In this exclusive, edited extract from The Installation Europe Guide to Voice Alarm Systems, Roland Hemming and Richard Northwood outline the four principles underlying the design and operation of voice alarm systems

Principle 1: Intelligibility and coverage

A system should be designed such that for an intelligible input there is an intelligible output.

The coverage of a building must include all areas where people might need to escape to safety, including the designated escape routes.

The risk of a person not hearing an intelligible message in an emergency must be assessed and compared to the likelihood of that space being occupied.

For areas that are small, rarely occupied, or both, such as store rooms, electrical plant rooms and roof-top plant areas, the assessment needs to be reasoned between pragmatism and percentage risk in a consistent manner.

The basic sound pressure level requirement is 65dB or 5dB above ambient noise and an objective intelligibility quotient or STI of 0.5. Dead spots in the audio coverage are permitted subject to the use of the building. Ninety or 95% coverage is a common requirement.

As well as your VA system providing sufficient level, it must not be too loud. A very loud sound can be just as unintelligible as one that is too quiet, as well as possibly causing panic.

There is a maximum level for sounders and the ‘attention drawing signal’ of VA at any point where people will hear it. There is technically no limit for voice messages. However, it has recently been suggested by some acoustic experts that more level does not necessarily increase intelligibility at levels above 100dBA because the brain shuts
it out.

It is also important to note that a single tone, such as a sine wave, is far more painful than a voice message at the same level.