Fire alarm sound level
requirements
The minimum sound levels required by BS 5839-1, how they are measured, what they mean in different environments, and how sounder coverage is calculated for a building.
A fire alarm that cannot be heard is no alarm at all. BS 5839-1 sets precise minimum sound levels that must be achieved throughout a building — not at the sounder itself, but at every point where occupants may be present. Meeting these levels in noisy workplaces, across large open spaces, and in areas where sleeping occupants need to be woken requires careful sounder selection and placement.
BS 5839-1 Sound Level Requirements
BS 5839-1 specifies minimum sound pressure levels that must be achieved at specific measurement points. The key figures are:
| Requirement | Minimum sound level | Where measured |
|---|---|---|
| General alarm audibility | 65 dB(A) | At any point where persons may be present, measured 1m above floor level |
| Noisy environments | 5 dB(A) above background noise, or 65 dB(A) — whichever is greater | At any point in the noisy area |
| Waking sleeping occupants | 75 dB(A) | At the bedhead, with all intervening doors closed |
| Maximum level to avoid discomfort | 120 dB(A) | At any point — levels above this may cause pain and disorientation |
The 75 dB(A) bedhead requirement is particularly significant for hotels, care homes, HMOs, and any premises where people sleep. It means the sounder output must penetrate closed bedroom doors and still achieve 75 dB(A) at the pillow — a requirement that often drives the need for sounders within or close to each bedroom rather than relying on corridor sounders alone.
Understanding dB(A)
What the Sound Level Figures Mean
Sound levels are measured in decibels — specifically dB(A), which is a measurement weighted to reflect the sensitivity of human hearing. The dB scale is logarithmic: every increase of 10 dB represents a sound that is perceived as approximately twice as loud. This means:
- 65 dB(A) — roughly the level of a loud conversation; adequate for waking buildings where occupants are alert
- 75 dB(A) — roughly the level of a busy road; sufficient to wake most sleeping people through a closed door
- 85 dB(A) and above — very loud; appropriate for industrial environments with high background noise
Sound also attenuates with distance. Every doubling of distance from a sounder reduces the sound level by approximately 6 dB. A sounder rated at 90 dB(A) at 1m produces approximately 84 dB(A) at 2m and approximately 78 dB(A) at 4m. This attenuation must be factored into sounder coverage calculations.
Sounder coverage
Calculating Sounder Coverage
The number and positioning of sounders required for a building is determined by calculating the sound level achieved at all points throughout the building, accounting for the rated output of each sounder, the distance from the sounder to each measurement point, and the attenuation caused by walls, floors, and closed doors.
A single sounder in a large open-plan office may achieve the required 65 dB(A) throughout the space. A hotel corridor sounder will not achieve 75 dB(A) at the bedheads of rooms further along the corridor with doors closed — additional sounders, or sounders within each room, will be needed.
Door Attenuation — the Critical Factor for Sleeping Risk
A standard commercial door with a self-closer attenuates sound by approximately 20–30 dB. A fire door in a hotel corridor may reduce a 90 dB(A) corridor sounder to 60–70 dB(A) at the bedhead — below the 75 dB(A) requirement. This is one of the most common design failures in fire alarm systems for premises with sleeping risk. The solution is either sounders within each room, or sounders positioned to achieve sufficient level even with the additional door attenuation. The designer must verify compliance by measurement or calculation for each door configuration.
Visual alarms
Visual Alarm Devices (VADs)
For occupants with hearing impairments, audible sounders alone may not be sufficient. BS 5839-1 recommends that the need for visual alarm devices (VADs) — flashing strobes — is considered as part of the system design. VADs are typically required where the building is used by members of the public, where employees have declared hearing impairments, or where the noise environment would prevent reliable audibility of sounders.
The Equality Act 2010 reinforces this requirement — failure to provide adequate warning to people with hearing impairments could constitute a failure to make reasonable adjustments.
Common questions
Frequently Asked Questions
The only way to verify compliance with the BS 5839-1 sound level requirements is to measure the sound pressure level at the required points using a calibrated sound level meter. At commissioning, the installing engineer should verify that the required levels are achieved throughout the building. If you have any doubt — particularly about bedhead levels in sleeping accommodation — arrange for a competent engineer to carry out sound level measurements and produce a test record.
Yes — BS 5839-1 sets a maximum as well as a minimum. Sound levels above 120 dB(A) can cause pain and disorientation, which could impede evacuation rather than assist it. In practice, the maximum level is rarely approached in standard commercial buildings, but in small plant rooms or areas very close to high-output sounders it is worth checking.
BS 5839-1 recommends the use of a specific temporal pattern — Temporal Pattern 3 (TP3) — for fire alarm sounders. This is the intermittent pattern of three short pulses followed by a pause that most people associate with a fire alarm. Using a consistent pattern throughout a building — and ensuring it is different from any other alert tones used in the premises — helps occupants recognise a fire alarm signal immediately without confusion.