The LWF Blog

Facilities Management – Voice Alarm Systems – Part 6

December 15, 2016 11:54 am

In this blog series for those who work in Facilities Management and who have an interest in and responsibility for fire safety, we have recently been looking at Voice Alarm Systems. In our last blog, we began discussing acoustics within the building itself and how this can impact upon sound clarity, reverberation and sound-absorption.


The sound quality of a voice fire alarm system is of paramount importance. The building occupants must be able to easily understand what instructions they are being given by the system and so reverberation and background noise are pertinent issues to investigate and mitigate before installing such a system.


In a building which has lots of hard surfaces and not many sound-absorbing materials, it would be said to have a long reverberation time which results in echoes bouncing from the hard surfaces which make the sound heard directly from the speakers incomprehensible. It is the case that such buildings, an acoustic specialist will be necessary for the system design.


In the case of loud background noise, the level at which the sound system alarm must operate to be heard over it would need to be 5 or 6dB above the background noise, however, this increase would not be sufficient for the occupant to understand the voice message itself and so the difference should be increased to at least 10dB to accommodate this.


Another potential issue which must be taken into account when considering the use of a voice alarm system in a building instead of a traditional fire alarm with sounder is that of obstructions. In a building, walls act as a partial barrier to sound which effectively filters out those sounds heard at high frequencies. In the case of a traditional alarm sounder, this really doesn’t matter providing the volume level of the sounder is sufficient – the sound level would still be such that it was recognisable even when the alarm sounder was separated from a building occupant by a wall.


However, in the case of voice alarm systems, this filtering through a wall means that the higher frequency elements of a spoken message may not be able to be understood by the recipient, necessitating the need to install a loudspeaker in each room or compartment of the building. In this way, a voice alarm system would be more costly to design, install and maintain than a traditional alarm system.


In next week’s blog, we will finish looking at the practicalities of voice fire alarm systems and how they work with background noise. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact Peter Gyere in the first instance on 0208 668 8663.


Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings


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