## 2 Experimental Procedure and Design  
### Experimental Subjects and Speech Data Collection  
In this study, the focus is on analyzing the formant features of Mandarin Chinese vowels produced by learners. Fifty Mandarin learners (equal number of males and females) were selected, all born in Xinjiang, with more than ten years of Mandarin learning experience. Their oral scores in the Chinese Proficiency Test were all above 45, and they had no language or hearing problems.  
The remaining ten subjects were standard Mandarin speakers. Based on MHK scores and expert auditory assessment, the learners were divided into three groups (Figure 1): the elementary group (UCB): 12 individuals (6 males/6 females), average score 56.0 \(\pm\) 1.41; the intermediate group (UCI): 20 individuals (10 males/10 females), average score 66.0 \(\pm\) 1.41; the advanced group (UCA): 18 individuals (9 males/9 females), average score 82.5 \(\pm\) 0.71; and the control group (MC): average score 88.0 \(\pm\) 1.41. Mandarin proficiency was significantly positively correlated with test scores ([PERSON]'s r = 0.98, \(p\) < 0.001), validating the effectiveness of the grouping.  
In this experiment, speech recording was conducted in a dedicated recording room. The equipment used included a laptop, external sound card, microphone, and interconnecting data cables. An external sound card was chosen for its ability to adjust volume, reduce noise, and monitor plosive sounds. The recording software was developed using Matlab R2019a, with a sampling frequency of 16 kHz. The reading material for the subjects consisted of monosyllabic words, with 98 data points per speaker, resulting in a total of 60 \(\times\) 98 = 5880 Mandarin monosyllabic word data points. Detailed information about the subjects in each group is shown in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Group**} & **Number of Male** & **Number of Female** & \multirow{2}{*}{**Total Number**} & **Mandarin** \\  & **Participants** & **Participants** & **Proficiency Level** \\  & **(Average Score)** & **(Average Score)** & & \\ \hline UCB (Elementary Level Group) & 6 (57) & 6 (55) & 12 & Beginner \\ UCI (Intermediate Level Group) & 10 (65) & 10 (67) & 20 & Intermediate \\ UCA (Advanced Level Group) & 9 (83) & 9 (82) & 18 & Advanced \\ MC (Mandarin Chinese) & 5 (87) & 5 (89) & 10 & Standard \\ \hline \hline \end{tabular}
\end{table}
Table 1: Detailed information about speakers in different proficiency groups.  
Figure 1: Speaker information for different proficiency groups.  
After obtaining the speech data, Praat software (version 6.1.16) was used to process the Mandarin vowels of standard Mandarin speakers and learners, extracting the first and second formants of the obtained data. SVM and ID3 algorithms were then employed for vowel classification.  
Feature Extraction: Each vowel segment was time-normalized to 200 ms using Dynamic Time Warping (DTW) in Praat to ensure uniform duration. The first and second formant trajectories (F1-F2) were then concatenated into a multivariate feature vector X\(\in\)R400, where dimensions 1-200 and 201-400 represent F1 and F2 values sampled at 10 ms intervals, respectively. This fixed-length representation preserves spectral dynamics while enabling direct comparison across proficiency levels.