The LWF Blog
Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 163
December 18, 2023 11:39 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 162, LWF talked about ceiling height limits and control equipment. In part 163, we will begin to discuss audible and visual alarms.
In the case of many fire alarm systems, the culmination of a properly working system is the alarm sounder. The alarm is usually a bell or electronic sounder which is audible throughout the building. The effect is to alert all building occupants (or those within a given area) to a fire and to the need to evacuate.
Where a fire-engineered strategy is in place for the building, the alarm system may have been installed as a part of that and a voice alarm may be used instead of more traditional alarm sounders. A voice alarm, while subject to very many requirements with regards to clarity and volume, is likely to result in an improved evacuation process.
BS 5839-1 and NFPA 72 have different recommendations for alarm sounders. BS 5839-1 is the standard in the UK and some other parts of the world, while NFPA 72 covers the US and some other areas.
BS 5839-1 provides guidance and recommendations as follows:
An alarm sounder should produce a minimum sound level of 65 dBA or 5 dBA above background noise which could persist for longer than 30 seconds (whichever sound level is greater should be applied) at any point in a building that could be occupied by a person.
Any single door could have the effect of reducing the sound level by 20 dBA or 30 dBA in the case of fire doors.
During fire alarm commissioning, it is likely that there will be areas of the building in which the sound level falls slightly below 65 dbA due to furnishings absorbing and attenuating the alarm sound. In such areas, additional sounders are often placed as a solution to the drop in sound level.
Where such sound-dampened areas are small and confined, or a small room, then a measured 2 or 3 dBA below the prescribed level may be acceptable as this would be imperceptible to the human ear.
In part 164 of LWF’s series on fire engineering, we will continue to look at the recommendations and guidance for fire alarm sounders as per BS 5839-1. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.