Fire alarm devices
explained
Motorised bells, electronic sounders, sounder beacons, and visual alarm devices — what each type does, how they differ, and where each one is used.
The alarm devices on a fire alarm system — the sounders, bells, and beacons — are the part the building’s occupants actually experience. Getting the type, placement, and volume level right is as important as the detection side of the system.
The Role of Alarm Devices
When a fire alarm system detects a fire, it must warn the building’s occupants quickly and clearly enough for them to evacuate safely. BS 5839-1 sets out requirements for alarm device coverage and sound pressure levels — see our dedicated guide to fire alarm sound level requirements for the full specification. The choice of alarm device type depends on the building, the occupancy, the noise environment, and whether any occupants have hearing impairments that require visual alarm devices.
Audible devices
Motorised Bells
How they work
Motorised bells — also called electric bells or alarm bells — use an electrically powered motor to strike a metal dome, producing a continuous ringing sound. They are one of the oldest and most familiar forms of fire alarm sounder, particularly associated with older conventional fire alarm systems in schools, industrial, and commercial premises.
| Characteristic | Detail |
|---|---|
| Sound output | Typically 85–95 dB(A) at 1 metre — loud and distinctive |
| Sound character | Continuous ringing — familiar and immediately recognisable as a fire alarm signal |
| Typical applications | Schools, factories, warehouses, older commercial premises — where the conventional ringing bell tone is preferred or already established |
| Advantages | Robust, reliable, simple, and low cost. Widely understood as a fire alarm signal by the general public. |
| Limitations | Fixed tone — cannot be used for multi-tone systems. Larger and heavier than electronic sounders. Less flexible for modern addressable systems. |
| Wiring | Typically wired on conventional sounder circuits — one circuit per zone or area |
Motorised bells remain common in schools and older industrial premises. In new installations, electronic sounders have largely replaced them due to their smaller size, lower current consumption, and tonal flexibility. If you have a sounder circuit fault, identifying which type of sounder is affected helps determine the correct diagnostic approach.
Electronic Sounders
How they work
Electronic sounders use a piezoelectric transducer or a small loudspeaker driven by an electronic circuit to produce alarm tones. They can be programmed to produce a range of different tones — continuous, pulsed, swept, or coded — and on addressable systems individual sounders can be controlled and set independently from the fire alarm panel.
| Characteristic | Detail |
|---|---|
| Sound output | Typically 85–105 dB(A) at 1 metre depending on type and model |
| Tone options | Wide range — continuous, pulsed, warble, sweep, coded tones (e.g. BS 5839 temporal pattern for evacuation) |
| Typical applications | The standard sounder type for most modern commercial, residential, and industrial fire alarm systems |
| Advantages | Compact, lightweight, low current consumption, highly flexible tone programming, suitable for both conventional and addressable systems |
| Addressable versions | Individual sounders can be switched on or off from the panel — enabling phased or staged evacuation and zone-specific alarming |
| Standard | EN 54-3 |
Electronic sounders are the default alarm device for the majority of new fire alarm installations. Their tonal flexibility makes them suitable for complex cause and effect evacuation strategies — for example, sounding an alert tone in adjacent zones while sounding a full evacuation tone in the affected zone.
Combined Sounder Beacons
How they work
Combined sounder beacons integrate an electronic sounder with a high-intensity strobe light in a single unit. The visual strobe flashes in synchronisation with or independently of the audible alarm, providing a visual warning to supplement the audible signal. They are available in red, white, and amber variants.
| Characteristic | Detail |
|---|---|
| Primary purpose | To provide both audible and visual fire alarm warning from a single device, reducing the number of separate units required |
| Typical applications | Noisy environments where audible warning alone may not be sufficient; areas where deaf or hearing-impaired occupants may be present; areas where BS 8300 requires visual warning |
| Flash rate | Typically 0.5–2 Hz — regulated to avoid triggering photosensitive epilepsy |
| Standard | EN 54-3 (sounder element), EN 54-23 (visual alarm device element) |
Visual alarm devices
Visual Alarm Devices — VADs
How they work
Visual alarm devices are standalone strobe or flashing light units without an integrated sounder. They provide a visual fire warning signal supplementary to the audible alarm — essential in areas where the audible signal may not be heard, or where occupants may have hearing impairments.
| Characteristic | Detail |
|---|---|
| When required | The Equality Act 2010 requires that fire alarm systems provide an accessible warning for deaf and hearing-impaired occupants. Where any area may be occupied by someone who cannot hear the audible alarm, a VAD should be provided. |
| Typical locations | Toilets, individual offices, sleeping areas in hotels, changing rooms, meeting rooms — anywhere a person may be isolated from the main alarm sound |
| Coverage | EN 54-23 specifies minimum coverage volumes — VADs must be positioned so the flash is visible from any point in the covered area |
| Synchronisation | Multiple VADs in the same field of view should be synchronised to avoid an unsynchronised strobe effect that could trigger photosensitive epilepsy |
| Standard | EN 54-23 |
VADs are increasingly required in new and refurbished buildings as awareness of accessibility obligations grows. They should be considered at the design stage rather than retrofitted as an afterthought — see our guide to fire alarm system design for the full design framework.
Sound levels
What Does BS 5839-1 Require for Alarm Audibility?
BS 5839-1 sets minimum sound pressure level requirements for fire alarm sounders throughout the protected premises:
| Location | Minimum sound level required |
|---|---|
| General areas (to alert occupants) | 65 dB(A) or 5 dB(A) above any background noise level if higher |
| Areas where sleeping is likely | 75 dB(A) at the bedhead — sufficient to wake sleeping occupants |
| Areas with high background noise | 5 dB(A) above the measured background noise level |
Achieving these levels throughout a building requires careful sounder placement — particularly in areas with high ceilings, absorbent surfaces, or complex layouts. Sounder positioning should always be determined by acoustic calculations or practical testing during commissioning, not assumed.