A siren system for first responder vehicles generally consist of two main items;
- Siren Amplifier
- Siren Speaker
The amplifier (amp) resides inside the vehicle and contains the electronic circuits needed to generate a tone. This tone is then transferred via a pair of wires to the speaker, which resides either behind the grille, or on a bumper (or push bumper).
The exception to this rule is the Federal Signal mechanical "Q" Siren, which is the big chrome siren you see on the bumper of most fire apparatuse. The Q siren is a stand-alone siren that has an electric motor that spins an impeller, nearly identical to the hand-cranked air raid sirens from 100 years ago. For the purposes of this blog, we won't be including the Q siren. Did you know that the Q siren tone is one of only 2 sounds in the world that are trademarked? The other is the exhaust sound of a Harley-Davidson.
On modern sirens, the siren amp is integrated with a lighting controller, so we often refer to the device as a light/siren controller.
There are lots of misconceptions about sirens and how they work. First, it's important to realize that there is no magic involved (sorry to burst your bubble) - it's all electronics and math.
Before we go much further, let's talk about the standards we need to meet. SAE (Society of Automotive Engineers) publishes various standards for vehicle performance, including things like brake light standards, etc. They also publish standards for the minimum performance of emergency lights , and emergency sirens. The siren standard is standard number J1849 . The standard is quite complex but for the sake of simplicity let's boil it down to simple Sound Pressure Level (SPL). SPL is a measurement of how loud the siren is, so the more SPL a siren produces, the louder the siren will sound.
SPL is expressed in Decibels (dB). Like the Richter scale for measuring earthquakes, the decibel scale is logarithmic. This means that loudness is not directly proportional to sound intensity. Instead, the intensity of a sound grows very fast. A sound at 20 dB is 10 times more intense than a sound at 10 dB, and would be perceived as twice as loud.
Image source; Hearinghealthfoundation.org
Under SAE J1849, the minimum standard for first responder sirens is 118dB. However, it's important to note that this measurement is taken at 10 feet from the face of the siren speaker, not right at the speaker's front surface.
If you're thinking of doing some testing on your vehicles' sirens with a sound meter you purchased from Amazon, keep in mind that you won't get an accurate reading. First, quality sound pressure meters are fairly expensive. Second, for accuracy, tests need to be performed in a labratory setting, inside an anechoic chamber. While holding a meter in front of your car in the parking lot might give you a sense of the dB level, you likely won't be able to get within 5 or even 10dB of the actual number.
This 5-minute video is an excellent overview of sound, and specifically anechoic chambers and how they are used for siren testing.
So, now we know that the amplifier generates a tone and feeds that to a speaker (just like your home stereo system), and that the amp has to push enough power to the speaker to allow it to achieve a minimum of 118dB when measured at 10 feet from the face of the speaker.
It's important to know how speakers work. Whether you look at a home stereo speaker or a siren speaker, the principle of operation is the same. The speaker contains a cone, a magnet, and a coil. The cone is the part that actually moves in and out to create sound pressure waves. The coil is connected to the amp and is essenitally an electromagnet system. The amp provides an AC (alternating current) signal to the speaker, and the coil is powered then de-powered, which energizes then de-energizes the magnet, which then forces the cone to move back and forth.
Speakers output sound in a certain pattern. For example, a Federal Signal AS124 speaker outputs sound out to about 45 degrees either way of centre, but not much vertically. If you are pushing 100 watts of power to the speaker (100 watts is a typical siren system speaker/amp power rating), this results is 'x' amount of sound, so the engineers use that sound as efficiently as possible (emergency vehicles need sound straight forward and to off-axis angles, not up and down). So, while the physics of a 100 watt siren system mean that you can get 'x' amount of power at the speaker cone, the design of the housing that the cone sits in can play a fairly significant factor in the overall volume of the speaker assembly(this means not all siren speakers emit the same volume or sound pattern!).
Should you use two speakers?
We often get questions about making siren systems louder, so as to better warn drivers as far ahead of the emergency vehicle as possible. One option is to add a second speaker to your siren system, though - as with most things - there are advantages and disadvantages to doing so.
First, if you want to add a second 100 watt speaker to your system, you need to ensure your amplifier is capable of powering it. Some amps are capable of 100 watts only. Others are capable of 200 watts (which means they can power one OR two siren speakers). The Federal Signal Pathfinder400 is capable of 400 (!) watts, so you can connect up to 4 x 100 watt speakers to it! If your amp is capable of only 100 watts of siren output, then you cannot use a second siren speaker (unless you replace your amp with one that is 200 watt capable).
