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Collaboration, Voice & AV · Advanced

Speech Transmission Index (STI)

A 0-to-1 objective measure of speech intelligibility through a sound system and room, defined in IEC 60268-16.

Definition

STI measures how well modulation of a test signal is preserved through the transmission path, capturing the effects of noise, reverberation and echoes. Higher values mean better intelligibility, and emergency voice systems often require minimum values around 0.5 or higher. STIPA is a simplified, faster version commonly measured with handheld meters.

Example

A paging system in a terminal is measured with STIPA and achieves 0.55 across the coverage area.

Exam tip

STI is a 0-to-1 scale; reverberation and noise both lower it.

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