Fire Alarm Alarm Devices
Explained
The alarm devices in a fire alarm system are what the occupants actually hear and see when a fire is detected. Getting the type, number, and placement right is just as important as the detection side of the system — an alarm that cannot be heard or seen is an alarm that fails at the critical moment.
Motorised Bells
Motorised bells were the traditional fire alarm sounder of choice for many decades and are still found in a large number of existing systems — particularly in older commercial and educational buildings. They work by an electric motor striking a metal bell at high speed to produce the familiar ringing alarm sound.
| Detail | |
|---|---|
| Sound output | Typically 80–90 dB(A) at 1 metre — the continuous ring tone is immediately recognisable as a fire alarm to most people |
| Typical applications | Schools, older office buildings, industrial premises — primarily in existing systems where replacement would be costly |
| Advantages | Robust and reliable, highly recognisable tone, relatively low cost |
| Limitations | Fixed tone only — cannot generate different alert signals or be programmed for cause-and-effect. Mechanical moving parts mean more maintenance than electronic sounders. Higher current draw than electronic equivalents |
| New installations | Rarely specified in new systems — electronic sounders offer greater flexibility, lower current draw, and programmable tones |
| Standard | BS EN 54-3 |
Electronic Sounders
Electronic sounders have largely replaced motorised bells in new fire alarm installations. They use a piezoelectric or magnetic transducer driven by an electronic oscillator to produce sound, with no moving mechanical parts. Most modern electronic sounders are programmable — they can produce a range of different tones and sound patterns, allowing one device to serve multiple alert functions.
| Detail | |
|---|---|
| Sound output | Typically 85–100 dB(A) at 1 metre depending on model — higher output models are available for noisy industrial environments |
| Tone options | Most units can generate multiple tones — including the standard BS 5839 evacuation signal (continuous tone), alert signal (intermittent), and various other patterns for cause-and-effect programming |
| Typical applications | All types of new commercial and residential fire alarm installations — the default choice for modern systems |
| Advantages | No moving parts — highly reliable, low maintenance. Low current draw compared to bells. Programmable tones. Wide range of models for different environments including weatherproof and high-output versions |
| Standard | BS EN 54-3 |
Audibility Requirements
BS 5839-1 requires that the fire alarm signal achieves a minimum sound level of 65 dB(A) throughout the premises, or 5 dB(A) above any background noise if the background noise level exceeds 60 dB(A). In sleeping risk areas the standard requires a minimum of 75 dB(A) at the bedhead. Achieving these levels in practice requires careful device placement and coverage calculations — not simply fitting one sounder per floor and hoping for the best.
Combined Devices
Combined Sounder Beacons
Combined sounder beacons incorporate both an audible sounder and a visual strobe light in a single unit. The visual element — a high-intensity xenon or LED strobe that flashes at a set rate — provides a fire alarm warning to people who may not be able to hear the audible alarm, particularly those who are deaf or hard of hearing.
| Detail | |
|---|---|
| Visual output | High-intensity strobe — typically white or red — flashing at a rate specified by BS EN 54-23 to ensure it is perceived as an alarm signal rather than a routine indicator |
| Where required | BS 5839-1 requires consideration of visual alarm devices wherever deaf or hard-of-hearing people may be present. This effectively means most public buildings, workplaces, and any premises where the occupant profile cannot be guaranteed |
| Typical applications | Open-plan offices, factories, shopping centres, leisure facilities, hotels, schools, healthcare premises |
| Ceiling vs wall mounting | Coverage area depends on mounting height and flash intensity — ceiling-mounted units generally achieve better coverage than wall-mounted ones at the same output level. Manufacturer coverage data must be checked against BS EN 54-23 requirements |
| Standard | BS EN 54-3 (sounder element), BS EN 54-23 (visual alarm device element) |
Visual Alarm Devices
Standalone Visual Alarm Devices (VADs)
Where a visual warning is needed but an audible sounder is not required at that point — for example in a toilet cubicle where a sounder is already providing coverage from the adjacent corridor — a standalone visual alarm device (VAD) can be used. These are strobe-only units with no audible element.
| Detail | |
|---|---|
| Typical applications | Toilet cubicles, shower rooms, changing rooms, areas where a sounder would be disproportionately loud but a visual warning is still required |
| Legal context | The Equality Act 2010 places a duty on employers and service providers to make reasonable adjustments for disabled people — ensuring a fire alarm can be perceived by deaf and hard-of-hearing occupants is part of that duty |
| Standard | BS EN 54-23 |
Design Considerations
Getting Alarm Device Coverage Right
The placement and specification of alarm devices is a design exercise that requires calculation, not guesswork. Key considerations include:
Audibility Calculations
The system designer must demonstrate that the required sound levels are achieved throughout the building — in all areas including toilets, stairwells, plant rooms, and areas with high background noise. This typically requires acoustic modelling or calculation based on sounder output, room geometry, and absorption characteristics.
Visual Coverage Calculations
BS EN 54-23 specifies minimum illuminance levels that must be achieved within the covered area of each VAD. Manufacturers publish coverage data for each device — the designer must ensure the specified device achieves the required coverage in each room or space.
Sleeping Risk Areas
In sleeping risk premises — hotels, care homes, HMOs, student accommodation — additional requirements apply. Sounders must achieve 75 dB(A) at the bedhead, and in some cases low-frequency sounders (producing tones around 520 Hz rather than the standard 3,150 Hz) may be required to wake deep sleepers or those with high-frequency hearing loss.
Tone and Signal Discrimination
Where a premises uses different alarm signals for different purposes — for example a first-stage alert signal and a full evacuation signal — the tones used must be clearly distinguishable from each other and from any other signals used in the building, such as intruder alarms or public address systems.
Further reading
Concerned about your alarm coverage?
If you are unsure whether your current system provides adequate audible and visual coverage — particularly for deaf or hard-of-hearing occupants — get in touch for independent advice.
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