Effect of speaker orientation on speech intelligibility in an automotive environment

Abstract

In addition to background noise, binaural effect, and the acoustic boundary conditions of a given space, speech intelligibility (SI) is affected by the source/speaker directivity or orientation, the latter of which has not been researched in small enclosed spaces, such as an automobile. Thus, this study aims to investigate the effect of speaker orientation on SI in automobiles, using both the speech transmission index (STI) and subjective experiments. Given the limited variation range for the orientation of rear-row speakers in an actual scenario, only the situation for front-row speakers was considered in this study. An artificial mouth was regarded as the speaker, which is placed at the driver’s seat. Binaural room impulse responses were measured for different listener locations and speaker orientations in an automobile and a listening room with negligible reflections. Subsequently, STIs were calculated and subjective speech reception thresholds (SRTs) in Mandarin Chinese were evaluated for the different listener locations and speaker orientations. The result shows that the SI in the listening room considerably decreases when the speaker orientation deviates from the listener due to the speaker directivity, with a best-ear STI (SRT) variation of up to 0.24 (7.4 dB). In comparison with the result in the listening room, early reflections improve SI in the automobile by a best-ear STI (SRT) increase (decrease) of up to 0.3 (7.7 dB). The speaker deviating from the listener signifies that the early reflections in the automobile afford a higher SI improvement. Thus, the variation range of SI with various speaker orientations in the automobile was greatly reduced compared with that in the listening room, with a best-ear STI (SRT) variation of no more than 0.1 (2.7 dB). Overall, the speaker orientation mostly does not affect SI except when the speaker is facing the listener due to rich early reflections inside an automobile. Moreover, the comparison between the subjective and objective results shows that the best-ear STI can well express the variation in the SRT in the automobile under various speaker orientations. This study bridges the gap in the research of SI considering various speaker orientations in an automotive environment and helps understand the combined effect of various factors on SI under the special acoustic condition in an automobile.