1. How loud should the installed sounder be?
2. How do I calculate the effective distance and coverage of a sounder?
3. How many sounders do I need?
4. What are the benefits of using higher output sounders?
5. When should I use mains powered sounders?
6. Where should I not use high output sounders?


Audible warning devices and Designing them Effectively in to Fire and Evacuation Alarm Systems.

All emergency systems contain audible and visual signals to alert people of danger once a hazard has been detected.

It is now accepted that in most systems a form of electronic sounder is initiated via a control panel in order to evacuate or alert people.

Many countries have a national ‘evacuate tone’, i.e. Germany, France, Holland, Australia etc. The UK does not. BS5839 Part 1 merely states that the evacuate tone should contain frequencies within the range of 500Hz to 1000Hz.

Fire alarm system designers are in the main very comfortable with designing sounders into normal environments such as offices, hotels etc. as they have a relatively low ambient background noise, most areas are fairly small and may be covered with a sounder(s) of approx. 100dB(A) at 1 metre or a 6” bell. The locations of these sounders are usually based on experience of previous systems or applications and common sense.

How loud should the installed sounder be?

There are 3 considerations:

1. The size of the area to be covered
2. The background noise
3. The frequency of tone (high frequencies DO get attenuated more severely in an industrial environment than lower BS5839 frequencies)

How do I calculate the effective distance and coverage of a sounder?

Most manufacturers state a dB(A) level at 1 metre*
The rule of thumb (inverse square law) is “every time the distance is doubled from a sounder subtract 6 dB(A)".
i.e. a sounder rated at 106db(A) will travel twice as far as a sounder rated at 100dB(A).

Please see below.

The effective distance of a sounder using this simple method is when the calculated dB(A) reaches 5dB(A) above the known ambient background noise (As stated in BS5839 Part 1).

For example the effective distance of a 100dB(A)@1 metre sounder in an ambient of 65dB(A) is the distance at which the sounder output level reduces to 70 dB(A) i.e. 100 dB – 30 dB = 70dB. From the above table (and using the inverse square rule / rule of thumb) a reduction of 30 dB means the sounder has an effective 70dB distance of 32 metres.

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 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.

More tips: In the open a sounder will spread in all directions, but in an enclosed space some of the sound will be reflected and an increased sound level will result. b) If the sounder, already mounted on a wall, is positioned near to a ceiling, more sound will be reflected.  Vice-versa for a ceiling mounted sounder. c) A sounder mounted on a wall is more effective than when mounted on a pillar. d) Sounders should be sited so as to avoid immediate obstacles and at an ideal height of approx. 2 to 2.5 metres. e) Sounders when installed that are synchronised will give a more effective overall effect.

How many sounders do I need?

When the area to be covered is large and / or noisy, many designers feel unsure and or / go for the ‘add-lots-more-bells / 100 dB sounders-than-normal-approach’. This usually leads to an inadequate coverage and / or the addition of sounders to achieve the bare minimum alarm level after the system has been evaluated during commissioning along with the costly addition of more cable, terminations and labour.

If a room with an area to be covered with an audible alarm is say 30 metres by 20 metres large with very little background noise (approx. 65dB(A)) then it would be reasonable to assume a 100dB sounder would cover this area as it has a 70dB(A) range of approximately 30 metres in low ambient background i.e. giving 5 dB(A) over the background noise (please see later).

Question: how many sounders would you require if the background noise were 85dB(A) say in a heavy machine shop?

Answer: One! The background noise has increased by 20dB(A) then install a sounder 20dB(A) louder i.e. a sounder rated at 120dB(A). Common sense! This principle may be used in all applications where noise and large areas are to be covered.


What are the benefits of using higher output sounders?

In short the use of higher output sounders in large and / or noisy areas will mean an effective alarm level giving a robust and unambiguous alarm dB(A) level. The use of these sounders will also save considerable installation of cable and labour.

Consider the following, an area 50m by 30m has a background noise of 75dB(A) so an alarm level of 80dB(A) is required. The figure compares one 120dB(A) sounder as equal (actually it would give a slightly better level) to twelve 100dB(A) sounders.


It is obvious which scheme is the most cost effective to install. It may be interesting to note; two, 100dB(A) sounders would be adequate if this was considered as a normal background noise i.e. to achieve a level of approx 70dB(A).


A quick comparison of the two 80dB(A) level installations is as follows:
 

100dB(A) sounder scheme :	120dB(A) sounder scheme :
12 sounders at 100dB(A)	1 sounder at 120dB(A)
Labour for installation of 12 sounders	Labour for installation of 1 sounder
Labour for installation 24 cable glands	Labour for installation 2 cable glands
Minimum 220 metres of cable	Maximum 50 metres of cable
Labour for installation 200 metres of cable	Labour for installation 50 metres of cable

Put your own costs down next to the above and find out which is most cost effective.

There is no need for calculations to compare the below. Both schemes will give an alarm level of 90dB(A) i.e the background noise is a maximum of 85dB(A). In practice the scheme with the two 120dB sounders will give a more effective alarm level.



Sounder siting notes:

The above assumes all sounders are synchronised
Using mid range sounders i.e 105 and 110 dB(A) sounders would require proportionally more / less sounders
Diagrams are for indicative purposes, no allowance has been made for obstacles or other sound barriers.
All sites should be subject to a test before installing sounders

When should I use mains powered sounders?

The use of mains powered sounders means these sounders will have no battery back up. It is difficult to understand why mains powered sounders are still considered when 24v dc sounders are available in versions up to a level of 140dB(A) with the ability to be powered from battery back up.

Where should I not use high output sounders?

High output sounders should not be used in low ambient back ground noises and as a means of “drenching” the area in sound. Alarm systems that are too loud may be dangerous and cause panic, discomfort and make communication very difficult resulting impedance of evacuation procedures. As guidance, the overall alarm level should be a maximum of 10 to 15dB(A) over the ambient background noise.