Fire Alarm Volume

It is essential that alarm signals are sufficient in nature and extent to warn all persons for whom the alarm signals are intended. In simple buildings, the alarm produced by a Category M or Category L system should be capable of alerting all occupants of the building regardless of their location. If people sleep in the building, the alarm signal must be sufficient to rouse them from sleep. In certain, usually more complex, buildings, a general alarm of the type described above may not be appropriate. For example, in hospitals and certain residential care premises in which occupants may need assistance to evacuate, the fire alarm system may not be intended to rouse people from sleep, and it may only be necessary for staff to be aware of the alarm signal. In some large or complex buildings, the extent of the area in which an evacuation signal is given may be restricted, at least initiall.

In Category P systems, the alarm signal may not be intended to alert all occupants; it might be sufficient for the system to alert key personnel in order to ensure that fire fighting action is taken. In practice, a Category P system is usually combined with a Category M system, in which case the considerations applicable to the Category M system take precedence as these will be more onerous.

In those areas in which audible alarm signals are intended to alert occupants, the sound pressure level and frequency of alarm signals must be adequate to provide unambiguous warning of fire. Particular care should be taken to ensure adequate sound pressure levels in small cellular spaces, such as cellular offices, toilets and plant rooms. If the alarm signal comprises a speech message, it is also necessary to ensure that the message(s) is intelligible.

Traditionally, a sound pressure level of 65dB(A) has been regarded as the minimum acceptable sound pressure level for fire alarm signals. However, a sound pressure level difference of 2-3dB is only just perceptible to the human ear. It is, therefore, considered unnecessary to install additional fire alarm sounders if, in open areas, 65dB(A) is generally achieved, but if, at specific points of limited extent, a sound pressure level of at least 60dB(A) is achieved. It is also considered acceptable for the sound pressure level to be as low as 60dB(A) in small enclosed spaces, such as cellular offices, and in staircases.

In order to prevent excessive sound pressure levels, which can cause disorientation or even damage to hearing, a larger number of quieter sounders is preferable to a few very loud sounders.

Once started, alarm signals should generally continue until manually silenced by use of a silencing control. In large sites comprising many buildings in single ownership (e.g. industrial sites), an external fire alarm device may be of value in directing fire fighters to the correct building; an external fire alarm device may also be useful in directing fire fighters to the correct entrance of a large building with many entrances. It is not necessary for an external fire alarm sounder to sound for a prolonged period; for it to do so is unlikely to be acceptable because of disturbance in neighbouring properties, particularly if false alarms occur when the protected premises are unoccupied.

In radio-linked systems, in which the power supply for sounders may comprise batteries. Prolonged operation of sounders as a result of false alarms when the premises are unoccupied may exhaust the batteries, leaving the premises without an operational fire alarm system when they are subsequently reoccupied. Since this creates greater risk than that associated with automatic silencing of alarm signals, the alarm signal may be automatically silenced in certain radio-linked systems, subject to the agreement of the enforcing authority.