Assuming your amp is capable of powering a second 100 watt siren speaker, there are a couple caveats to consider;
- depending on your siren amp, you may be able to tee the new speaker's wiring off the existing speaker's wiring. On some amp systems (particularly, newer amps) you may need to run another pair of wires from the amp to the location of the new speaker.
- If you are able to "T" the new speaker off the existing speaker's wires, make sure you connect the two speakers in parallel, not in series. This simply means, connect all the like-coloured wires together. For example, most siren speakers have black and white wires coming out of them. If the wires from your amp to the existing speakers are also black and white coloured, then connect the black on the new speaker to the black on the existing speaker. Same with the other wire. If you connect them incorrectly, your speakers will not be 'in phase' (cones moving in and out at the same time) and your total sound output will drop , as the speaker outputs will fight each other.
- While adding a second speaker will increase your volume output, the resulting increase will be 3dB-5dB. Technically 3dB is double the sound (the jump from 120dB->123dB is twice as loud), the human ear can only detect an increase of about 3dB. So, while the siren will get a bit louder, it won't be drastically louder.
- Having a second siren speaker also gives some redundancy as, if one speaker fails, the system can still operate off the second speaker. What usually happens, though, is that a speaker will fail and go undetected, and then when the second speaker fails a year or two later, the customer will bring the vehicle into an upfitter and say their amp has failed... "it couldn't be the siren speakers because I have two and it's unlikely they failed at the same time". Some siren amps, like the Federal Signal "Pathfinder" system, have speaker failure indication lights on the light/siren control pad so the user can see a red blinking light to indicate a speaker problem (assuming your users don't ignore flashing red lights on their dash/console!).
Placement of a second siren speaker is also important. If you recall your Physics 11, the term "destructive interference" may sound familiar. The speaker's output waves can interfere with one another and result in a decrease in sound overall (if speakers are wired out of phase) or in certain locations (if speakers are improperly mounted). Likewise, if the speakers are mounted and wired properly, the speakers can work together and result in constructive interference , which is where one speaker's output 'helps' the other to achieve higher sound output.
In the image below you can see there are areas of Constructive and areas of Destructive interference. The goal for our purposes is to minimize the Destructive and increase the Constructive interference areas.
This is quite difficult to do if your speakers are mounted behind the grille of your vehicle. However, if you mount the speakers on a bumper/push-bumper of a vehicle then you can mount the speakers so they are spaced as far apart from one another as possible (ideally 16" apart, measured from the inner most edges of each speaker). This system should result in a +3dB increase in sound at all angles. On some vehicles it's not possible to mount the speakers with 16" of space between them (the header image of this page shows such an application), so the best guideline is just to get them as far apart as possible.
If you mount the speakers side-by-side, the resulting sound will be +3-5dB increase in sound straight in front of the vehicle, but -3dB at 45 degree angles (3dB sound reduction vs. 1 speaker). If you're using the vehicle primarily for highway patrol, maybe this is a desired charactaristic for you! However, in an urban environment, it would be a detrimental setup.
Warning Distance
Another aspect of sirens that we get a lot of questions about is warning distance. In a theoretical discussion (not accounting for ambient noise, air temperature/wind, etc.) a 100 watt siren speaker will provide about 400 feet (about 133m) of warning distance. The physics of speakers dictate that if you double the power, you get 50% more sound (see reference above to adding a second speaker will add 3dB of sound). So, in theory if you add a second siren speaker your warning distance will increase to 600 feet (about 200m).
"Outdriving" your siren
This is another good question, "when I'm going faster in my police/fire vehicle, I'm not getting the same warning distance that I do in town - why?". Keep in mind that people need time to detect your siren, then react. If you are travelling "x" speed with 600 feet of warning distance, you'll have a certain number of seconds before you overtake the vehicle in front of you. For the sake of argument, let's say you'll have 6 seconds. If you are now travelling at twice that speed, you will still have the same 600 feet of warning distance, but now your 'overtake' time is 3 seconds. So, it's not really that the warning distance of your speaker is decreasing, it's that your vehicle speed is faster so the motorist in front of you has less time to react since the relative speed from your cars to theirs has halved.
Other Siren Options
Adding a second siren speaker may not be the answer to your warning woes. You may want to consider a Low Frequency Siren such as the Federal Signal "Rumbler". These systems use lower sound frequencies to better penetrate vehicles. They also work quite well warning pedestrians who are wearing earbuds or headphones. Rather than adding a second 100 watt speaker to your existing speaker, you may want to consider adding a LFS instead, especially if you are responding in an urban environment (they don't work well in open areas). We'll cover Low Frequency Sirens in another blog entry.
We hope this helps you understand sirens a bit better. If you're in Canada and would like more information about sirens / siren speakers, please feel free to connect with us. You can also hop over to our webstore to purchase siren speakers, brackets, and more.