apparent
cation of data scatter and deviation from an ideal constant that better correlation could not be expected for these sites.
offset ("1-to-1" fit). A 1-to-1 line (slope of 1) can also be con- When the PCC site is included (right columns), this is not the
structed and deviations about that line considered. These are case. Although the OBSI results hold some small advantage
presented in Table 1 for the non-porous AC pavements (S1, S5, over the CPX, the r2 and standard deviations for both are not
and W3), for all of the AC pavements (including the porous very acceptable. From the sound propagation tests, the PCC
Section S4), and for all five sites grouped together. site was found to be substantially different (2 to 4 dB) than
For the first grouping of sites (left columns), there may be the others due to sound absorption from the acoustically soft
a slight advantage in favor of the OBSI measurements as the ground at the site and/or because the pavement was slightly
standard deviation about a 1-to-1 fit of the data is smaller than depressed below the grade of the adjoining ground. These
84
AC S1
AC S4
82
Passby Sound Pressure Level, dBA
AC S5
80 AC W3
Waverly PCC y = 0.80x - 3.23
78 1-to-1 Line R2 = 0.79
Linear Regression
76
74
72
70
68
66
87 89 91 93 95 97 99 101 103 105 107
CPX Sound Pressure Level, dBA
Figure 5. Controlled vehicle passby levels versus CPX sound pressure level for
all test sites and both test tires.
