Speech Intelligibility in a Voice Alarm System: What It Depends On and How to Achieve It Through Equipment Selection

Hearing the alert signal is not enough — during an evacuation, every word of the voice message has to be understood. Let's look at what actually drives speech intelligibility and how to choose equipment that delivers it.

What has the biggest effect on speech intelligibility?

It comes down to how far the signal rises above the background noise, the room acoustics (reverberation), how even the coverage is, the frequency response of the loudspeakers across the speech band, and how much power headroom you have. No single factor works in isolation — it's the balance between them that matters.

Is there a specific intelligibility figure you have to hit?

Threshold values are set by the applicable codes of practice for voice alarm and evacuation — in Russia, the SP (Svod Pravil) building rules — and are verified for each specific site at the design stage. In broad terms, the goal is simple: the voice message must be understood at every point within the alarm zone. The exact numerical requirements are set by the licensed design organization in line with the governing standards.

Which loudspeakers work best in a tall, reverberant hall?

Usually column loudspeakers: their narrow vertical directivity pattern focuses the sound onto the listeners and cuts down harmful reflections off the ceiling and far walls. For low-ceilinged spaces, ceiling loudspeakers are often the better fit; for noisy areas, horn loudspeakers.

Why can't you just fit a more powerful loudspeaker?

Excess power at a single point doesn't improve intelligibility — more often it makes it worse, driving up reflections and pushing the room into overload. An even sound field comes from a uniform grid of moderate-power sources, not from one loud emitter.

Does VPA design the voice alarm system?

No. VPA is an equipment supplier: we help you select loudspeakers and amplifiers to suit your acoustics and put together an approximate bill of materials. The system design, installation, and formal sign-off of code-compliant intelligibility are handled by a licensed specialist organization.