## 2 Acquisition and Processing of Noise Samples
### Subjective Evaluation  
As presented in Table 1, we evaluate the sound quality according to the degree of discomfort and adopt the equal interval direct one-dimensional evaluation method [12]. There are five levels of discomfort in Table 1, where 0 indicates extreme discomfort with the highest level of noise annoyance and 10 indicates no discomfort. Each of the three remaining discomfort levels contains three scores, each score representing the discomfort degree at the same level. We selected twelve healthy testers (ten males and two females) with no ear problems, all testers were between the ages of 20 and 30 and had driving experience. The subjective evaluation test was conducted in an indoor environment with a maximum sound pressure level of no more than 20 dB. The tester sits on the chair with earphones and the audio is played by Groove software (version number: 11.2402.6.0). The process of the evaluation test is as follows: The tester randomly played the first audio, which was repeated three times and then stopped. After listening to the first noise sample, the tester had a hearing perception of the sample. Based on Table 1, the tester could give the final score of the \begin{table}
\begin{tabular}{c c c c c c} \hline
**Uncomfortable Level** & **Extremely Uncomfortable** & **Very Uncomfortable** & **Moderately Uncomfortable** & **Little Uncomfortable** & **Not Uncomfortable** \\ \hline Scores & 0 & 1–3 & 4–6 & 7–9 & 10 \\ \hline \end{tabular}
\end{table} 
Table 1: Subjective evaluation scoring table. Figure 3: Time-domain waveform ((**A1,A2**) Hy-Vo chain system, (**B1,B2**) silent chain system). sample. The score was recorded in an Excel spreadsheet, and the tester randomly switched to the next audio until all 48 noise samples were completely evaluated. The twelve testers took turns participating in the evaluation test, and the subjective evaluation results are illustrated in Figure 4.  
In Figure 4, the horizontal line in the middle of each box line represents the median score, and the upper and lower boundaries of the box line represent the upper and lower quartiles. Taking the median score as a benchmark, we can find that the score generally presents a step-down trend with the increase in rotational speed. When the rotational speed is in the range of 500-1500 rpm, the score is reduced with the increase of the rotational speed. When the rotational speed is in the range of 2000-3000 rpm, compared with the value at 1500 rpm, the score is steady and unchanging. When the rotational speed is at the range of 3500-4500 rpm, compared with the value at 3000 rpm, the score drops and remains stable at 3500 rpm and 4000 rpm and then drops again at 4500 rpm. The score of 5000-6000 rpm has the same downward trend as the 3500-4500 rpm. According to the above analysis, we can know the relationship between the noise performance of the Hy-Vo chain system and the rotational speed. At low speeds, the noise performance is negatively correlated with the rotational speed. At middle speed, the chain system is in optimum working condition and the noise performance remains relatively stable. At high speeds, the noise performance is not good and continues to deteriorate with the increase in rotational speed. For the same sample, twelve testers must have relatively consistent feelings. From Figure 4, we use box line length to evaluate the score fluctuation at the same speed. Obviously, the score fluctuates least at low speed, less at medium speed, and more at high speed. To test whether the subjective evaluation score of each sample is reasonable, we calculate the correlation coefficient \(R\) between the twelve groups of evaluation results.