Field Story
Failing BB93: The Auditorium That Could Not Graduate
A new school auditorium failed its BB93 acoustic assessment with RT60 of 1.9 seconds at 500 Hz against the 1.0-second requirement. The building could not receive its occupancy certificate. Impulse response measurements across 6 receiver positions confirmed the issue. Installing Class A absorbers on 35% of the rear wall and ceiling clouds above the seating brought RT60 to 0.95 seconds, passing the standard.
School AuditoriumSTI Measurement per ISO 3382-2
TL;DR
ISO 3382-2 reverberation time measurement in ordinary rooms directly feeds into STI prediction for speech-critical spaces like classrooms, offices, and hospitals. By measuring RT60 per octave band as specified in ISO 3382-2, you can estimate STI using the IEC 60268-16 indirect formula without a separate STIPA measurement. SonaVyx combines both workflows, letting you measure RT60 per ISO 3382-2 and automatically predict STI from the same impulse response data. This is particularly valuable for classroom acoustics compliance where both BB93 RT60 limits and minimum STI requirements must be met.
RT60 and STI in Ordinary Rooms
Ordinary rooms per ISO 3382-2 are where speech communication typically matters most: classrooms, meeting rooms, healthcare spaces, and open offices. The RT60 measured per this standard directly predicts speech intelligibility.
Classroom Acoustics
- BB93 (UK) requires RT60 below 0.8 seconds for classrooms up to 350 cubic meters
- This corresponds to STI around 0.55-0.65 for unamplified speech
- SEN (special educational needs) rooms require RT60 below 0.6 seconds
- ANSI/ASA S12.60 requires RT60 below 0.6 seconds for core learning spaces
RT60 to STI Estimation
- Measure RT60 per octave band (125 Hz to 8 kHz) per ISO 3382-2
- Measure ambient noise per octave band with the SPL meter
- Apply the IEC 60268-16 indirect formula: MTF = f(RT60, SNR)
- Calculate the weighted STI from the 7-band MTF values
Target Values by Room Type
Recommended RT60 values to achieve STI above 0.50:
- Classrooms: RT60 below 0.8 s (target STI above 0.55)
- Meeting rooms: RT60 below 0.6 s (target STI above 0.60)
- Hospital wards: RT60 below 0.8 s (target STI above 0.50)
- Open offices: complex, depends on masking and distance
Common Mistakes
- Using a single broadband RT60 value instead of per-band values for STI prediction
- Ignoring ambient noise, which may dominate STI degradation in occupied rooms
- Measuring in unoccupied rooms and reporting STI without noting that occupancy improves absorption
- Confusing ISO 3382-2 (ordinary rooms) with ISO 3382-1 (performance spaces) requirements
SonaVyx Tools
Measure RT60 with the SonaVyx RT60 tool per ISO 3382-2. Get STI predictions from the STI calculator. Measure ambient noise with the SPL meter. Capture impulse responses with the IR tool for direct STI derivation. Calculate treatment with the treatment advisor. Visit AcousPlan for design-stage predictions.
Standard Reference
ISO 3382-2:
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Last updated: March 19, 2026