## 2 Materials and Methods  
### Experimental Measurements  
In this study, measurements were conducted in a quiet underground parking lot on a university campus in a left-hand-drive automobile, with dimensions of 4733 mm \(\times\) 1839 mm \(\times\) 1673 mm (length \(\times\) width \(\times\) height). To represent real-life usage scenarios, eight speaker-listener position configurations were chosen for this study, and these can be divided into two groups (Figure 1). Group 1 (see Figure 1a) represents the situations in which the speaker is located in the driver's seat (D seat) while the listener is seated in one of the four passenger seats, i.e., the front passenger seat (FP seat), back-right seat (BR seat), back-middle seat (BM seat), or back-left seat (BL seat). In each case, both the speaker and listener were facing forward. Group 2 (see Figure 1b) represents situations in which the listener is located in the driver's seat and facing forward while the speaker is seated in one of the four passenger seats (FP, BR, BM, or BL), with the front of the speaker always facing toward the listener. When the speaker/listener was positioned in the FP seat, they were 0.70 m away from and directly behind the listener/speaker. When the speaker/listener was positioned in the BR seat, the distance between them was 1.15 m, and the angle to the right of the listener/speaker was roughly 53\({}^{\circ}\). When the speaker/listener was positioned in the BM seat, the distance from the speaker to the listener was 0.97 m, and the angle to the right of the listener/speaker was roughly 69\({}^{\circ}\). When the speaker/listener was positioned in the BL seat, which was directly behind the listener/speaker, the distance from the speaker to the listener was 0.85 m.  
In practice, the acoustic effects of adding occupants will also be affected by their arrangement [36]; thus, each group included eight occupancy modes that were arranged and combined into eight permutations for no passengers (one case), one passenger (three cases), two passengers (three cases), and three passengers (one case), as listed in Table 1. For instance, when the speaker is located in the driver's seat and the listener is in the FP seat, there will be eight permutations including situations for no other passengers (None), one other passenger in one of three seats (BR, BM, or BL), two other passengers in two of three seats (BM+BL, BR+BL, or BR+BM), and three other passengers (BR+BM+BL) filling all of the occupiable area. The occupants were recruited from the students at Guangxi University. To simplify the analysis, factors such as each passenger's body size, height, and clothing were not considered in the present study. Most of the occupants had similar body-shape characteristics, with an average weight of \(62\pm 2.9\) kg. Each was wearing a thin summer T-shirt. As noted above, the listener was represented using a dummy head designed to represent the typical physiological characteristics of a Chinese adult male [53]. The speaker used in the measurements was an artificial mouth (GRAS 44 AB, GRAS Sound and Vibration, Holte, Denmark), which had dimensions of 0.104 m \(\times\) 0.104 m \(\times\) 0.114 m (length \(\times\) width \(\times\) height). The artificial mouth was chosen because it had a directivity and radiation pattern that were comparable to those of an average human mouth, and it had an effective frequency range of 100 Hz to 16 kHz. The speaker and the mouth of the dummy head were aligned on a single plane situated at a height of 0.889 m from the floor of the automobile. The ear-canal entrance height of the dummy head was 0.942 m. These values were determined based on