# Parameter Conversion between Controlled Pass-By Method and Alternative Close Proximity Method
## 3 Results and Correlation Analysis
### Parameter Conversion of Noise Experiment Results between CPB and A-CPX
#### 4.3.2 Considering the Shielding Effect of the Vehicle Body on Noise Propagation  
Considering the blocking effect of the vehicle body during the experiment, the speakers were placed in the preset sound source positions (T1, T2, T3, and T4) in order, and the noise experiments were performed sequentially to analyze the tire noise sound field propagation relationship between the measured points S1 and S2 in the semi-free field. According to the S2 experiment point data when the sound sources on the left and right sides of the vehicle produced sound separately, the ratio of the radiating sound energy of the left sound source to the right sound source at S2 was found. By multiplying the predicted value of the sound energy radiated to the S2 measuring point at T1 and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tire Type** &
\begin{tabular}{c} **CPB Experimental** \\ **Value/dB** \\ \end{tabular} & **CPB Predicted Value/dB** & **Difference Value/dB** \\ \hline a & 76.1 & 75.0 & 1.1 \\ b & 78.0 & 77.1 & 0.9 \\ c & 79.4 & 78.1 & 1.3 \\ d & 77.1 & 75.8 & 1.3 \\ e & 78.0 & 77.0 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Difference between the measured value and the predicted value of CPB tire noise when ignoring the shielding effect of the vehicle body on noise propagation. T3 by the ratio, the predicted value of sound energy radiated to the S2 measuring point at T2 and T4 was obtained. During the experiment, we ensured the consistency of the sound power of the power amplifier and made the speaker sound for a long time to ensure that the sound source signals of the four positions were as consistent as possible. We can suppose that the total energy of the sound source at T1 and T2 is E. The sound energy radiated by the sound source at T1 and T2 to the S2 measurement point is \(\mathrm{E_{T1}}=\alpha\mathrm{E}\) and \(\mathrm{E_{T2}}=\beta\mathrm{E}\), where \(\alpha\) is the propagation coefficient of the sound energy at the T1 position radiating sound energy to S2 and \(\beta\) is the propagation coefficient of the sound energy at the T2 position radiating sound energy to S2. \(\mathrm{E_{T1}}\) and \(\mathrm{E_{T2}}\) can also be expressed as follows [22]:  
\[\mathrm{E_{T1}}=\alpha\mathrm{E}\propto\frac{\mathrm{P_{CPB1}}^{2}}{\mathrm{P_ {e}}^{2}}=10^{0.1\mathrm{SPL_{T1}}} \tag{10}\]  
\[\mathrm{E_{T2}}=\beta\mathrm{E}\propto\frac{\mathrm{P_{CPB2}}^{2}}{\mathrm{P_ {e}}^{2}}=10^{0.1\mathrm{SPL_{T2}}} \tag{11}\]  
where \(\mathrm{P_{CPB1}}\) is the sound pressure measured at S2 when the loudspeaker is placed at T1 to make sound alone (dB), \(\mathrm{P_{CPB2}}\) is the sound pressure measured at S2 when the loudspeaker is placed at T2 to make sound alone (dB), and Pe is the effective value of sound pressure, \(\mathrm{Pe}=2\times 10^{-5}\) Pa. \(\mathrm{SPL_{T1}}\) and \(\mathrm{SPL_{T2}}\) are the noise values measured at the S2 measurement points when the speakers were placed at T1 and T2 to make sounds individually. Dividing Equation (11) by Equation (10) and substituting the experimentally obtained values of \(\mathrm{SPL_{T1}}\) and \(\mathrm{SPL_{T2}}\), the propagation coefficients ratio \(\beta/\alpha\) at different frequencies was obtained. Similarly, by placing sound sources at T3 and T4 positions individually, the propagation coefficient ratio (\(\beta\)1/\(\alpha\)1) of the T3 and T4 sound sources radiating sound energy to S2 was obtained (where \(\alpha\)1 and \(\beta\)1 are the propagation coefficients of T3 and T4 sound sources radiating sound energy to S2, respectively). \[\beta/\alpha=\frac{10^{0.1\mathrm{SPL_{T2}}}}{10^{0.1\mathrm{SPL_{T1}}}}=10^ {0.1\mathrm{SPL_{T2}}-0.1\mathrm{SPL_{T1}}} \tag{12}\]  
\[\beta_{1}\alpha_{1}=\frac{10^{0.1\mathrm{SPL_{T4}}}}{10^{0.1\mathrm{SPL_{T3}} }}=10^{0.1\mathrm{SPL_{T4}}-0.1\mathrm{SPL_{T3}}} \tag{13}\]  
Table 6 shows the ratios of the sound energy propagation coefficients \(\beta/\alpha\) and \(\beta\)1/\(\alpha\)1 at each frequency.