Voice Alarm (VA/VAS) Systems: BS 5839-8, Intelligibility Scores, Zoning and When VA Is Required vs Standard Alarms

Summary

A conventional fire alarm sounds the same tone everywhere: everyone hears it and everyone evacuates. This works well in small buildings where simultaneous evacuation is practical and safe. It does not work in a 30-storey office tower, a large hospital, a stadium, or a shopping centre, where simultaneous evacuation would itself create casualties through crushing, panic, and blocked escape routes. In these environments, a voice alarm system replaces or supplements the tone alarm with pre-recorded or live spoken messages that tell specific zones what to do and when.

Voice alarm systems are more complex to design, install, and maintain than conventional tone alarms, but they are not a luxury add-on. In any building where phased evacuation is the life-safety strategy, a voice alarm is not optional — it is the mechanism through which the phased strategy is delivered. A phased evacuation strategy that relies on staff telling people what to do verbally in a major fire is not a credible plan; it depends on staff remaining calm, being heard above a noisy alarm, and being in the right place at the right moment. Voice alarm eliminates this dependency.

BS 5839-8:2013 is the code of practice that governs the design, installation, commissioning, and maintenance of voice alarm systems. It is closely related to, and in many applications integrated with, BS 5839-1 (fire detection and alarm systems) and BS 5588 (fire safety in buildings — phased evacuation). Contractors working on VA systems need to understand both the acoustic engineering requirements (intelligibility testing) and the fire engineering requirements (zoning, phased evacuation logic, interface with detection panels) — or work closely with specialists in both disciplines.

Key Facts

• BS 5839-8:2013 — current UK code of practice for voice alarm systems; issued by BSI; not a statutory instrument but referenced by fire engineers, building control, and insurers
• CIS score — Common Intelligibility Scale; a logarithmic scale from 0 to 1 where 0 = completely unintelligible and 1 = perfect intelligibility; minimum requirement for VA systems is 0.7
• STI — Speech Transmission Index; the underlying measurement on a 0–1 linear scale; CIS 0.7 corresponds approximately to STI 0.45
• STIPA — Speech Transmission Index for Public Address systems; a faster, simplified measurement method suitable for on-site commissioning testing; the standard field measurement method for VA commissioning
• Measurement method — STIPA measurements using a calibrated test signal and receiver (e.g., Norsonic Nor140, Brüel & Kjær 2250, or similar) at representative measurement points throughout each zone
• Minimum measurement points — BS 5839-8 requires measurements at a minimum of one point per 25m² of floor area, with at least one measurement in every discrete area (corridor section, lobby, seating block)
• Speaker types — ceiling-mounted, wall-mounted, and column speakers are all used; the design must account for ceiling height, room absorption coefficient, and background noise levels
• Amplifier redundancy — BS 5839-8 requires that the failure of any single amplifier should not reduce the sound level in any occupied area by more than 6dB below the design level; in practice this means N+1 amplifier configurations for critical zones
• Emergency microphone — a dedicated, prioritised emergency microphone at a permanently staffed location (typically a control room or fire brigade access point) must allow live announcements to override pre-recorded messages in any or all zones
• Voice message priority — the message hierarchy must be: (1) live announcement via emergency microphone, (2) fire evacuation messages, (3) alert messages, (4) general PA. Lower priorities must not override higher priorities
• Zoning — VA zones should align with fire alarm zones for phased evacuation to work correctly; a VA zone can cover multiple fire alarm zones but a fire alarm zone must not span multiple VA zones
• Integration with fire detection — the voice alarm system must receive signals from the fire detection panel and automatically select and broadcast the correct pre-recorded message for the zone in which the detector has activated
• Standby power — the same BS 5839-1 battery backup requirements apply: 24-hour standby plus 30 minutes of full alarm operation
• Background noise — the minimum sound level in any area must be at least 10dB above the measured ambient background noise level (typically 65dB(A) minimum in most spaces; 75dB(A) minimum in areas with high background noise such as plant rooms)

