E2S complete product catalogue (v10a) - E2S - #4

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How do I calculate the effective distance and coverage of an alarm sounder? Loudness decreases as the listener gets further from the source of the sound, mainly due to “divergence”. The intensity decreases because the energy is spread over a larger area. It decreases inversely with the square of the distance from the source at a rate of 6dB for each doubling of the distance. So the sound output from an alarm rated at 106dBA) will travel twice as far as a sounder rated at 100dB(A). If a sounder is rated at 100dB(A) at 1 metre, at two metres it will be 94dB(A), at 4 metres it will be 88dB(A) and so on. How ambient background noise impacts on the effectiveness of the sounder Note: The effective distance of a sounder is when the calculated dB(A) reaches at least 5dB(A) above the known ambient background noise. For example the effective distance of a 100dB(A)@1 metre sounder in an ambient noise 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) a reduction of 30 dB means the sounder has an effective 70dB distance of 32 metres. A 120dB(A) @ 1 metre sounder has a 70dB distance of approximately 300 metres i.e. ten times the effective distance and, more importantly 100 times the coverage area. Remember • In the open, a sounder will spread in all directions. In an enclosed space some of the sound will be reflected and increase the sound level. • all-mounted sounder is positioned near a ceiling, more sound will be reflected. The same is true for a ceiling mounted sounder near a wall. • A sounder mounted on a wall is more effective than one mounted on a pillar. • Sounders should be sited to avoid immediate obstacles, ideally at a height of approx. 2 to 2.5 metres. • Synchronized sounders will give a more effective overall effect. • Personnel may be wearing ear protection. Sound Output Attenuation: Frequency and Tone Pattern Sound output is also affected by the frequency of the sound. Lower frequencies tend to travel further, penetrate structures better and are less likely to be attenuated by obstructions. A further adjustment to the range of a sounder may be made according to the frequency of the tone as shown below. Frequency of sounder Adjustment Up to 500Hz 0dBA 500Hz to 1000Hz -3dBA 1000Hz to 2000Hz -5dBA However Perception of a tone is not entirely dependent on frequency and sound level. An output with differing frequencies and/or temporal pattern will have a more distinct sound. This can be useful in areas of background noise where hearing protection may be worn. Usually two-tone frequencies, intermittent, ramp-up frequencies or ramp-down frequencies are the most effective. Basics of sound E2S manufacture an extensive range of alarm horns or sounders suitable for many applications. The efficiency of any audible signaling device depends on: • The size and / or distance of the area to be covered • The background noise • The pattern & frequency of tone Typical Sounder rated at 100dBA at 1m Distance (metres) 100 dBA 1 2 3 4 5 6 7 8 9 10 90 80 Reduction in sound - Based on sounder rated at 1m 0 dBA Distance (metres) 1 2 3 4 5 6 7 8 9 10 -10 -20 Distance (metres) Reduction (dB(A)) 1 0 2 (1m doubled) -6 4 (2m doubled) -12 8 -18 16 -24 32 -30 64 -36 128 -42 256 -48 512 -54 100dB(A) 105dB(A) 110db(A) 112db(A) 117dB(A) 118dB(A) 120dB(A) 140dB(A) 10,000 metres 1,000 100 50 10 5 2 Very noisy areas, ambient 80-90dB(a) Noisy areas, ambient 70-80db(a) Normal areas, ambient <65db(a)