## 3 Results and Discussion
### Effect of the Occupants on Magnitude Spectrum  
There could be considerable variations in the speech levels at different target ears for the various different speaker-listener position configurations and occupancy modes due to the additional sound absorption provided by the occupants. Figure 3 shows the relative magnitudes of the BRIRs for various speaker-listener position configurations and occupancy modes, including the result at both ears. As noted above, the definition of the STI [52] states that the SNR values from \(-15\) to \(+15\) dB are directly proportional to the STI, which ranges from 0 to 1. The conversion from SNR to STI is linear, meaning that a 3 dB increase in SNR corresponds to a 0.1 rise in STI. The JND of 0.03 for the STI [66] is roughly equivalent to a JND of 0.9 dB for SNR. Table 2 lists the variation ranges in magnitude among different occupancy modes under various speaker-listener position configurations; values greater than 1 JND (0.9 dB) are displayed in bold. Figure 2: Reverberation times (\(T_{30}\)) in the 250–4000 Hz octave bands 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 maximum and minimum \(T_{30}\) values, as well as the reference \(T_{30}\) value with no other passengers. As can be seen in Figure 3b, when the listener is in the driver's seat and the speaker is in one of the passenger seats, the magnitudes at the right ear are always larger than those at the left ear, especially for the FP seat. It seems that the level at the ipsilateral ear is always larger than that at the contralateral ear, which is in line with expectations based on binaural effects [24]; however, this was not the case when the speaker was located in the driver's \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline \multirow{2}{*}{**Speaker**} & \multirow{2}{*}{**Listener**} & \multicolumn{6}{c}{**Variation Range in Magnitude (dB)**} \\ \cline{3-10}  & & & **125 Hz** & **250 Hz** & **500 Hz** & **1 kHz** & **2 kHz** & **4 kHz** & **8 kHz** \\ \hline \multirow{6}{*}{D seat} & FP seat & Left & 0.49 & **1.68** & **1.41** & **0.90** & 0.54 & 0.60 & **0.91** \\  & Right & 0.55 & 0.73 & 0.68 & 0.88 & 0.74 & 0.39 & 0.41 \\ \cline{2-10}  & BR seat & Left & **2.15** & 0.88 & **1.09** & 0.67 & **1.95** & **4.92** & **7.46** \\  & Right & **2.20** & 0.77 & **2.88** & **2.21** & **2.00** & **1.87** & **1.48** \\ \cline{2-10}  & BM seat & Left & **1.63** & **0.96** & **1.75** & **3.21** & 0.86 & **2.09** & **3.35** \\  & Right & **2.62** & **2.96** & **1.83** & **1.95** & **2.29** & **3.90** & **2.86** \\ \cline{2-10}  & BL seat & Left & **1.13** & **6.91** & **5.23** & **2.00** & **2.01** & **1.65** & **1.50** \\  & Right & **4.90** & **10.80** & **3.53** & **1.86** & **2.79** & **1.88** & **4.05** \\ \hline FP seat & \multirow{6}{*}{D seat} & Left & 0.64 & 0.73 & **1.53** & **1.00** & **1.08** & 0.40 & 0.49 \\  & Right & 0.83 & 0.37 & 0.64 & 0.57 & 0.52 & 0.32 & 0.35 \\ \cline{2-10}  & Left & **2.09** & **1.12** & **1.41** & **1.51** & **2.02** & **0.96** & 0.88 \\  & Right & **1.47** & **1.39** & 0.84 & **2.39** & 0.50 & **1.07** & **1.48** \\ \cline{2-10}  & Left & **1.33** & **1.28** & **1.16** & **1.93** & 0.74 & 0.55 & **1.09** \\  & Right & **1.19** & **1.17** & **1.00** & **1.52** & 0.85 & **1.67** & 0.71 \\ \hline BL seat & \multirow{6}{*}{D seat} & Left & 0.34 & 0.88 & 0.53 & **1.35** & **0.92** & 0.64 & 0.57 \\  & Right & 0.67 & **1.23** & **2.04** & **2.43** & **0.92** & **2.64** & **1.50** \\ \hline \hline \end{tabular} Note: Values greater than 1 JND (0.9 dB) are displayed in bold.  
\end{table} 
Table 2: Variation ranges in magnitude among the different occupancy modes under various speaker–listener position configurations.