Quick Reference Table

Detailed Guidance

CIS Score: What It Means and How It Is Measured

The Common Intelligibility Scale was developed to provide a single, accessible intelligibility metric derived from the Speech Transmission Index. The STI models how effectively a room's acoustics transmit speech — measuring the degradation caused by reverberation, background noise, and signal distortion. The CIS translates the linear STI scale into a logarithmic form that better corresponds to perceived intelligibility:

CIS = 1 + log₁₀(STI)

CIS 0.7 → STI ≈ 0.45 → "Fair" intelligibility
CIS 0.8 → STI ≈ 0.63 → "Good" intelligibility  
CIS 1.0 → STI = 1.0  → Perfect intelligibility (theoretical)

In practice:

• CIS < 0.5 — speech is largely unintelligible; occupants may hear noise but cannot reliably understand instructions
• CIS 0.5–0.7 — marginal; occupants can understand some words but may miss critical information
• CIS 0.7–0.8 — acceptable for emergency communications; most occupants will understand the message
• CIS > 0.8 — good intelligibility; appropriate for environments with vulnerable occupants or high safety criticality

The minimum CIS of 0.7 set by BS 5839-8 is a floor, not a target. For hospitals, schools, and care homes, many fire engineers specify CIS 0.8 as the design target to account for the higher proportion of occupants who may have hearing impairment, cognitive difficulties, or language barriers.

Field measurement using STIPA:

STIPA (Speech Transmission Index for Public Address systems) is the standard commissioning measurement method. It uses a test signal — a modulated noise that simulates the statistical properties of speech — broadcast through the VA speaker system. A calibrated meter at each measurement point captures the received signal and calculates the STI value. The process:

1. Position the measurement microphone at 1.5m height (ear height for a standing person) at the measurement point
2. Set the VA system to broadcast the STIPA test signal at the intended alarm level
3. The meter calculates the STI/CIS automatically
4. Record the result for that measurement point
5. Repeat at all required measurement points throughout the zone

Results are recorded on a zone-by-zone basis. Every measurement point must achieve CIS ≥ 0.7. If any point fails, speaker position, level, or orientation must be adjusted and the measurement repeated.

Zoning Differences from Standard Alarm Systems

Standard fire alarm zoning (BS 5839-1) divides a building into detection zones, each typically no larger than 2,000m² with a maximum dimension of 100m, to allow fire service personnel to locate a detector activation quickly. VA zoning serves a different purpose: it defines the groups of people who will receive the same voice message at the same time.

Key differences:

In a phased evacuation building, the VA zoning is determined by the fire engineer's evacuation strategy, which defines which zones are evacuated first (the fire zone), which are put on alert (adjacent zones above and below), and which remain in normal occupation. The VA system must be capable of addressing any combination of zones independently.

Phased Evacuation: When VA Is Required

Phased evacuation is a strategy where occupants are evacuated in a controlled sequence rather than all simultaneously. It is used where:

1. The building is too large for simultaneous evacuation to be completed within the Available Safe Egress Time (ASET) before conditions in escape routes become untenable
2. The structure provides sufficient fire resistance and compartmentation to hold occupants in unaffected zones safely while others evacuate
3. The number of occupants or the condition of vulnerable occupants makes simultaneous evacuation more dangerous than the fire itself

A tone alarm cannot deliver phased evacuation — it tells everyone to go, regardless of which zone they are in. A voice alarm can deliver different messages to different zones:

• Alarm zone: "This is a fire alarm. Please evacuate the building immediately using the nearest staircase. Do not use the lifts."
• Adjacent zones: "This is a fire alert. Please stand by for further instructions. Do not evacuate unless instructed to do so."
• Remote zones: Normal background or silence

BS 7974 (Application of fire safety engineering principles) and BS 9999 (Code of practice for fire safety in design, management and use of buildings) both recognise phased evacuation as a valid strategy, subject to the building meeting the necessary compartmentation standards and having an appropriate management plan.

