16.1 Commentary

General Requirements:

Alarm signals must be adequate to warn all intended persons.
In simple buildings (Category M or L systems), alarms must alert all occupants regardless of location.
If occupants sleep in the building, alarms must rouse them.

Complex Buildings:

General alarms might be inappropriate (e.g., hospitals, care premises) where staff-only alerts may suffice.
In large or complex buildings, evacuation signals might be initially limited to specific areas.

Category P Systems:

Alarms might only alert key personnel for firefighting actions.
Often combined with Category M systems, where Category M considerations take precedence.

Audible Alarm Signals:

Sound pressure level and frequency must provide a clear fire warning.
Ensure adequate levels in small spaces (offices, toilets, plant rooms).

Speech messages must be intelligible.

Sound Pressure Levels:

Minimum acceptable level: 65 dB(A).
Acceptable at specific points: at least 60 dB(A).
In small spaces and stairways: 60 dB(A) is acceptable.
Prefer more quieter sounders to prevent excessive noise.

Alarm Continuity and Silencing:

Alarms must continue until manually silenced.
External fire alarm devices can help direct firefighters on large sites.

Prolonged external alarms should be avoided due to disturbance risks.

Radio-Linked Systems:

Battery-operated sounders may be automatically silenced to prevent battery exhaustion, subject to authority approval.

This summary highlights the critical aspects of fire alarm signal requirements, ensuring effective and appropriate alerts for all building types and situations.

16.2 Recommendations

16.2.1 Recommendations applicable to Category M and L systems (other than in hospitals and residential care premises)

The following recommendations are applicable.

Sound Pressure Levels:

Minimum 65 dB(A) throughout accessible areas, except:
60 dB(A) in stairways, small enclosures (≤ 60 m²), and specific limited points.
Where background noise > 60 dB(A), the fire alarm signal must be 5 dB above background noise.
Minimum 75 dB(A) at bedhead in rooms meant to rouse sleeping individuals.
Maximum 120 dB(A) at any accessible point.

Frequency and Characteristics:

Fundamental frequency range: 500 Hz to 1,000 Hz.
Continuous output is preferred, but frequency and amplitude may vary (e.g., warbling note).
All fire alarm sounders in a building should have similar sound characteristics, distinct from other alarms.

NOTE 1 Background noise that is unlikely to persist for longer than 30 s may be ignored.

NOTE 2 This recommendation does not apply to noise created by running water in bathrooms and shower rooms.

NOTE 3 Typical noise levels in buildings are given in Annex B.

Special Conditions:

In public entertainment and retail premises, music > 80 dB(A) should mute automatically when a fire alarm is triggered.
In noisy areas, primary sounders can be supplemented by secondary mains-powered sounders that silence noisy machines if power is lost.

NOTE 4 Experience has shown that this normally necessitates the provision of a fire alarm sounderwithin the room in question.

External Sounders:

External sounders and/or visual alarm devices may be used to direct firefighters, subject to consultation with the fire service.
External sounders should be clearly marked with “FIRE ALARM”.

NOTE 5 All these figures are arbitrary, but are judged to be generally appropriate.

NOTE 6 In carrying out measurements to verify compliance with these recommendations, other than in the case of 16.2.1a)3), accounts need not be taken of sound pressure levels within 500 mm of any walls or partitions.

NOTE 7 Measurements need to be made with all doors shut.

NOTE 8 An instrument conforming to BS EN 61672-1, set to slow response and A-weighting, is suitable for measuring the sound pressure level of the alarm signal.

NOTE 9 It is often necessary to measure the sound pressure level of the fire alarm signal in the presence of background noise. The sound pressure level of the fire alarm signal (in isolation) can be deemed to be 5 dB above background noise if, when the background noise is present, a sound pressure level increase of 6 dB occurs on operation of the fire alarm system.

NOTE 10 On a site where occupants move regularly between buildings, there is benefit to adopting a fire alarm signal that has similar sound characteristics throughout all buildings on the site.

NOTE 11 In premises with two (or more) stage alarm systems, consideration ought to be given as to whether the music is to be muted at the “Alert” or the “Evacuate” stage.

NOTE 12 Where the sound pressure level of the music is likely to be between 60 dB(A) and 80 dB(A), the recommendation of 16.2.1a)2) applies.

Alarm Silencing:

Manual operation required for silencing.
Visual signals at control equipment should not be canceled by silencing.
Sounders should reactivate if a new zone goes into alarm or a previous zone alarm is reactivated.
Silencing should not prevent correct operation of starting/restarting sounders or alarm transmission to an ARC.

Automatic Silencing:

External sounders in Category L and P systems should automatically silence after 30 minutes unless continuously occupied.
Battery-powered radio-linked sounders should also silence after 30 minutes unless continuously occupied, with possible extensions for phased evacuation.
In multi-stage systems, alert signals may auto-silence after a set period.

Additional Sounders:

At least two sounders required, with at least one in each fire compartment, even if sound pressure levels can be met with one sounder.

Speech Messages:

Follow BS 5839-8 for message content, sound pressure levels, and speech intelligibility.
Voice sounders should comply with BS 5839-8 and standard recommendations.

Exclusive Use:

● Fire alarm sounders should only warn of fire unless the response is identical to a fire (immediate evacuation).

● In schools, fire alarms may signal the start or end of periods, not exceeding 5 seconds.

This summary encapsulates the key requirements for fire alarm signal levels, characteristics, and operational guidelines, ensuring effective and compliant fire warning systems.

Figure 4 — Sound pressure levels [see 16.2.1a)]

16.2.2 Recommendations applicable to Category P systems

The recommendations of 16.2.1 apply in respect of those areas where audible alarm signals are required.

16.2.3 Recommendations applicable to hospitals and residential care premises

The following recommendations are applicable.

a) Audible alarms in hospitals should conform to the recommendations of the Department of Health

publications HTM 05-03 Part B [N1] (in England and Wales) or SHTM 82 [N2] (in Scotland).

b) Audible alarms in residential care premises should conform to 16.2.1, except that, if the alarm signal is not intended to rouse occupants of some or all rooms from sleep, 16.2.1a)3) does not apply.

16 Audible alarm signals