Voice-evacuation intelligibility: STI per seat in a real evacuation, not a clean room

Voice-evacuation is the most consequence-bound PA system in any building. The acceptance criterion is intelligibility during an evacuation event — when the alarm is sounding, the doors are opening, the population is in motion, and the announcement has to compete against everything else for cognitive bandwidth. STI ≥ 0.50 at every seat in the empty room is the floor; STI ≥ 0.50 during the actual evacuation is the engineering target.

## Zone-loudness margin against evacuation-event ambient

The catalogue zone-loudness specification is typically ≥ 10 dB above standing ambient (the room's background noise floor in normal use). The evacuation-event ambient is much higher — alarm sounders at 80-90 dBA, audience movement noise at 65-70 dBA, door operation noise at 60-65 dBA. The voice-evacuation PA must deliver ≥ 10 dB above the evacuation-event ambient, which means 95-105 dBA at every seat under evacuation conditions. Specifying against standing ambient and discovering the gap during the event is operationally late.

## Multi-language message ordering

Public-occupancy buildings in India routinely require evacuation announcements in 2-4 languages (English, Hindi, regional language, sometimes additional for transit hubs and airports). The ordering and timing of the languages is part of the engineering: each language takes 12-18 seconds for a complete evacuation message; the message library must be sequenced so the most-comprehended language plays first, followed by additional languages on a rotating cycle, with sufficient pause for the population to act on the message rather than waiting for the next language. Generic preset libraries miss this.

## Hardware-enforced priority over routine paging

PA voice-evacuation priority must be hardware-enforced, not software-configured. The fire-alarm panel's voice-evac module wires into the highest-priority input on every zone amplifier; on a fire-alarm trigger, the zone amplifiers switch to the voice-evac source regardless of what the routine PA is broadcasting. A software-configured priority that a routine paging operation can over-ride is a procedural failure waiting to happen — and we test the hardware priority on every commissioning by triggering a fire-alarm event during a routine page.

## Supervised cabling with per-zone fault reporting

IS 16102 / EN 54-16 voice-evacuation requires supervised cabling — the system monitors every cable run between the panel and the amplifier, between the amplifier and every speaker, with per-zone fault reporting on cable open, cable short or amplifier failure. Supervision is engineered at the cable layer with end-of-line resistors or active line-monitoring; without it, a single cable fault collapses a whole zone silently and the operator discovers the fault during the evacuation event.

## Battery-backed amplification for 30 minutes minimum

Voice-evacuation amplification must continue through power failure — the standard requires 30 minutes minimum battery autonomy under full-load voice-evacuation discharge. The battery bank is sized against the full-discharge load profile, not the standing-charge load; sealed maintenance-free batteries on a 3-year refresh cycle for the 30-minute guarantee.

## Re-measurement after any room-acoustic change

STI is a function of the room's acoustic envelope — carpet, ceiling treatment, audience-chair upholstery, layout reconfiguration all shift the RT60 and therefore the achievable STI. Any room-acoustic change triggers an STI re-measurement on the AMC schedule; the original commissioning STI is the starting baseline, not the lifetime guarantee.

## Callout — what evacuation-PA procurement most miss

**STI per seat at commissioning is the starting point, not the operational guarantee.** The evacuation-event STI is what decides whether the population evacuates safely; engineer the zone-loudness margin, the multi-language sequencing, the hardware priority, the supervised cabling, the battery backup and the AMC re-measurement discipline against the event, not the clean-room test.

## Reference deployment context

Capital Cultural Hall Kohima (1,800 seats) and Town Hall Auditorium Dimapur (374 seats) both run voice-evacuation overlay on the PA chain per IS 16102, with hardware-enforced priority on the zone amplifier, supervised cabling per zone, battery-backed amplification for ≥ 30 minutes and per-zone fault reporting on the fire-alarm panel.

## References

1. IS 16102:2014 — Voice Alarm Systems — Sound systems for emergency purposes.

2. IEC 60849 — Sound systems for emergency purposes (international reference).

3. EN 54-16 — Fire detection and fire alarm systems — Voice alarm control and indicating equipment.

4. NBC 2016 Part 4 — fire and life-safety provisions for assembly occupancy.