Research On Speech Intelligibility In Small Eclosed Space As Automotive Cabin

Since the 21 st century,the definition of an automobile has gradually evolved from a simple "driving tool" or "travel tool" to the "third space" in people’s daily life.Especially with the integration of advanced technologies such as new energy automobiles and self-driving automobiles,people put forward higher requirements for the performance of speech communication or conversation in automotive cabins.Therefore,speech intelligibility(SI)has become an important index to evaluate the automotive acoustic design.SI in an automotive cabin is not only affected by the internal and environmental noise,but also closely related to the sound field characteristics of the automotive cabin.It involves many fields or issues such as architectural acoustics,automotive acoustics and environmental noise,and has attracted the joint attention of researchers in various fields.In particular,due to the small size of the interior space and complex boundary conditions,the interior sound field in automotive cabins shows substantial low-frequency resonance and rapid high-frequency attenuation,and is mainly composed of direct sounds and early reflections.The SI in an automotive cabin is very different from that of the traditional indoor acoustic space.It is necessary to systematically study the sound field characteristics and speech transmission characteristics in automotive cabins from the perspective of architectural acoustics.The SI in the automotive cabin has been studied since the 1990 s,but the relevant research is still imperfect due to the particularity and complexity of its internal sound field,and there are few reports in China.The problems that have not been solved in the existing research mainly include:(1)The existing research paid more attention to the variation law of SI in automotive cabins under different noise conditions,but rarely mentioned the near-field sound source conditions in automobiles,especially for the influence mechanism of the speech transmission characteristics from the speaker or electroacoustic playback equipment to the listener’s ears under near-field conditions;(2)There are significant differences in speech communication experience at different positions in automotive cabins,so that passengers often have to turn their heads to listen or speak at a high level to have effective speech communication.This is essentially the binaural hearing difference caused by the listener head orientation and the change of sound source radiation characteristics caused by the sound source orientation,but these two important factors have not been paid attention to in the existing research;(3)In particular,the interaction between the above factors and the complex sound field in small spaces such as an automotive cabin makes its influencing mechanism on SI more complex,and it is worthy of further exploration;(4)The existing research was mostly based on objective indicators,and rarely combined with subjective and objective means.To this end,this paper has carried out the following work:Firstly,according to the condition of near-field sound source in automotive cabins,the near-field head-related transfer function(HRTF)was used to explore the dependence of speech transmission index(STI)and binaural STI(BSTI)on the sound source azimuth and distance in the non-reflective sound environment,i.e.,the variation law of STI and BSTI with the sound source azimuth and distance.Secondly,the individual HRTF data of 56 Chinese subjects were used to estimate the BSTI under different sound source locations to explore the impact of individual HRTF differences on BSTI.The results show that the impact of individual HRTF differences on BSTI can be ignored.On this basis,the azimuth-dependent BSTI model of KEMAR manikin at different sound source distances was fitted,which can be used for BSTI’s approximately analysis and prediction with different sound source locations under the condition of weak reflection environment.The above works provide important clues and basis for the study of BSTI in automotive cabins with abundant reflected sounds and near-field sound source.Thirdly,the BSTI variation with different listener head orientations in the complex reflected sound field in the automotive cabin was explored,considering different sound sources(virtual speakers)locations.To explore the comprehensive effect of the reflected sound and the seat occlusion in the automotive cabin as well as the binaural effect on SI,three kinds of experimental scenes with different reflection intensity,including the automotive conditions with left-front window closed and open and the listening room condition,were set respectively.The results show that the early reflections greatly improved BSTI,especially for the contralateral ear with lower direct sound energy.The left-front window was a strong reflection boundary,which usually plays a role in improving BSTI(especially for the left-back seat).Because of the the seat occlusion,the BSTI in the automotive environment under the rear-row sound source was lower than that in the listening room.Overall,in the automotive environment,when listeners in the driver’s seat turned their head from the left to the front passenger,the BSTI of the four virtual speaker positions was improved to varying degrees.Fourth,subjective experiments on speech intelligibility were conducted to verify the above analysis results based on BSTI.Speech reception thresholds(SRTs)under different listener head orientations were measured.The results show that with the variation of the listener head orientation,the closer the sound source azimuth was to the front lateral,the smaller the SRT value and the higher the SI would be in the automotive cabin,which was slightly different from the BSTI result.To explain the difference between subjective and objective results,the SRTs obtained directly through subjective experimements and the SRTs estimated indirectly through different STI-SI models were compared and analyzed.It was found that the SRT obtained based on the STI-SI model derived from traditional indoor could only partially express the variation law of the SRT obtained directly through subjective test.It reveals the limitations of calculating SRT based on BSTI and the necessity of further exploring the mapping model between subjective SI score and BSTI in the special sound environment in automotive cabins.Fifthly,in the near-field non-reflected sound environment,the effects of sound source frequency response(equalization)and sound source orientation related to the sound source directivity on STI and BSTI results were analyzed.Firstly,it was confirmed that the frequency response characteristics of the selected sound source had a significant impact on STI,and the frequency response equalization could effectively correct the results.Using the sound source transfer functions after equalization,the variation law of BSTI caused by sound source directivity with different sound source orientations was analyzed,and the sound source orientation correction functions of three sound source types were fitted.A BSTI model considering the sound source orientation,sound source distance and azimuth was constructed.Last but not least,the influence of sound source orientation on SI was studied,and the BSTI differences under different sound source directivity and listener locations were analyzed.The results show that when the sound source orientation deviated from the listener,the BSTI in the automotive cabin decreased,and remained in a small range after the sound source behind the listener.At the same time,the BSTI results of the listening room under the same sound source and listener configuration were also measured,so as to compare with the results in the automotive cabin.The interaction between the complex sound field in the automobile and the directivity of the sound source is analyzed.It was found that the more the sound source deviates from the listener,the greater the BSTI difference between the two measured conditions will be,especially for the contralateral ear with weak direct sound.Finally,the subjective SRT experiment further verified the effectiveness of the above conclusions.This dissertation comprehensively and deeply studied the binaural effect related to the listener head orientation,the sound source characteristics related to the speaker(sound source)orientation and frequency response,and the comprehensive influence mechanism of special sound field conditions such as early reflections and seat occlusion in automotive cabins on SI,breaking through the limitations of existing studies that focus on the analysis of noise characteristics on SI in automotive cabins.It makes up for the blank of the comprehensive influence of the orientation of the listener and the speaker on the SI in automotive cabins,and helps to further understand the comprehensive effect of the automotive acoustic factors(acoustic boundary conditions)on the sound field and SI in automotive cabins,to provide an essential basis for its acoustic design and SI evaluation,which has the dual significance of theoretical research and practical application.