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Fire Safety for Facilities Management Personnel – Voice Alarm Systems – Part 214

August 14, 2023 11:00 am

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 213, LWF discussed what factors should be considered in the design of a voice alarm system. In part 214, we talk about emergency microphones and message sources.

A message given by a voice fire alarm system as a fire alert should be pre-recorded. The message generators which store the recorded messages and control broadcast should use only solid-state electronics. Any storage media involving the use of moving parts, such as tapes, compact discs or hard disks would not be considered reliable for this purpose. In other words, the voice alarm system should work as intended if the system is unmanned.

In a simple building, a pre-recorded message may suffice in giving warning of fire. In a more complex building or a very large building, it will be necessary to have emergency microphones installed to assist with ongoing evacuation information and to be used by the building management or representatives of the Fire Service. When such systems are installed, it is usual to ensure an emergency microphone has the highest priority of all inputs. This means that any transmission from the emergency microphone will override a pre-recorded emergency broadcast message.

The sound pressure of a message should be at least as high as that for conventional fire alarm signals. In the case of alarm sounders, the only requirement is for the sound to be audible, however, when considering a voice alarm system, it is essential that the message is intelligible and easily understood.

For this reason it is important that the sound system and loudspeakers are of sufficient quality to prevent distortion at the required sound level. There are other factors likely to affect intelligibility – reverberation time within the building and ambient noise levels.

Some buildings may be subject to long reverberation times, resulting from a lack of sound-absorbing materials. Echoes arise from sound waves bouncing off hard surfaces and the late arriving echoes can render the direct sound from loudspeakers unintelligible to the hearer.

Where echoes will be an issue due to hard surfaces, an acoustic specialist’s advice in the design phase is likely to be necessary.

In situations where the background noise is likely to be particularly loud, the output from the voice alarm system can be increased to a level 5 or 6 dB above the background noise. However, this may also mean the broadcast may be intelligible. The ratio of speech signal level to background noise level should normally be increased least 10 dB to ensure intelligibility.

In part 215 of this series, LWF will continue discussing how to avoid the potential sound pitfalls when designing a voice alarm system. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

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.

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