Speech Intelligibility STI & C50 Calculator
Auditorium acoustics & electroacoustics: Calculate Speech Transmission Index ($STI$), Early-to-Late Clarity index ($C_{50}$ and $C_{80}$), and articulation loss of consonants ($\%AL_{cons}$).
Acoustic Environment
Mid-frequency RT (500–2,000 Hz)
Direct speech SPL minus ambient noise
Depends on listener distance from speaker
Enclosure volume
IEC 60268-16 STI Scale
- 0.75 – 1.00: Excellent (Recording studio, conference room)
- 0.60 – 0.75: Good (Classrooms, lecture halls, theatres)
- 0.45 – 0.60: Fair (PA paging, airport concourse)
- 0.30 – 0.45: Poor (Hard to comprehend, fatigue)
- < 0.30: Bad (Unintelligible)
Intelligibility & Clarity Scores
Speech Transmission Index (STI)
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Speech Clarity (C₅₀)
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Music Clarity & Consonant Loss
Music Clarity (C₈₀)
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Target: -2 to +4 dB
Consonant Loss (%AL_cons)
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Target: < 10% for classrooms
Performance Rating
IEC 60268-16 Qualification:
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Common Intelligibility Scale (CIS):
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Speech Privacy / Intelligibility:
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