# Parameter Conversion between Controlled Pass-By Method and Alternative Close Proximity Method
## 5 Conclusions  
In this study, we constructed a new method called the alternative close proximity (A-CPX) method. This method was designed to evaluate the far-field sound pressure level emitted by a rolling tire mounted on a car at a constant speed through near-field sound pressure level measurement. The test results showed that the order of the tire noise obtained by the two test methods is consistent, and the tire noise obtained by the two test methods has a strong linear correlation. Theoretical research showed that the analysis method based on the propagation relationship between near-field and far-field noise is not feasible, and the average deviation between the theoretically predicted value and the measured value of far-field tire noise is large. Using the white noise sound signal as the noise source, the semi-free field noise propagation rule test obtained the correction value of the test results between the two test methods. When considering the occlusion effect of the vehicle body, two parameter conversion relationships of the measurement results between test methods were identified. The results showed  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Frequency/Hz**} & \multicolumn{2}{c}{**Transmission Coefficient Ratios**} & \multirow{2}{*}{**Frequency/Hz**} & \multicolumn{2}{c}{**Transmission Coefficient Ratios**} \\ \cline{2-3} \cline{5-5}  & \(\beta\)/\(\alpha\) & \(\beta_{\text{T}}\)/\(\alpha_{1}\) & & \(\beta\)/\(\alpha\) & \(\beta_{\text{T}}\)/\(\alpha_{1}\) \\ \hline
100 & 0.92 & 0.96 & 1.25 k & 1.12 & 0.89 \\
125 & 0.85 & 0.96 & 1.6 k & 0.94 & 0.90 \\
160 & 0.98 & 0.89 & 2 k & 0.82 & 0.90 \\
200 & 0.93 & 0.91 & 2.5 k & 0.85 & 0.84 \\
250 & 0.96 & 0.90 & 3.15 k & 0.90 & 0.71 \\
315 & 0.92 & 0.99 & 4 k & 0.95 & 0.88 \\
400 & 0.93 & 0.94 & 5 k & 0.91 & 0.86 \\
500 & 0.96 & 1.08 & 6.3 k & 0.78 & 0.88 \\
630 & 1.1 & 0.86 & 8 k & 0.88 & 0.80 \\
800 & 0.91 & 0.76 & 10 k & 0.83 & 0.77 \\
1 k & 0.91 & 0.87 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Transmission coefficient ratios at different frequencies.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Tire Type**} & \multicolumn{2}{c}{**CPB Experimented**} & \multirow{2}{*}{**CPB Predicted Value/dB**} & \multirow{2}{*}{**Difference Value/dB**} \\ \cline{3-3} \cline{5-5}  & & **Value/dB** & & \\ \hline a & 76.1 & 73.5 & 2.6 \\ b & 78.0 & 75.5 & 2.5 \\ c & 79.4 & 76.6 & 2.8 \\ d & 77.1 & 74.2 & 2.9 \\ e & 78.0 & 75.5 & 2.5 \\ \hline \hline \end{tabular}
\end{table}
Table 7: The difference value between the measured value and the predicted value of CPB tire noise when considering the shielding effect of the vehicle body on noise propagation.  
that the deviation between the predicted value and the measured value of the far-field tire noise is very small, verifying the technical feasibility of the method.  
Since the experimental tire has only four all-season tires and one ordinary tire, and the test road is an ordinary cement road, the experiment has certain limitations. To further verify the experimental results, more types of tires and roads need to be tested in the future.  
Design experiment and conceptualization, B.S.K. and H.J.; methodology and performed experiments, B.S.K., H.J., and M.Z.; investigation and editing, H.J. and B.S.K.; writing paper H.J. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This paper was supported by WonkWang University in 2020.  
The authors declare no conflict of interest.