## 3 Results and Discussion
### Effect of the Occupants on Magnitude Spectrum
Figure 3: Relative magnitudes of the BRIRs 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), including the results at each ear. The results for the left and right ears are denoted by dashed and solid lines, respectively. The inset diagram in each panel shows the positional relationship between the speaker and the listener. seat (Figure 3a) or BL seat (Figure 3b). This was mainly because, in these cases, due to the directivity of the speaker or the influence of seat occlusion, the direct sound received by both ears was weak and mainly comprises the reflected sound from multiple random directions, so the ipsilateral and contralateral ears defined by the relative direction of the speaker to the listener were meaningless. Conversely, the direct sound was relatively strong when the listener was in the driver's seat and the speaker was in one of the passenger seats facing toward the listener. It is also worth mentioning that, in all cases, the level at 2 kHz was always underestimated; this was mainly due to the lower-frequency response of the artificial mouth near 2 kHz, which is similar to the characteristic of human voice production, as shown in ref. [26]. As can be seen in Figure 3, the magnitude levels in the 1-8 kHz bands mostly decreased after occupants were added, and these differences were mostly greater than 1 JND (i.e., 0.9 dB), as listed in Table 2. This trend is mostly consistent with the findings in previous studies [35]. However, this was sometimes not the case, such as for lower-frequency bands, which may be influenced by resonance effects in the automotive cabin. Furthermore, the magnitude levels not only varied with the number of occupants, but also with their distribution. The fluctuation in levels across different occupant distributions at a fixed number of occupants appeared to be comparable to the fluctuation across different numbers of occupants. For instance, when the speaker was in the driver's seat and the listener was in the BM seat (Figure 3a), the level difference between no occupants (None) and one occupant in the FP seat was lower than the differences among one occupant in different passenger seats (FP, BM, and BL). These observed phenomena differed considerably from the results of previous research in traditional indoor environments such as classrooms [36]. There were significant differences in the impact of the occupants on magnitude levels under the different speaker-listener position configurations. When both the speaker and listener were in the front-row seats--i.e., the front-to-front scenarios--the differences in magnitude level caused by the additional occupants were smaller (no more than 1.7 dB) than in other speaker-listener position configurations, especially for the right ear (no more than 1 JND), as listed in Table 2. For the front-to-back scenarios--i.e., the situations in which the speaker was in the driver's seat and the listener was in a rear-row seat--the differences in magnitude level caused by additional occupants were relatively larger, mostly close to or greater than 2 dB, with values that sometimes even exceeded 7 dB (see Figure 3a and Table 2). In comparison to the front-to-back scenarios, the variation range of the magnitude under different occupancy modes was smaller (mostly no more than 2.5 dB) when the listener was in the driver's seat and the speaker was in a rear-row seat, i.e., the back-to-front scenarios (see Figure 3b and Table 2). Overall, when the listener was in less favorable acoustic conditions, the addition of occupants had a greater impact on sound levels. This was mainly because when the direct sound received by the listener was weak, the gain of the early reflected sound became more important.