Why STI matters more than reverberation time, and how slatted timber systems balance warmth with clarity in unamplified halls.

Reverberation time has been the dominant metric for hall acoustics for over a century — but for spoken word performance, Speech Transmission Index (STI) tells a more honest story.

In our recent retrofit of the National Opera House, we paired curved slatted oak panels with perforated rear walls to bring STI from 0.48 to 0.74 without dampening the orchestral warmth singers depend on.

The key is targeted absorption in the 250–500 Hz band — where speech consonants live — while preserving reflections in the 2–4 kHz vocal formant region.