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Speech Transmission Index ("STI")

The Speech Transmission Index (STI) is a standardised metric used to evaluate speech intelligibility (how clearly speech can be understood) within communication systems, such as public address and emergency warning systems. STI quantifies speech intelligibility by measuring the effects of factors like background noise, reverberation, and distortion on the speech signal.

STI is calculated using modulation transfer functions (MTFs), which analyse how well variations in speech intensity (modulations) are preserved as the signal passes through a system.

The STI scale ranges from 0 (unintelligible) to 1 (perfect intelligibility). A higher STI score indicates better speech clarity and intelligibility. 

This measure is widely used in designing and testing systems in environments like auditoriums, transportation hubs, and buildings, ensuring effective communication during both normal and emergency conditions.

    Disclaimer

    In preparing this definition, we have drawn from various sources including Legislation, Codes, Standards and industry information, research and knowledge.  Like the english language, these definitions may subtly change from time to time. As such these definitions are provided solely on the basis that users will be responsible for making their own assessment of the definition and and are advised to verify all relevant representations, statements and information.