Amplifier Redundancy: N+1 Configuration

BS 5839-8 requires that the failure of any single amplifier must not reduce the sound level in any zone below 6dB of the design target. In practice, this means:

• For each power output channel, there must be a standby amplifier ready to take over on failure
• The system must automatically switch to the standby amplifier without manual intervention
• The failure of the primary amplifier must generate a fault indication at the system controller and at the fire alarm panel
• Standby amplifiers must be capable of carrying the full load of the failed unit

A system with four power amplifiers serving a four-zone building would require at least one standby amplifier (N+1 = 5 total) to meet this requirement, assuming the standby can cover the load of any one of the four. Some designs use a matrix configuration where each amplifier can route to any zone, and one redundant amplifier is shared across all zones.

Emergency Microphone Requirements

The emergency microphone must be:

• Located at a permanently staffed position or at a defined fire brigade access point
• Protected against accidental or unauthorised use (typically key-switch or PIN activation)
• Capable of addressing any single zone, any combination of zones, or all zones simultaneously
• Given absolute highest priority — any active emergency microphone transmission must override all pre-recorded messages in the selected zones
• Connected to the system via a supervised, monitored circuit so that a cable fault is detected and alarmed

In practice, the emergency microphone position is typically within the building's control room or security desk, with a secondary position at the fire brigade entrance. The system configuration must make operation simple and unambiguous under the extreme stress of an emergency — large, clearly labelled zone selectors and a single "speak" button rather than a complex menu system.

Integration with Fire Detection Panels

The VA system controller receives activation signals from the fire detection panel. The integration must be designed and tested as a system, not two independent systems that happen to share a cable. Key integration requirements:

• The detection panel sends a signal to the VA controller identifying the activated zone (typically via a relay output or digital interface)
• The VA controller maps the detection zone to the correct VA zone(s) and activates the appropriate pre-recorded message sequence
• Faults in either system (detection panel fault or VA system fault) must generate a fault indication at the other system's panel or at a jointly monitored point
• The integration must be tested with live activation of detection devices to confirm the correct message sequence is broadcast in the correct zones
• The cause-and-effect matrix (which detection zone triggers which VA message in which VA zone) must be documented and included in the commissioning record

PAVA: Public Address and Voice Alarm

In many large buildings, the fire voice alarm function is integrated with the building's public address (PA) system. This combined system is known as PAVA (Public Address and Voice Alarm). PAVA systems must comply with BS 5839-8 for the voice alarm function while also meeting the building's PA requirements (background music, general announcements, etc.).

Critical requirement for PAVA: the fire/emergency function must have absolute priority over all PA functions, automatically interrupting any ongoing PA broadcast when a fire alarm is triggered. This priority must be hardware-enforced, not software-enforced — a software failure must not prevent the fire alarm from overriding PA.

BS 5839-8 requires that PAVA systems:

• Meet all the intelligibility requirements (CIS ≥ 0.7) in emergency mode even when the system is configured for PA broadcast levels that differ from alarm levels
• Maintain the fire alarm function even if components of the PA system fail
• Have a clear physical separation or fail-safe design between PA circuits and fire alarm circuits so that a fault in the PA system cannot impair the fire alarm function

Frequently Asked Questions

How is a voice alarm different from a standard fire alarm with recorded messages?

A standard fire alarm may play a recorded message as the alarm tone (the familiar "Please evacuate the building" recording), but this is typically the same message broadcast everywhere simultaneously. A true voice alarm system as defined by BS 5839-8 has:

• Zone-selective messaging (different messages to different areas simultaneously)
• Verified intelligibility (STIPA measurements to confirm CIS ≥ 0.7)
• Amplifier redundancy
• Emergency microphone override
• Supervised integration with the fire detection system
• Documented cause-and-effect matrix A simple recorded message played through a conventional alarm sounder circuit does not meet BS 5839-8 and is not a substitute for a VA system where one is required.

Can I use a voice alarm in a small building where it isn't technically required?

