# Parameter Conversion between Controlled Pass-By Method and Alternative Close Proximity Method
## 1 Introduction
[PERSON] and [PERSON] [9] studied the relationship between the CPX method and the OBSI method in the same category, and the CPB method and the OBSI method in different categories, and the correlation coefficients were 0.93 and 0.79, respectively. [PERSON] and [PERSON] [10] compared the OBSI and SPBI sound levels of 15.24 \(\times\) 1.524 m (50 \(\times\) 5 ft) microphone positions at 97 km/h (60 mph). The results showed that the sound level of SPBI was 28.2 dB (A) lower than that of OBSI. [PERSON] [11] conducted a large number of experimental comparisons on each tire noise measurement method and fully analyzed the relationship between different measurement methods using material analogy, force analogy, and acoustic analogy. [PERSON] et al. [12; 13] used analysis and experiments to identify the driving factors of the maximum noise level (MNL). They also built a vehicle test system to collect the near-field sound level contribution of a single vehicle. The relationship between the near-field measurement level and the level extrapolated to the receiver position was discussed. [PERSON] et al. [14] used the CPX and DR methods when they used similar or identical surfaces. The results showed that the difference between the A-weighted sound level measured by the DR method and that measured by the CPX method is no more than \(\pm\)1 dB as long as the drum of the drum device is covered with a simulated road surface similar to the real road surface. The tire grades of the two test methods usually remain the same and the spectrum also shows good similarity, but the level of some frequencies may differ by 4 dB. [PERSON] and [PERSON] [15] analyzed the correlation between the measurement of CPX and OBSI on the whole sound level and the one-third octave spectrum at a speed of 60-110 km/h on various concrete and asphalt wear-resistant pavements. After considering the average correction factor between the two, they concluded that the difference between them was minimal. [PERSON] and [PERSON] [16] used SPB and CPX methods to explore and analyze the acoustic characteristics of asphalt concrete and cement concrete surface types. The test showed that the noise generated by the cement concrete surface is 5 dB (A) higher than that of asphalt concrete surface, and the noise difference between SPB and CPX methods is about 5 dB (A) for cement concrete road sections; for asphalt concrete pavement, the noise difference between the two methods is about 10 dB (A). [PERSON] et al. [17, 18] defined the alternative close proximity trailer method (A-CPX) to evaluate the sound power level emitted by a specific tire/road combination and to extrapolate the sound pressure level obtained by taxiing on the roadside. The CPB method used for measuring the maximum sound pressure level of a vehicle passing by is simple, and uses a microphone placed on the side of the road. Its disadvantages include a long experiment cycle and high experiment cost. The CPB method cannot obtain a stable and accurate noise spectrum. The alternative close proximity (A-CPX) method considers the accuracy and cost-effectiveness of the experiment. Its advantage is that it can exclude or ignore other noise components except tire noise. Through data processing, it can obtain an accurate and stable spectrum. The experiment cycle is short, and the experiment cost and the dependence on the experiment road surface form are low. If the relationship between the experiment results of these two methods can be determined [1, 7], the CPB experiment results can be predicted from the A-CPX noise experiment data. This would shorten the experiment cycle and reduce the cost of tire noise experiments [2]. Based on previous research, we conducted CPB and A-CPX experiments on five mainstream passenger car tires on the market. The correlation between the noise data of the two experimental methods was analyzed. To study the parameter conversion between the experiment results of the two experiment methods, we conducted acoustic radiation propagation experiments in the half-free field.