Application Guide
2.5. ALARM SIGNALS
2.5.1.Audible Alarm Signals
Audible fire alarm signals must provide a clear warning of a fire to all those for whom the signal is intended. For category M and L systems this would normally imply all occupants
of a building, however in some sites this may not apply, for example in hospitals or rest homes, residents might need assistance to evacuate, in which case it may be sufficient to alert staff.
The general requirement for the volume of audible alarm signals is that they should provide a Sound Pressure Level (SPL) of at least 65dB(A), but not more than 120dB(A) throughout all accessible areas of a building. See figure 2.5.1.
Minimum 65dB(A)
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| 65dB(A) | 65dB(A) |
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Area<60m² | Min60dB(A) | Minimum | Minimum |
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65dB(A) | 65dB(A) |
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| Min 60dB(A) |
Figure 2.5.1. General Fire Alarm Sound Pressure Levels
Exceptions to this general rule are as follows:
•In stairways the SPL may be reduced to 60dB(A)
•Enclosures less than 60m² may be reduced to 60dB(A)
•There is no minimum for enclosed areas less than 1m²
•At specific points of limited extent the SPL may be reduced to 60dB(A)
Where a continuous background noise level greater than 60dB(A) is present the fire alarm signal should be 5dB above the ambient, but not greater than 120dB(A).
Sounder
Minimum
85dB(A)
Machinery
Generating
80dB(A)
Where the alarm is intended to wake people, an SPL of 75dB(A) is required at the bed head. Generally this will require a sounder to be placed within the room.
Volume at
Bed Head
75dB(A)
Where it is not possible to place a sounder within a room, there will be a loss of approximately 20dB(A) through a standard door, and 30dB(A) through a fire door.
| Fire Door | Standard Door |
| Reduces by | Reduces by |
| 30dB(A) | 20dB(A) |
Sounder | 115 - 30 = | 85 - 20 = |
Volume | ||
115dB(A) | 85dB(A) | 65dB(A) |
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Warning: Volumes greater than 120dB(A) will cause damage to hearing.
SOUND REDUCTION AGAINST DISTANCE
Based on a sounder rated at 1m
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reduction |
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Sound |
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Distance (Metres)
In open space, as the distance from a sounder doubles, the sound level will be reduced by 6dB(A), as shown.
It is preferable to use multiple quieter sounders to achieve the required sound level, rather than a smaller number of loud devices. This is to prevent points of excessive volume, which may lead to disorientation or damage to hearing. Two sounders providing equal sound levels will combine to add 3dB(A) to the SPL.
2.5.2.Visual Alarm Signals
25m
| 69dB(A) | |||
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| 4m |
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12.6m |
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0dB | 63 + 3 | 85dB(A) | ||
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| = 66dB(A) | |||
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| 62 + 3 | 69dB(A) | |
| = 65dB(A) |
Note: dB(A) figures are for example only. Left side represents attenuation; right side indicates typical sound pressure level
Visual alarms are normally used only as a supplement to audible alarms where they are likely to be ineffective, for example in areas of high background noise levels where hearing protection is likely to be worn. They can however be used alone where audible warnings are undesirable for example operating theatres and recording studios.
Visual alarms should be clearly distinguishable from other warning lights, preferably red and should flash at a rate of 30 to 130 flashes per minute. The recommended mounting height is above 2.1m, however they should not be mounted closer than 150mm from the ceiling. They should be positioned so that any alarm is clearly visible from all locations within the area protected.
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Note: This document is based on the recommendations of BS5839 Part 1: 2002. It is intended only as a guide to the application of fire detection systems.
Reference must be made to relevant national and local standards.