Yes, and it can be a good choice even in buildings where simultaneous evacuation is practical. For buildings with non-English-speaking occupants, elderly or vulnerable persons, or where the building operator wants to reduce panic during evacuation, a voice alarm that gives clear instructions improves evacuation outcomes. The main constraint is cost — VA systems are more expensive to design, install, and commission than tone alarm systems. For small buildings, the benefit typically does not justify the additional cost unless there is a specific risk driver.

Who carries out the STIPA measurement at commissioning?

The STIPA measurement must be carried out by a competent person using calibrated measurement equipment. This is typically an acoustic consultant, a specialist commissioning engineer from the VA system manufacturer, or a third-party fire alarm commissioning engineer trained in STIPA methodology. The measurement must be carried out with the building in its intended occupied state — with furniture, fitted carpet, and people present, as these significantly affect the acoustic characteristics of the space. Measurements on a bare, unfurnished floor will not represent operational conditions.

Does a voice alarm system need weekly testing like a conventional fire alarm?

Yes. BS 5839-8 requires the same periodic testing regime as BS 5839-1. The responsible person must test the system weekly by activating at least one call point or initiating a test alarm from the panel, confirming that the correct message is broadcast in the correct zones. The test must confirm both the detection function and the voice broadcast function. Intelligibility measurements are not required at weekly test — these are carried out at commissioning and at annual inspection.

What happens to the voice alarm if the mains power fails?

The battery backup requirement for VA systems is the same as BS 5839-1: 24-hour standby followed by 30 minutes of full alarm. The battery backup system must power all amplifiers, the system controller, and all active speakers at full alarm level for the 30-minute alarm period. Battery sizing for a VA system is particularly critical because power amplifiers draw significantly more current during full alarm than conventional sounders, and the calculation must account for the full amplifier load, not just the controller.

Regulations & Standards

• BS 5839-8:2013 — Fire detection and alarm systems for buildings. Code of practice for the design, installation, commissioning and maintenance of voice alarm systems. Primary standard for VA design and commissioning in the UK.

• BS 5839-1:2017 — Required reading alongside BS 5839-8 for systems integrating voice alarm with fire detection; sets requirements for battery backup, maintenance, and false alarm management

• BS 9999:2017 — Code of practice for fire safety in design, management and use of buildings; defines when phased evacuation strategies are appropriate and what systems are required to support them

• BS 7974:2019 — Application of fire safety engineering principles; framework for performance-based fire safety design including ASET/RSET calculations that justify phased evacuation

• BS EN 60849:1998 — European standard for sound systems for emergency purposes; provides technical performance requirements for amplifiers and loudspeakers in emergency sound systems; underpins the equipment specifications in BS 5839-8

• Regulatory Reform (Fire Safety) Order 2005 (SI 2005/1541) — Articles 14 and 17 require appropriate fire detection/alarm systems and their maintenance; applies to VA systems

• Building Regulations Approved Document B: Fire Safety (2019 edition) — references voice alarm systems in the context of phased evacuation strategies for high-rise residential and complex non-residential buildings

• BS 5839-8:2013 — BSI, primary standard for voice alarm systems

• BS EN 60849:1998 — sound systems for emergency purposes — BSI, European technical standard for emergency sound system equipment

• FIA guidance on voice alarm systems — Fire Industry Association technical bulletins on VA design and commissioning

• Approved Document B Volume 2 — GOV.UK, Building Regulations fire safety guidance for buildings other than dwellinghouses

• CIRIA guidance on BS 9999 phased evacuation — CIRIA guidance documents on fire safety engineering and evacuation strategy

• bs 5839 1 fire alarm standard — Foundation standard for fire detection and alarm systems that VA systems integrate with

• fire alarm zoning design — Zone design principles applicable to both detection and VA zoning

• fire alarm battery backup — Battery backup sizing, which applies directly to VA amplifier systems

• fire alarm commissioning procedure — Commissioning documentation requirements including STIPA test records