## 1 Introduction
Additionally, binaural listening phenomena such as binaural interactions [31] and the head shadow effect [22; 23; 32] will produce an \"effective\" SNR that is greater in one ear than the other, and this will have a direct effect on the speech intelligibility in automobiles. The speech intelligibility in an automotive cabin is strongly influenced by the speaker's direction in relation to or distance from the listener's ears [22; 25]. The near-field acoustic conditions exacerbate the head shadow effect, and the special acoustic conditions, such as the sound-field nonuniformity, distribute early reflections and seat-back occlusions, further complicating the sound field. These factors make speech intelligibility under binaural listening in an automobile cabin more complex than that in a traditional room [22; 25]. From the perspective of indoor acoustics, occupants, being an integral part of the indoor environment, are also regarded as a significant variable affecting acoustic conditions. The addition of occupants to a space leads to a decrease in its effective volume and an increase in sound absorption [33; 34]. Hence, careful consideration of the effect of occupancy on acoustic environments is crucial during the initial stages of acoustic design for building spaces. Research has shown that the acoustic effect of occupancy varies with factors such as the occupation density and distribution [35; 36], the surface area of the body exposed [37], standing or sitting [33], height and weight [38], clothing [39], and the room itself [35]. Due to the \"edge effect\" in auditoria, assessing the sound absorbing properties of occupants using a typical reverberation chamber may not yield precise outcomes [33; 40]. Consequently, previous studies have focused on experimental measurements and modeling of the sound absorption characteristics of occupants, chairs, and occupied pews [34; 37; 40; 41] using methods such as that based on perimeter-to-area ratios [41; 42]. The additional sound absorption provided by occupants is expected to absorb more reflected sounds and reduce room reverberation [35; 36; 43; 44]. Furthermore, sound absorption by occupants can eliminate the possibility of a flutter echo between the ceiling and floor (known as the seat-dip effect), thus reducing sound-focusing phenomena [43; 45]. However, the effect of occupancy on acoustic conditions is contingent upon the fundamental acoustic features of the space, such as the total sound absorption. As an example,Choi [35] demonstrated that adding occupants to more absorbing classrooms led to smaller fractional increases in the total sound absorption and smaller incremental changes in the room acoustic parameters. More specifically, the sound absorption provided by the occupants resulted in reduced reverberation times and late-arriving reflected sounds, with the largest effects observed in highly reflective spaces, but minor effects were also observed in absorptive classrooms [35]. Thus, occupants are typically observed as the principal sound absorption component, significantly influencing the acoustic properties of the space, particularly in spaces with many rigid or reflective boundaries [33; 35]. Speech intelligibility is primarily affected by the acoustic conditions of a space, which are mainly determined by the SNR and reverberation time [22; 31]. Consequently, occupancy can also affect speech intelligibility. On the one hand, the presence of occupants can increase the background noise level and weaken the sound level with distance within the crowd, leading to a decrease in the SNR and a subsequent decrease in speech intelligibility [35; 43]. [PERSON] [35] found that, while adding absorption can reduce late-arriving speech sounds, too much absorption can also reduce useful early-arriving speech sounds, thereby decreasing speech intelligibility. On the other hand, additional sound absorption by occupants reduces the reverberation time, increasing speech intelligibility [35; 36; 46; 47].