## 1 Introduction
[PERSON] [46] found that decreasing occupancy leads to decreased acoustic quality in terms of the room-averaged speech intelligibility in classrooms. [PERSON] [43] demonstrated that occupancy, based on measurements in six churches, induces a mean speech transmission index (STI) increase of 0.05 with the use of a public address system and 0.035 without a public address system. [PERSON] [35] reported that occupancy causes a mean increase of 0.05 in STI values in six reflective classrooms but a smaller decrease of 0.02 in STI values for six absorptive classrooms. Therefore, the effect of occupancy on speech intelligibility is dependent on the acoustic conditions of the room and can either increase or decrease speech intelligibility. It can be seen that the acoustic effects of occupancy on acoustic conditions and speech intelligibility are dependent on the primitive acoustic properties of the space [35; 43]. However, previous studies have paid more attention to how occupancy affects acoustic conditions and speech intelligibility in large spaces, such as classrooms [35; 36; 46; 47], churches [37; 43; 48; 49], and theaters [33; 34; 40; 41; 50]. Given the unique acoustic conditions in automotive cabins, the effects of occupants (or passengers) on speech intelligibility may differ from those in larger spaces. The physical volume occupied by passengers is comparable to the overall volume of the automotive cabin, and this is a fundamental difference from traditional large spaces. Thus, occupants can disrupt geometric symmetry in an automotive cabin, further altering modal behavior; human activities also generate sound waves that interact with the room acoustics, potentially changing the sound field and its decay characteristics [8; 9]. Furthermore, an automotive cabin contains substantial sound-absorbing materials, particularly the seats, and the presence of occupants will also modify the sound absorption of the seats. Hence, the sound absorption caused by occupants could be different from that occurring in larger spaces, making the effect of occupancy more complex. Despite these important differences, to the best of our knowledge, the effect of occupants on speech intelligibility in an automotive cabin has not been studied. While internal noise is the primary factor reducing speech intelligibility in an automotive cabin, it is also important to discuss other aspects of speech intelligibility in specific noise conditions from the perspective of room acoustics. These factors motivate us to systematically explore the ways through which occupants affect speech intelligibility in an automotive cabin. Based on the above, this study comprehensively investigated the effects of adding occupants on the speech intelligibility in an automotive cabin. In practice, previous studies have mostly used percentage occupancy [36] and the perimeter-to-area ratio [42] to discuss the impact of occupants on the sound field and speech intelligibility; however, while these variables are suitable for large spaces, they are not appropriate in the special sound field inside an automotive cabin. Therefore, this study specifically considered the differences in speech intelligibility between different occupancy modes. First, binaural room impulse responses (BRIRs) were measured in an automotive cabin with different speaker-listener position configurations, with an artificial mouth regarded as the speaker and a dummy head designed to represent the typical physiological characteristics of a Chinese adult male, who was regarded as the receiver. Different occupancy modes were considered for a fixed speaker-listener position configuration. Then, the magnitude spectra of BRIRs were analyzed, and the corresponding STIs were experimentally examined based on the measured BRIRs in the same manner as the approach demonstrated in [51; 52]. Finally, the subjective speech-reception thresholds (SRTs, i.e., the SNR required for 50% intelligibility) in Mandarin Chinese were assessed via headphones based on the measured BRIRs.