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 an 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.

Fire Alarm Siren System

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 building.

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 1 m²
  • At specific points of limited extent the SPL may be reduced to 60dB(A)

Where a continuous background noise level greater the 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 place 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).

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 sounder providing equal sound levels will combine to add 3dB(A) to the SPL.

Visual alarms are required in order to satisfy the Disability Discrimination Act (DDA) as well as being used in areas of high background noise where hearing protection is likely to be worn. Just as audible alarms should be placed through all accessible areas of a building, visual alarms should be placed such that they can be seen in order to alert the hearing impaired.

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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