Lecturers often inadvertently modify their position and orientation while lecturing in classrooms. However, data on speech intelligibility for various speaker orientations and positions in classrooms is limited. This study examines how speaker orientation, position, and acoustic treatment affect speech intelligibility within classrooms. Measurements were made in two university classrooms, one with acoustic treatment (T30 ≈ 0.4 s) and the other without (T30 ≈ 1.0 s), each with an area of 80 m2. Speech level, speech transmission index (STI), and speech intelligibility scores in Mandarin were assessed in both classrooms for different speaker orientations and positions. The results show that varying speaker orientation can modify the speech level by up to 3 dBA and the STI by up to 0.11. A speaker at the podium typically improves intelligibility when the speaker is oriented toward the majority of the audience or directly ahead. Acoustic treatment reduces speech level but improves intelligibility scores by 5%–10%. In the classroom without acoustic treatment, although speech level is mostly independent of speaker orientation, the STI variation among various speaker orientations is comparable to that in the classroom with acoustic treatment. Speech intelligibility is larger than that at the podium when the speaker is in the center of the classroom and more sensitive to speaker orientation and receiving position. The classroom with better acoustic conditions has satisfactory intelligibility scores for any speaker orientation, whereas speaker orientation can improve intelligibility scores by up to 10% in classrooms with inferior acoustic conditions, especially for listeners in an adverse condition.
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