## 1 Introduction  
In recent years, the interior spaces of cabins have become an essential component of individuals' daily lives. The concept of an cabin has evolved from a basic mode of transportation to an integral part of people's lifestyles, and it is sometimes referred to as a \"third space\". With increasing consumer demand, acoustic comfort has become a significant concern. As a example, the acoustic comfort inside ships has attracted a great deal of attention [1; 2; 3]. However, studies on the acoustic comfort inside an automotive cabin is limited. Speech intelligibility in an automotive cabin is closely related to the level of acoustic comfort experienced by the passengers, and this can directly affect the safety of driving and the overall ride experience. However, when compared to traditional indoor spaces, the acoustical environments within automotive cabins exhibit unique characteristics [4], meaning that understanding speech intelligibility in these environments is a particular challenge. In general, speech intelligibility is primarily affected by the acoustical conditions of a space--such as its reverberation time--and the signal-to-noise ratio (SNR) [5; 6]. Due to the much smaller spatial volume of an automotive cabin when compared to building interiors and the large number of absorbing boundaries (such as seats, carpets, and the roof) in close proximity to a listener, the sound field exhibits significant low-frequency modal phenomena and a rapid attenuation of high-frequency sound waves [7; 8; 9]. The density of sound resonance modes inside an automotive cabin is much lower than that in a traditional interior,and this results in low-frequency sound fields having significantly uneven distributions in both the frequency and spatial domains. Furthermore, the rapid attenuation of sound waves by absorbing the boundaries inside an automotive cabin result in an early decay time that is generally within 0.15 s [7; 9; 10]; this essentially means that there is no \"reverberation\" process, and even the concept of reverberation time is not applicable. Almost all reflected sounds in an automotive cabin arrive within 50 ms, meaning that they are early reflected sounds, and many indeed arrive within 1 ms [9; 10]. These early reflected sounds can be combined with direct sound [11; 12], thereby significantly improving speech intelligibility. As such, speech intelligibility in an automotive cabin is almost unrelated to the time-domain attenuation characteristics of the transfer function (i.e., the reverberation) [13; 14; 15; 16] and mainly depends on the SNR. Many factors influence passengers' speech intelligibility in an automotive cabin. Previous studies have shown that, in this context, background noise--such as that from the engine, tires, air conditioning, and turbulent air--significantly affects speech intelligibility because it is unique and fluctuates depending on speed, operating conditions, and road conditions, unlike in traditional indoor environments [15; 16; 17; 18; 19; 20]. Furthermore, seat backs have a significant role in absorbing sound within an automobile, effectively separating the cabin into two compartments [17; 21]. Consequently, seat occlusions reduce the speech energy transmitted from a speaker to a listener, and this greatly reduces speech intelligibility, particularly when a front-row passenger is listening to a passenger in the back [22; 23]. Furthermore, due to the extremely small dimensions of an automotive cabin, both the speaker and listener are within the near-field region (i.e., separated by a distance of no more than 1 m), and the near-field acoustic transmission properties further complicate the variations in speech intelligibility within a cabin [24; 25; 26]. For instance, speech intelligibility may be more sensitive to the directivity or orientation of the sound source in an automotive cabin than in a traditional indoor space [27; 28; 29; 30].