Examining speech transmission index thresholds for Mandarin speech intelligibility in university classrooms

The speech transmission index (STI) is the most widely used objective metric for speech intelligibility evaluation in enclosed spaces. STI thresholds specify the minimum values needed for specific intelligibility levels and serve as the basis for relevant room-acoustics standards. Although the International Electrotechnical Commission [IEC 60268-16, Sound System Equipment-Part 16: Objective Rating of Speech Intelligibility by Speech Transmission Index (2020)] permits application-specific thresholds, systematic investigations remain scarce. In this study, university classrooms were examined as representative lecture rooms. Listening conditions with continuously varying STI values over a broad range were established using acoustic characteristics measured at 24 locations across 12 university classrooms. The classic five-point qualification scale framework was employed to derive Mandarin speech intelligibility thresholds using the noninteger rank sum ratio and binary response methods. Results showed notable differences between the two methods under poor listening conditions (maximum difference of 0.07), whereas the thresholds converged under better conditions. An STI threshold of 0.59 is recommended as the minimum acceptable criterion for university classrooms, corresponding to 89.6% intelligibility on phonetically balanced syllable lists. For optimal conditions, an STI value of 0.72 (94.5% intelligibility) is proposed. These thresholds provide practical guidance for classroom acoustic design and contribute to the refinement of room-acoustics standards tailored to university classrooms where Mandarin is spoken.