Smoke Alarm User Manual

16
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.
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.
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).
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.
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.
Warning: Volumes greater than 120dB(A) will cause
damage to hearing.
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
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.
d
o
w
n
Area < 60m²
Min 60dB(A
)
Minimum
65dB(A)
Minimu
m
65dB(A
)
Minimum
65dB(A)
Minimum
65dB(A)
Minimu
m
65dB(A
)
Minimu
m
65dB(A)
Stairwell
Min 60dB(A)
SOUND REDUCTION AGAINST DISTANCE
Based on a sounder rated at 1m
-30
-2
5
-2
0
-15
-1
0
-5
0
0 5 10 15 20
Distance (Metres
)
Sound reduction (dB(A)
dB(A) Reduction
69dB(A)
69dB(A)
63 + 3
= 66dB(A)
62 + 3
= 65dB(A)
1
4m
12.6m
-16dB
-23dB
-16dB
0dB
Note: dB(A) figures are for example only.
Left side represents attenuation; right side
indicates typical sound pressure level
25m
1
4m
85dB(A)
Machinery
Generatin
g
80dB(A)
Sounder
Minimu
m
85dB(A)
Volume at
Bed Head
75dB(A)
Sounder
Volume
115dB(A)
Standard Door
Reduces by
20dB(A)
85 - 20
=
65dB(A)
Fire Door
Reduces by
30dB(A)
115 - 30
=
85dB(A)
Application Guide