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Technical Information

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Outdoor Warning Siren Technical Information

Douglas County has approximately 120 outdoor warning sirens installed and operating. The sirens are a pre-incident warning device. Although not required by law, they are an important device to warn citizens of impending tornadoes and are tested monthly in the spring, summer and fall. Most but not all of the populated areas of Douglas County are within the audible range of a siren. It should be understood that the sirens are designed and intended to be an audible warning device for those persons who are out-of-doors. When sounded, sirens may not be an effective warning device for those persons who are in dwellings or automobiles.

There are many factors that determine the ability of a siren to activate and the ability of the listener to hear the sound. These factors are explained below.

1. The presence or absence of an auxiliary battery power source for the siren radio and horn determines its operation.  All of the sirens within Douglas County have a battery back-up power source for the siren radio. The auxiliary battery back-up power source in the radio allows the radio to maintain its frequency settings in its memory so that it can recognize the radio signal that activates it.  It should be noted that since most community power losses are “flickers” or short power interruptions, the sirens typically remain functional and in a ready stand-by status.  During extended time periods when community electrical power is lost the siren will not activate but when the power comes back on, the siren will activate once again.

2. The location and condition of the power line that feeds the siren is important.  Many of the sirens were installed with an overhead power line to an adjacent power source. This line may be vulnerable to falling tree limbs, flying debris or fatigue.  Where and when feasible, Douglas County is relocating these lines underground and enrolling them in the “One Call – Diggers Hotline” system.

3. The geographical location of the siren from the listener is significant.  Sound power decreases as distance from the horn increases. Hills, valleys, buildings, trees and other obstructions may have an overall negative effect on the sound level perceived by the listener as well.

4. The sound power level for all sirens in Douglas County is the same.  The sirens purchased by Douglas County have an overall effective design radius of approximately one mile from the horn in an open field. The volume of the siren tone is fixed and cannot be increased or decreased.

5. The rotation speed of the horn is checked throughout storm season.  Siren effectiveness is improved as the horn rotates during activation. Due to the design of the horn and a phenomenon known as the Doppler Effect, the sound from the rotating siren horn will oscillate in pitch relative to the listener. The sound produced by a rotating siren can usually be discerned from other noises. The wind also affects the sound heard by the listener.

6. The ambient noise level around the listener effects the ability for a siren to be heard.  If the listener is near a loud radio, television or other sound or noisy source, the siren may not be heard.

7. The composition and quantity of the shielding between the siren horn and the listener has an impact on the siren's effectiveness.  If the listener is inside a car or home, the sirens may not be heard. Concrete and brick structures can also have a negative effect by reducing the siren sound level to the listener. Weather alert radios are strongly recommended for all dwellings, businesses, schools, mobile homes, and occupied trailers.  Weather radios may be a desk-top, hand-held portable or a compact clip-on type that can be clipped on to purses or belts.  All are effective and may be purchased at many grocery stores in the local area.

8. The existing wind conditions during siren activation influence its effectiveness.  As the wind increases so does the resultant noise that it produces. People have often stated that a tornado sounds similar to an approaching freight train by producing a roaring sound that can mask any other sound or audible communication. During a high wind condition if you are located upwind of a siren, it can be difficult to hear a siren that is located down-wind.

9. The listener’s ability to consciously hear the siren is very important.  Persons with varying degrees of hearing loss may have difficulty hearing the sirens. This group may include the elderly and those with hearing impairment. Additionally, persons at rest or sleeping may not be awakened by the sirens.

10. Although unlikely, it is possible that interference can occur with the radio signal to the siren.  Sirens are designed such that the duration of the radio signal that initiates the siren is extremely short, practically instantaneous.  The possibility for interference of the radio signal to randomly occur at some location(s) across Douglas County during attempted siren activation is possible but highly unlikely.  For those of us living in the Midwest, the value of a weather radio can not be overemphasized.    

Most of the newer sirens in Douglas County are the American Signal Tempest™ brand series of emergency alerting sirens and they are used for notification of severe weather conditions.

T-128 Rotational Siren

This 128dB rated siren includes:

  • Pole Mount & Mtg. Hdwr.
  • Rotation Sensor.
  • 60º Sound Pattern 360º Coverage.
  • Weather resistant Fiberglass housing.
  • Gear Driven Rotation (No Belts).

Back-up Battery Power for Outdoor Warning Sirens

For those communities in Nebraska including Douglas County and throughout the United States that have outdoor warning sirens, the greater majority are not equipped with back-up battery power supplies. This is due to several considerations which are primarily based on priority, cost and benefit.  It should be kept in mind that our sirens are intended as a pre-incident device.  This means that by the time the community has experienced a sustained power outage which would render our sirens inoperable, the incident has most likely already occurred and the value of the siren to warn of impending weather has passed.

The use of available funding for outdoor warning sirens in Douglas County is focused on three priorities. It should be understood that outdoor warning sirens are considered to be “another means” of notification that should not take precedence over the use of NOAA weather radios or other sources of professional weather alerting and notification.

Maintenance is a priority to maintain the satisfactory operation of the outdoor warning sirens currently in use.  Outdoor warning sirens are not a maintenance-free device.  Depending on a siren’s frequency of use and the severity of the winter weather along with other factors, the siren’s dependability may be affected with corresponding maintenance costs increasing with age.

On occasion, the effect of seasonal winds may cause the outdoor warning siren to lose power due to a failure in the power line to the siren. Burying the power line to the siren reduces the vulnerability of a siren to power interruptions caused by falling tree limbs and normal wear and tear on power lines and their attachment points. Where and when appropriate, Douglas County buries the siren power line to increase the siren's resiliency.

Another priority is for increased siren coverage throughout Douglas County. Not all, and especially the more rural locations in Douglas County, are within audible sound of a siren even under the most ideal conditions.  A planning objective to help alleviate this situation is to place sirens in the more populated but "uncovered" areas.

The cost associated with battery back-up systems for the siren horn is not insignificant. The initial cost for battery installation is greater than for line-powered sirens.  This cost is magnified when considering the continual monitoring, annual maintenance and replacement of batteries and hardware associated with back-up battery sirens.

Battery back-up power for outdoor warning sirens allows the siren to be used as a post-event notification device. For example, after a tornado, fire, or other incident occurs and should the power be interrupted, the outdoor warning siren would still activate.  The Omaha Public Power District states that most power interruptions are brief in nature which people often refer to as “flickers.” This is significant since an electrical flicker will not permanently disable a line-powered, outdoor warning siren.

An example of a common siren activation protocol is when tornado-forming atmospheric conditions are first identified and characterized by the National Weather Service, local media meteorologists or private weather services. A Tornado Warning may then be issued for any portion of Douglas County and a geographic tornado warning polygon will be drawn by the forecaster that outlines the warned area.  The outdoor warning sirens are then sounded for the entire county prior to the tornado event.  

The prioritization and use of available funds as previously described will be used to improve the siren system and obtain the greatest overall coverage and benefit for Douglas County residents due to the most frequently occurring tornado severe weather scenario. 

Similar to a traffic signal or other safety device, no outdoor warning siren system is 100% fail safe or effective even if it is equipped with back-up battery power. Furthermore, it should not be exclusively relied upon as the only means of notification of severe weather.  It is up to the individual to be aware of his/her surroundings especially during severe weather season and be prepared to take proper and appropriate protective measures.

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