## 3 Results and Discussion
### Effect of the Occupants on STI Results  
Figure 4 gives the STI values for various speaker-listener position configurations and occupancy modes. Note that the values for the front-to-front scenarios are considerably larger (greater than 0.7, in the speech intelligibility rating of \"Good\") than for the front-to-back and back-to-front scenarios (less than 0.6, in the speech intelligibility rating of \"Fair\"). This is mainly due to the combined effects of seat occlusion, binaural effects (i.e., head and ear occlusion), and speaker directivity during front-rear speech transmission [22,32]. When the positions of the listener and speaker are reciprocal, the STI values for the front-to-back scenarios (Figure 4a) are always lower than those for the back-to-front scenarios (Figure 4b); this is mainly due to the weak radiation behind the speaker [32]. It is also worth noting that there is a higher STI for the driver listening to the passenger in the front passenger seat compared to when the positions of the listener and speaker are reciprocal. This is mainly because, in this scenario, the front passenger always faces the driver directly. As shown in Figure 4a, when the speaker is in the driver's seat, the STI slightly decreases after adding other occupants, but there is a variation exceeding 1 JND (i.e., 0.03) [66] only when the listener is in the BR or BM seat, and this sometimes reaches 0.07. In other words, when a person in the FP seat or BL seat is listening to the driver, the addition of other passengers will mostly not affect the STI (with a variation of no more than 1 JND); conversely, for a listener in either the BR or BM seat, there could be a perceptible difference after other passengers are added to the automotive cabin. It is worth mentioning that the influence of the occupant in the FP seat on the STI values of the rear-row listeners can always be ignored. When the listener is in the driver's seat (Figure 4b), the additional occupants will have less of an effect on the STI values than the situation in which the speaker is in the driver's seat (Figure 4a); the STI differences among various occupancy modes are mostly lower than 1 JND, except for the situation with the speaker in the BL seat and full seat occupation (0.046). Figure 4: STIs for various speaker–listener position configurations and occupancy modes ((**a**) represents situations in which the speaker is in the driver’s seat; (**b**) represents situations in which the listener is in the driver’s seat). The inset diagram in each panel shows the positional relationship between the speaker and the listener, which is represented by a red pattern. The range of \(\pm\)1 JND (\(\pm\)0.03) is marked as a light red area in the figure referring to the result of “None” (red solid line). Note that there are small differences in the STI values among different arrangement configurations, and these are sometimes comparable to the differences among the STIs under various numbers of occupants. This indicates that, when considering the influence of occupants within an automobile on its acoustic environment and speech intelligibility, it is insufficient to focus solely on occupancy rates, as was done in a previous study [35]. This is because there are notable variations depending on the specific occupant distribution. In fact, the STI accounts for both the effects of reverberation and the SNR. On the one hand, the addition of occupants provides additional absorption, thereby slightly reducing the reverberation time and increasing the speech intelligibility (see Figure 2); on the other hand, as the occupancy increases, the SNR value decreases (see Figure 3) and the speech intelligibility thus decreases. Additional occupants also reduce the sound level of the early reflected sounds that are beneficial to speech intelligibility [35].