Note: An increase of 3db is the equivalent to doubling the volume. For example a siren with an output of 110dB is approximately 8 times louder than one with an output of 100dB. The effective distance of a siren is when the calculated dB(A) is at least 5dB(A) above the known ambient background noise. For example the effective distance of a 100dB(A) @ 1m siren in an ambient noise of 65dB(A) is the distance at which the sounder output reaches 70dB(A) (ie 100dB-30dB=70dB). From the table above (or using the inverse square rule) a reduction of 30dB means the sounder has an effective distance of 32m.
A 120dB(A)@1m siren has a 70dB(A) distance of 300m ie. Ten times the effective distance and more importantly 100 times the coverage area of a 100dB(A) siren.
Two identical sirens placed together will produce a combined output of 3dB greater than their individual outputs. Four identical sirens will produce an increase of 6dB. ie 4x 100dB(A) @ 1m sirens will together produce 106dB@1m.
a) In the open a siren will spread the sound in all directions, whereas in an enclosed space some of the sound will be reflected from hard surfaces and increase the sound level.
b) The closer a wall mounted siren is positioned near a ceiling the more sound will be reflected. The same is true of a ceiling mounted siren positioned near a wall.
c) A siren mounted on a wall will be more effective than one mounted on a pillar.
d) Sirens should be positioned to avoid immediate obstacles. Ideally at a height of 2 to 2.5m above the floor.
e) Large outdoor sirens should be mounted 10-12m above ground level or 2-3m above any nearby obstacles such as buildings or trees.