Sounders and Visual Alarm Devices
Detection is only half of a fire alarm system's job. The other half is alerting people effectively and getting them moving. That depends on the alarm being loud enough and, where needed, visible enough to reach everyone in the building. This guide explains the alerting principle behind sounder and visual alarm device (VAD) coverage — and is deliberately clear that the exact sound levels and coverage figures belong to the current standards and a calibrated measurement on site, not to memory.
Alerting is easy to get wrong in a way that only shows up in a real evacuation, so understanding the principles is worth the effort.
Who this is for
This is for competent fire alarm engineers commissioning or servicing sounders and VADs, or advising on audibility problems. The experience level assumed is competent engineer. Use it for the reasoning; use the current standards and a calibrated sound level meter for the numbers.
The alerting principle
The purpose of the alarm signal is to be recognised. Occupants must be able to distinguish it clearly from the normal sounds of the building and understand that it means evacuate. That requires the signal to stand out above the background noise everywhere people may be, and it requires the sound to reach through the building's structure, doors and partitions.
Audibility and background noise
Because the alarm has to be heard above background noise, the required level depends on how noisy the space normally is. BS 5839-1 sets recommended minimum sound levels for general areas, with different treatment for stairways and small enclosures. These are recommendations applied by a competent designer, and they are verified in practice by measurement. This article deliberately does not quote the figures — take them from the current edition and confirm them with a calibrated meter.
Sleeping risk
Where people sleep, the alarm must be loud enough to wake them, so a higher sound level is recommended at the bedhead than in general areas. Sleeping-risk premises — hotels, care settings, residential accommodation — therefore need particular attention to sounder provision. Again, the specific level is set by the standard and confirmed by measurement.
Visual alarm devices
Visual alarm devices provide a flashing light alert where sound cannot be relied upon: noisy plant areas, spaces used by people who are deaf or hard of hearing, and locations such as toilets or lone-working areas. VADs manufactured to BS EN 54-23 carry a defined coverage volume — a stated cuboid the device can illuminate to the required intensity — which the designer uses to lay them out. Whether VADs are needed, and where, follows the risk assessment and the standard.
Voice alarm and phased systems
In larger or complex buildings, a voice alarm system delivering intelligible messages can improve response and support phased evacuation. The choice between tone sounders and voice alarm is a design decision driven by the building, its occupants and the evacuation strategy, and voice systems carry their own standards and commissioning requirements.
Verifying coverage on site
Audibility is verified by measurement. Take readings with a calibrated sound level meter at representative points, compare against background noise and the recommended levels, and record the results. From field experience, shortfalls commonly appear behind closed fire doors, in noisy plant rooms, and in rooms added or repurposed since the original design. If levels fall short, the fix is a design one — more or relocated devices — not overdriving devices beyond their rated output.
When not to rely on this alone
When not to use this article: do not use it to set sound levels, VAD coverage or device counts. Those come from the current BS 5839-1 and BS EN 54-23, applied by a competent designer and confirmed by measurement. Sleeping-risk levels, stairway allowances and small-room provisions all carry specific values in the standard.
Relevant standards
Audibility recommendations sit in BS 5839-1, a code of practice; visual alarm device performance is defined in BS EN 54-23, a product standard. The legal duty to warn occupants in most non-domestic premises derives from the Regulatory Reform (Fire Safety) Order 2005. When advising a client, separate the legal duty to provide effective warning from the recommended methods and figures, and always work to current editions.
Professional disclaimer
This is an educational resource for competent engineers and does not replace the current British Standards, manufacturer data, the site fire strategy, or competent design judgement. Verify all audibility and coverage decisions against current documentation and calibrated measurement.
Related documentation
Use this with the current BS 5839-1 and BS EN 54-23, the device manufacturer's data, and the site design. Record all sound level readings and VAD coverage checks in the commissioning and service documentation.