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INDUSTRY & MARINE AUDIBLE
Audible warning devices and Designing them How do I calculate the effective distance and
Effectively in to Fire and Evacuation Alarm coverage of an alarm sounder?
Systems. Most manufacturers state a dB(A) level at 1
metre*
All emergency systems contain audible and visual
signals to alert people of danger once a hazard The rule of thumb (inverse square law) is “every
has been detected. time the distance is doubled from a sounder
subtract 6 dB(A)”.
It is now accepted that in most systems a form of
electronic alarm sounder is initiated via a control i.e. a sounder rated at 106db(A) will travel twice
panel in order to evacuate or alert people. as far as a sounder rated at 100dB(A).
Many countries have a national ‘evacuate tone’, Distance (metres) Reduction (dB(A))
i.e. Germany, France, Holland, Australia etc. The 1 0
UK does not. BS5839 Part 1 merely states that 2 (1m doubled) -6
the evacuate tone should contain frequencies 4 (2m doubled) -12
within the range of 500Hz to 1000Hz. 8 -18
16 -24
Fire alarm system designers are in the main very 32 -30
comfortable with designing fire alarm sounders 64 -36
into normal environments such as offices, 128 -42
hotels etc. as they have a relatively low ambient 256 -48
background noise, most areas are fairly small 512 -54
and may be covered with a sounder(s) of approx.
100dB(A) at 1 metre or a 6” bell. The locations of The effective distance of a fire alarm sounder
these fire alarm sounders are usually based on using this simple method is when the calculated
experience of previous systems or applications dB(A) reaches 5dB(A) above the known ambient
and common sense. background noise (As stated in BS5839 Part 1).
How loud should the installed For example the effective distance of a
alarm sounder be? 100dB(A)@1 metre sounder in an ambient of
There are 3 considerations: 65dB(A) is the distance at which the sounder
output level reduces to 70 dB(A) i.e. 100 dB – 30
1. The size of the area to be covered dB = 70dB. From the above table (and using the
2. The background noise inverse square rule / rule of thumb) a reduction of
3. The frequency of tone (high frequencies DO 30 dB means the sounder has an effective 70dB
get attenuated more severely in an industrial distance of 32 metres.
environment than lower BS5839 frequencies)
Likewise for a 120dB(A) @ 1 metre sounder it has
a 70dB distance of approximately 300 metres i.e.
ten times the effective distance and even more
importantly 100 times the coverage area!
*TIP: make sure you know the rated dB level
and tolerance of the actual tone you intend to
use on a multi-tone sounder. dB levels of the
various user selectable tones available on multi
tone electronic alarm sounders vary drastically
depending on the tone selected. In general the
lower the frequency of tone (< 1000Hz) the lower
the dB level and the higher the frequency (>
1000Hz [not BS 5839 compliant]) the higher the
dB level and the bigger the attenuation.
