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| Jun 23, 2010 1:49 pm |
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The Real Issue with current outdoor Warning Systems: Frequency |
Richard Weisenberger
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The real issue here isn't that the newer sirens are necessarily cheaply made. The manufacturers are doing their best to provide the maximum dB rating at 100 feet using the least HP or kW. It is much easier to accomplish this at higher frequencies. By using higher frequencies you don't need as large of horns with low cutoff frequencies and generally the power requirements can also be lower, as greater directivity indices can be achieved in a smaller space. This is particularly the case with the 360 degree radial horn arrays seen on some electronic units, as they are already of immense size.
The problem is that achieving an x number of dB at 100 feet really tells you very little of how well the unit will perform in real world conditions at a distance, involving frequency dependent losses related to atmospheric absorption and penetration of common construction materials. We already know that some manufacturers only test their units in anechoic chambers and interpolate how they might perform from a distance.
Even when you reach the usable outdoor 70 dB radius, you still have to contend with additional losses of up to 55 dB or more due to trying to penetrate masonry walls of buildings. Due to this, a 70 dB outdoor radius is no longer effective for indoor penetration. To rate all units on an equal basis, we might consider a distance, such as 1 mile, in which the measured SPL is in excess of 70 dB. It will start becoming apparent at this distance that lower frequencies are in fact more effective. It will become even more apparent from greater distances, such as 2 miles, for any source that can still supply the required output. In all cases the higher frequencies are always the first to go. It is first noticeable in the upper harmonics.
A frequency spectrum that peaks below 500 Hz will lose significantly fewer dB with distance to begin with than a frequency spectum that peaks around 1 kHz, especially when the distance is great. We are not simply talking about the fundamental frequencies here, but the effective frequency due to the spectrum of the tone with all of its harmonics, which effectively increases the fundamental frequency by up to a factor of 1.4.
There is also at least 5 dB less loss for 500 Hz penetrating common construction materials than at 1 kHz. This is the very reason the older sirens can be heard in places the newer ones cannot. It's not a matter of manufacturing quality, but a matter of acoustics and physics.
Private Reply to Richard Weisenberger (new win) |
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