E2S complete product catalogue (v10a) - E2S - #5

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How many Sounders do I need? When the area to be covered is large and / or noisy, designers often add more sounders. This could lead to an inadequate coverage, if the alarm sounders were positioned incorrectly or require the addition of more sounders to achieve the bare minimum alarm level. Example: Question: A 30m by 20m room, with very little background noise (approx. 65dB(A)), is satisfactorily covered by a 100dB(A) sounder with a 70dB(A) range of approximately 30m. How many sounders would you need if heavy machinery meant the background noise was 85dB(A)? Answer: One! If the background noise increases by 20dB, install a sounder 20dB louder i.e. a sounder rated at 120dB(A). This simple principle is often forgotten in the need to cover large and noisy areas. The effective distance of a 100 dB(A) sounder in a very noisy environment is 1.8m, the distance for a 120 dB(A) sounder is approx 18m (10 times the distance). The effective coverage of a 100 dB(A) sounder in a very noisy environment is x m2, the distance for a 120 dB(A) sounder is approx Xm2 (XX times the coverage). Note: Alarm sounders that are too loud may be dangerous and cause panic, discomfort and make communication very difficult. As guidance, the overall alarm level should be a maximum of 10 to 15 dB(A) over the ambient background noise. To achieve 90bB(A) in an area 50 x 30m More Design Considerations Sounders aren’t generally effective at an output lower than 65 to 70dB(A) or less than 5dB above the ambient, background noise. Additional sounders or louder sounders may be necessary after calculating the maximum distance and coverage required. Any adjustment according to the output frequency should also be factored in. Internal fire doors attenuate sound by at least 30dB, and normal doors at least 20dB. It’s advisable that any sounder isn’t required to be heard through more than one partition. In the above example; The attenuation caused by distance d1, the door & partition and distance d2 must be calculated (with an adjustment for tone frequency). The final dB(A) level should be not less than 65 to 70dB(A) or not less than 5dB above the background level in area 2. Care should be taken not to use a sounder with too higher output in area 1 simply to achieve an acceptable level in area 2 as this may make levels in area 1, particularly close to the sounder, unacceptably loud. Sound Output of Multiple Sounders Two sounders together with an equal output increases the total output by 3dB. So two 100 dB(A) sounders together will provide 103dB(A) total. Four 100dB(A) sounders will deliver 106dB(A) in total. It’s important to establish the most suitable sounder at the design stage as simply adding more of the same sounders may only increase an overall alarm level by a few dB. Disaster warning / Wide area signaling Large sounders with high outputs of typically 140dB(A) and above have additional considerations such as: • Attenuation caused by ground effects, barriers such as buildings • Vertical temperature gradients • atmospheric refraction • sound absorption in the atmosphere • people’s perception • building construction It is important to realise predicting coverage can only be an estimate. A combination of these factors attenuating sound in the atmosphere, is both complicated and unpredictable. Strong winds will influence the effectiveness of the sound coverage. It will tend to make the sound travel further in the direction it is blowing, i.e. in the same direction as gas will be blown in the event of a leak. In general, disaster warning sounders should be mounted horizontally 10 to 15 metres above the ground, preferably at the highest point on the site to be covered (although not so high that the sound travels over the top of the area). As a general guide, take the height of any obstruction within 50m and keep the sounder at least 2m higher than this for best sound coverage - ideally the source to target should be aimed or “line of sight”. Area 1 Area 2 d1 = x metres d2 = y metres -20dB(A) Normal Door -30dB(A) Fire Door Either fit eighty A100 (100dB(A) at 1m) units... ...or fit just two A121 (121dB(A) at 1m) units Basics of sound (cont)