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Outdoor/Public Warning F.A.Q.
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Discussion on Sirens
While many communities still have mechanical sirens, they are being replaced by electromechanical sirens, or state of the art electronic sirens. The old mechanical sirens are no longer available, but our competition still refers to them. What is available today is electromechanical and electronic.
Q. What is the difference between electronic and mechanical sirens? Are there other types of sirens?
A. According to The American National Standard Institute (ANSI), the definitions are as follows:
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Definitions
Electronic Siren: A siren that produces tonal sounds by amplifying the output of an electronic signal generator and broadcasting the amplified signal from one or more electrodynamic loudspeakers.
Mechanical Siren: A siren that produces tonal sounds by periodically interrupting
a flow of compressed air. Mechanical sirens may be motor or engine driven, and the
air compressor may be integral with or separate from the flow interrupter.
Mechanical/Electronic Siren: A siren that uses a tone generator driven by mechanical
means, and the tone generator output is applied by direct connection to loudspeakers,
or through one or more electronic amplifiers to loudspeakers.
Horizontally Omnidirectional Siren: A siren that radiates sound approximately
uniformly in all horizontal directions from the siren at the fundamental frequency
of
Directional Siren: A Siren that radiates most of its sound in a beam pointing
in a specific horizontal direction.
Rotating Siren: A directional siren that contains a mechanism to slowly rotate
its beam of sound about a vertical axis.
Two-Tone Siren: A siren designed to produce tonal sound with two simultaneous
fundamental frequencies. The two fundamental frequencies are not harmonically related.
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Discussion on Siren Sounds
Q. What is the difference between high and low frequencies?
A. The frequency of a warning tone used in rating a siren's Sound Pressure Level is critical in evaluating the performance of an outdoor warning siren, where the siren's warning tones are expected to carry over great distances.
Whelen siren systems are designed to optimize the production and projection of low frequency warning tones. It is documented that higher frequency tones attenuate (lose power) more over distance than low frequency tones.
A warning siren capable of producing 124 dB at 100 feet with a 500hz warning tone will be more effective than a siren producing 124 dB at 100 feet using a 1,000hz warning tone. This factor should be considered in planning a warning system.
Consider this example of the attenuation of higher frequency tones that is apparent in our everyday life...
As you are standing out in your yard, your neighbor's teenage son has the stereo in his car blasting as he is coming down the road toward your house. The earth seems to shake as the "boom, boom, boom" of the low frequency bass notes are felt beneath your feet and on your ears. It is not until the car is very near to you that you can hear the high frequency acoustic guitar and voices. As the car passes you, these high frequency tones seem to disappear, but the pounding and thumping of the "boom, boom, boom" bass can still be heard and felt.
This scenario clearly demonstrates that ability of the lower frequency tones to be heard at a farther distance than the higher frequency tones.
In short, remember that not all 124 dB (at 100 feet) sirens are alike.
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Questions on Solar Power
Q. What should I be concerned about when using solar power as the only source of power for a public warning siren?
A. The most important thing is that the combination of solar panel output and battery capacity must handle the siren standby power requirement, while also maintaining enough charge on the batteries to meet the run time requirements of the siren. These requirements varies with location, seasons and weather.
Q. What will happen during inclement weather?
A. A solar powered siren depends on a properly sized battery bank for power during bad weather such as rain, snow or fog. A Whelen siren has a battery capacity of 230 Amp-Hours. Typical standby current is about 50mA, therefore, after two weeks of total darkness, only 16.8 Amp-Hours of capacity has been drained from the batteries. There is still plenty of capacity for a number of siren tone activities.
Q. How will solar power affect the 30 minutes of run time?
A. All Whelen sirens are equipped with a large battery bank. The batteries actually power the siren during any warning tone or message. Therefore, any Whelen nsiren will meet its specified run time, regardless of whether it is powered by AC or solar.
Q. How long will it take to recover to full charge?
A. Realistically, this is only critical if the batteries have been drained significantly, from considerable use. For example, ten activations at 3 minutes each.
A more likely scenario might be: Four activations at 3 minutes each, with a WPS2810. This means that 44 Amp-Hours of capacity has been used, therefore 44 Amp-Hours need to be replaced. An SBC260 solar option will supply approximately 3.37 Amps in good sunlight, therefore after 13 hours the batteries will be fully recharged. Remember that even without recharging the siren has battery capacity left for six more 3 minutes activations, in this example.
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Questions on Lightning
Q. Are Whelen electronic sirens more susceptible to lightning than our competitor's mechanical sirens?
A. No. Any siren sitting on a pole in the middle of an open area is susceptible to lightning. The key is how well the siren is protected from the inevitable strike.
Q. How will lightning damage a siren?
A. There are two basic ways that lightning can damage a siren. One way is from energy that is picked up through the atmosphere by a nearby strike. This energy isoften coupled through the radio antenna or radio coax cable. The other way is from energy that is coupled onto the AC power lines, in the form of a voltage transient. In either case, the siren must be protected from the inevitable.
Q. How does Whelen protect its sirens from lightning strikes?
A. Whelen does a number of things to protect against lightning damage.
1. Clearly, proper grounding is essential in protecting the siren. We recommend a minimum of two separate 4 AWG copper ground wires, one from the electronics cabinet and one for the steel pole top mounting bracket and pole mounted radio antenna. The copper wires act as low resistance paths, which help divert lightning energy away from the siren, to ground. The ground path from the pole top mounting bracket makes a nice lightning rod.
2. All Whelen sirens include a Lightning Arrestor for the AC line voltage. This is a gas discharge type of device that provides a low resistance path, to ground, for high voltage transients.
3.The battery charger, on all Whelen sirens, is equipped with a line filter and transient protection circuit that exceeds IEC801 requirements. All Whelen battery chargers are UL listed.
4. A gas discharge type of device is included in every Whelen siren to protect the electronics, in particular the radio, from transients entering via the radio antenna path. This device is often referred to as a "polyphaser".
5. Whelen recommends rigid steel conduit to enclose the wiring from the cabinet to the speaker array or rotor mechanism. This reduces atmospheric coupling of lightning energy into the wires, and therefore, into the electronics cabinet.
Q. Will lightning damage Whelen's non-metallic speakers more than a conductive, metal speaker?
A. No. Whelen's omni speakers are made of fiberglass reinforced Lexan® and Whelen's directional speakers are fiberglass. In either case this is a non- conductive material. Lightning will seek the path of least resistance, which is the steel pole mounting bracket and its associated ground wire. The pole top mounting bracket is a nice lightning rod.
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