Wave Dynamics GuidesSpeech Intelligibility and STI Explained
What the Speech Transmission Index (STI) is, why voice alarm and PA systems are tested for it, typical targets and how to improve intelligibility.
In Short
The Speech Transmission Index (STI) is a measure of how clearly speech can be understood through a sound system or in a room, on a scale from 0 (unintelligible) to 1 (perfect). It is widely used to test voice alarm and public address systems, where emergency messages must be understood.
- STI is measured to IEC 60268-16.
- Voice alarm systems commonly target an STI of 0.50.
- Reverberation and background noise are the main causes of poor intelligibility.
- Testing on completion confirms the system performs as designed.
Why Intelligibility Matters
In an emergency, a voice message is only useful if people understand it. Stations, airports, stadia, shopping centres, offices and schools rely on voice alarm and PA systems, and fire safety standards require those messages to be intelligible.
How STI Is Measured
A test signal is played through the system and measured at positions across each area with calibrated equipment. The STI value accounts for how reverberation, echoes and background noise blur the speech signal. Results are compared with the target set in the specification or the relevant fire safety standard.
What Affects STI
- Long reverberation times in hard, large spaces.
- High background noise from crowds, plant or traffic.
- Loudspeaker type, number, spacing and aiming.
- System equalisation and level settings.
Improving Results
Improvements can come from adding acoustic absorption, using more directional loudspeakers closer to listeners, adjusting system tuning or zoning. Considering acoustics and system design together at the outset is the most effective route. See our guide to reverberation time.
Frequently Asked Questions
What STI value is required?
Voice alarm systems commonly target a minimum STI of 0.50 in most areas, but the required value depends on the standard and specification for the project.
Can STI be predicted at design stage?
Yes. Acoustic modelling can predict STI so loudspeaker layouts and room finishes can be optimised before installation.
This guide gives general information and is not a substitute for project-specific advice. Requirements change, so always check the current regulations, guidance and your planning conditions. Ask our team about your project.
Jon Maguire (BSc Hons MIOA) brings over 8 years of expertise in acoustic consultancy, specialising in environmental acoustics with a focus on 3D modelling, assessments, and detailed design for industrial, commercial, and energy projects. He has lead noise assessments for key clients while thriving in multidisciplinary environments. Jon’s diplomatic communication skills are ideal for stakeholder consultation.
Jack is a Field Engineer at Wave Dynamics, supporting the delivery of environmental monitoring and acoustic consultancy services through detailed on-site fieldwork. He has hands-on experience in noise monitoring, vibration monitoring, and dust monitoring across a variety of infrastructure, construction, and environmental projects. Jack is involved in the setup, operation, and maintenance of monitoring equipment, ensuring accurate data collection in line with relevant standards and project requirements. He works closely with consultants and project teams, providing reliable field support that underpins high-quality assessments and reporting.
Aidan is the Director for all U.K Operations and has worked in multiple industries including for clients in the UK and internationally operating in the residential, industry, healthcare, education, and construction sectors.
