Voice Alarm Intelligibility and Speech Transmission Index
A voice alarm exists to tell people what to do in a fire. If the message is loud but no one can make out the words, the system has failed at the one thing it is for. That is why voice alarm design is judged on intelligibility, not just sound level. This guide explains why intelligibility matters, how it is assessed, what degrades it, and what engineers should check.
The central point is simple: people must be able to understand the message, not merely hear that something is being said.
Who this is for
This is for competent fire alarm and voice alarm engineers working with voice alarm and PAVA systems. The experience level assumed is competent engineer. Use it for the principles; the specific intelligibility targets and design come from BS 5839-8, the product standards and the acoustic design of the space.
Why intelligibility matters
A voice alarm only works if people can actually understand what it is telling them to do. A message that is plenty loud but unclear can be as useless as silence, because occupants cannot act on words they cannot make out. So intelligibility — not just loudness — is a core design and commissioning requirement for voice alarm and public address / voice alarm (PAVA) systems, addressed by BS 5839-8 and the relevant product standards. The design has to deliver understandable speech throughout the areas it covers.
The Speech Transmission Index
Intelligibility can be measured. The Speech Transmission Index (STI) is a recognised measure of how intelligible speech is in a space, expressed on a scale, and is commonly used to assess whether a voice alarm meets an intelligibility target. It captures the effect of factors such as reverberation and background noise that degrade speech, giving an objective figure rather than a subjective impression. Whether and how STI is applied, and the target chosen, follows the design and BS 5839-8 rather than a single universal number.
What degrades intelligibility
Several things work against clear speech. Reverberation in hard, echoey spaces smears the words; high background noise masks them; and loudspeaker type, spacing and aiming all affect what actually reaches the listener. Large, reverberant spaces — atria, halls, concourses — are the hardest to get right. From field experience, intelligibility problems usually trace back to reverberation and loudspeaker layout rather than raw power. Good loudspeaker selection and layout, and sometimes acoustic treatment, are how intelligibility is achieved, as a design matter under BS 5839-8.
Servicing for intelligibility
Intelligibility can drift over a building's life. Confirm the system still delivers clear, understandable messages across the covered areas, that loudspeaker circuits and the controller are healthy, and that changes to the space have not degraded speech. From field experience, new partitions, added hard finishes, or a change of use that raises background noise are common reasons a once-clear system becomes hard to follow. Record findings and flag any area where the message is no longer clear, so it can be assessed properly rather than assumed adequate.
Common points to check
Recurring issues include reverberant spaces where speech is unclear, loudspeaker layout that leaves poorly-covered areas, and building changes that add reverberation or background noise. Confirming the message remains understandable throughout is the essential check.
When not to rely on this alone
When not to use this article: do not use it to set intelligibility targets or design loudspeaker coverage for a specific building. Those come from BS 5839-8, the product standards and a competent acoustic and voice-alarm design.
Relevant standards
Voice alarm design and intelligibility follow BS 5839-8, a code of practice, together with the relevant product standards; the fire alarm interface follows BS 5839-1. The legal duty for fire precautions in relevant premises sits under the Regulatory Reform (Fire Safety) Order 2005, with Building Regulations statutory guidance in Approved Document B applying to building work. Separate the legal duty from the recommended methods, and always work to current editions.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British Standards, the manufacturer's data, the acoustic design, or competent judgement. Verify voice-alarm intelligibility against current documentation.
Related documentation
Use this with the current BS 5839-8 and the relevant product standards, the acoustic and voice-alarm design, and the manufacturer's data. Record intelligibility results and loudspeaker coverage, and re-assess after any change to the space.