Q. What is the difference between electronic and mechanical sirens? Are there other types of sirens?

A. Over the past 50 years outdoor warning siren technology has drastically changed to meet the demands of the purpose and use of the equipment. Sirens were originally manufactured for civil defense purposes. During World War II, the most common use of sirens was for signaling the threat of an air raid. Due to the nature of this type of warning and the lack of technology available at the time, the use of commercial power was the most effective means of producing high-powered sound.

An electro-mechanical siren is a fairly simple device. It consists of an electric motor that turns a fan called the “rotor” or "impeller", spinning inside a slotted drum called the "stator". However, over the years these sirens have evolved to combine the use of electronics to control the tonality of the moving parts in the form of timers, relays and radios thus rendering them reliable on technology.

The first job of the rotor is as a centrifugal fan. It pulls air into the siren axially through the intake, and blows it out radially through the holes in the stator. The second job of the rotor is to chop the incoming air stream into impulsive bursts. Vanes that periodically cover and uncover the holes in the stator segment the rotor. Each time the rotor and stator holes align, a burst of air is forced through. The frequency of these bursts is the pitch of the siren. The faster the motor turns the louder the noise. The resulting tone has a very unique sound. These devices are usually connected to a three phase, 208, 220 or 240VAC utility service and served their intended purpose very well but are costly to maintain due to the high volume of moving parts and are extremely limited in their ability to incorporate future technological advances.

During the “Cold War Era” the threat of a nuclear attack was at the forefront of concern. During this period, a civil defense fund was established by the United States Government to provide money for many cities throughout the U.S. to purchase sirens for civil defense threats (nuclear attack). Most cities updated their existing WWII era, air raid sirens to a newer version of the same technology, which is one reason that there are so many electro-mechanical sirens still in service today. As the availability of parts began to diminish, a new era of siren technology emerged.

In the early seventies, Whelen's engineers designed a 24-volt, DC powered siren that had a minimal amount of moving parts in order to provide uninterrupted siren communication in any weather scenario without the high cost of maintenance of its predecessor. These sirens produce their sound electronically via a tone generator that transmits a tone or voice signal into an amplified circuit, which passes it on to a speaker driver configuration thus resulting in an audible signal. These devices work exactly the same as your stereo, the more powerful the amplifier, the louder the sound. This design allows for both siren tones and voice announcements therefore making it’s application much more versatile than that of the electro-mechanical variety. The Whelen sirens have undergone many modifications, have been constantly updated with the latest technology and are considered to be the cutting edge siren technology on today's market. Although our competition has tried to embrace the technology, Whelen manufactures and sells more electronic sirens that all of our competitors combined!

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.

Q. What should I be concerned about when using solar power as the only source of power for a public warning siren?

A. Solar power is a very good solution for power in electronic sirens due to their low consumption of power and high battery capacity. Solar power also allows for more freedom in system design. When designing an AC powered system one of the criteria used in determining feasibility of a location is whether or not a power source is available nearby. Therefore, in some instances, optimal locations have to be passed up for less desirable locations with access to utilities. Solar systems have become more and more prevalent over the past ten years as more of the old style electro-mechanical sirens have been replaced by electronic versions.

The most important consideration of using solar power as a primary source of power for a public warning system 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. This requirement varies with location, seasons and weather.

A solar powered siren depends on a properly sized battery bank for power during bad weather such as rain, snow or fog.

Whelen sirens have 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 activations.

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 siren will meet its specified run time, regardless of whether it is powered by AC or solar.

In order to prevent damage to the solar panels from large, damaging hail, a Lexan® solar shield can be installed over the panels to allow for maximum protection without prohibiting the amount of light reaching the panels.

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 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.

Q. Are there any advantages to utilizing AC power as a primary means of power?

A. Absolutely not! On the contrary, today’s siren systems are primarily used as warning devices for severe weather and other emergency conditions. Due to the volatility of high wind and lightning, which is almost always associated with severe weather, it is no longer feasible to depend on commercial (AC) power as a single source of power for sirens for obvious reasons. To overcome this concern, manufacturers of older-style (electro-mechanical) sirens redesigned their systems to allow for 48 volt operation which can be backed up by a 48 volt battery supply (four 12 volt batteries in series). This system typical operates as a back up power source whereas the primary source of power is provided by commercial (AC) power. In the event of a loss of AC power, the siren would switch to battery operation. As long as the batteries are in good condition, the siren system will operate until the batteries loose their capacity.

This capacity is the troubling part of this whole process whereas batteries that are not exercised routinely and maintain a constant charge have a proven tendency to loose their capacity under load after a very short period of time. This condition can only be diagnosed under load, as batteries will usually show acceptable voltage values at rest if they are being charged.
Therefore, if the system has been up and operating for several months powered only by commercial (AC) power the battery may show acceptable voltage. However, if a storm was to take away the commercial (AC) power, and the system switched to battery power, upon activation of the siren, the amperage or capacity of the batteries may immediately drop out due to their lack of use. Most of us who use rechargeable batteries in our cell phones and cordless home phones have probably experienced a similar scenario with a battery that only lasts for a few minutes because of over charging on a routine basis.

The battery charger in the Whelen, electronic siren utilizes a computer-controlled thermistor connected to the battery series, which measures battery temperature, and voltage as well as cabinet temperature and applies the proper amount of charge accordingly. This method of charging known as “float charging” assures optimal battery life. Furthermore, as the electronic siren is DC only, the radios and other standby systems are constantly drawing on the batteries as are any tests, silent or audible. This constant drain on the batteries exercises the system, which maintains the amperage of the batteries. This is in stark contrast to a series of batteries that never see any drain and maintain a constant trickle charge.

The most common type of battery charger used in siren systems is known as a trickle charger. The trickle charge method is more likely to overcharge the batteries due to constant charging, which can cause early failure or even battery combustion hence the use of a separate battery compartment in most electro-mechanical sirens.

The battery charger's primary use in the Whelen system is to maintain the system battery supply to full capacity.

The Whelen siren utilizes two (2) twelve volt batteries in series to achieve it’s required voltage of 24 volts. These batteries are locally available, sealed and maintenance free. DC powered systems are highly advantageous for life safety applications due to there lack of dependence of commercial power and regular use of batteries.
In either scenario, the batteries will have to be maintained and changed out on a routine basis. Therefore it makes a whole lot more sense to purchase a system that actually utilizes it’s batteries on a daily basis and requires one-half of the money to change out (two batteries vs. four per siren).

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 is often 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. 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.