Building a loud siren

[sebastian.salmhofer]

Hi everyone,

I live in a gated community settlement project in Paraguay and we want to implement a warning system with sirens. The settlement area is around 4km² / 1000 acres. In Paraguay we cant find any large sirens and importing is very expensive and/or takes a long time. So I got the idea to build something myself. I have a machine shop with manual lathe and mill, I can weld and we can get CNC plasma and laser cut parts. Electric motors of any size are no problem either.
My problem is that I have no experience with the design and I can't find any plans online, except for simple wooden or 3d printed sirens, but I guess they won't nearly be loud enough.
Can some of you help me or point me in the right direction where I can find something? Maybe there are drawings available for some old sirens that we could replicate. We don't even know what commercial siren or multiple sirens we would need to cover the area so that people can hear it inside their homes too.

Sebastian

[TheVintageLouisiana]

Depends. Whats the budgert?

[sebastian.salmhofer]

There isn't a real budget yet because we all don't know anything about the topic. It should be as inexpensive as possible while doing it's job and being reliable. My time is free if I am going to build something, so it would just be materials in this case.

[Eclipse DDS]

Before anything else is said, I would briefly like to clarify that whilst sirens can in some cases be heard from indoors, it is very hard to ensure that this will be the case for all buildings at all times. Whether a siren can be heard from indoors depends on a variety of factors, some of which would be difficult or near impossible to combat. In short, whilst sirens can occasionally be heard indoors, generally they are more effective working as outdoor environments to alert people who are outside.

I know that there are (or were) a few people on this site that machined their own sirens, but generally I think that they were making scale models rather than their own full-scale design. Sirens chopper assemblies are pretty complex shapes, and require a lot of accuracy and precision to work. They are definitely possible, though.

Another question would be if you are planning on making this system a durable, long-term solution? If so, you will want to ensure corrosion-resistance on your sirens. Alerting Communicators or America (A.C.A.) had a useful habit of making most of the exposed parts of their sirens out of fibreglass, which obviously doesn’t rust. This allowed their models to stay in good condition for longer than some of Federal Signal’s units, most if not all of which used metal housing.

Here in the UK, we didn’t always have any type of housing on our sirens, although a fibreglass cover was an option offered. Despite the lack of housing, a lot of the existing sirens from World War II and the Cold War are still intact, although there is still clearly a difference in the condition of units with or without some form of shelter. A lot of UK units are also equipped with de-icing heaters, which may be something to think about if you get particularly cold winters (although I’m not sure you would in Paraguay).

I hope this has been useful, I’ll admit that I’m not the greatest source of knowledge for something like this, but it should give you some points to think about at least

[sebastian.salmhofer]

Thank you for the clarification, that is indeed good to know. But at least outside it should be easy to hear even with wind.

I think corrosion won't be an issue for us. Paraguay is great in that regard, almost no rust on cars. First of all no ice and therefore no salt on the roads and I have the feeling that the red dirt/mud is actually protecting metal from rust. So I would just paint any steel parts and that should be fine for a very long time.
We don't really get any frost, sometimes on the coldest days of the year we get around 0°C in the morning for a couple of hours, but we won't need any de-icing.
But I can also work with fiberglass, so that is an option too if it has any other benefits.

The main thing is I need some plans or guidelines for the design. I think the execution is not an issue, but I have no idea how to design the fan (is it called that), the slots and horns, if we need horns. I don't know what type of design would be good in general for our use case.

[Synther]

The main thing is I need some plans or guidelines for the design. I think the execution is not an issue, but I have no idea how to design the fan (is it called that), the slots and horns, if we need horns. I don't know what type of design would be good in general for our use case.

The design of a siren is fairly straight forward, and as mentioned above, relies heavily on precision. Most choppers (turbines) in sirens are made of cast aluminum, however, replicating the design with the machinery you listed isn't impossible. The spinning chopper has evenly spaced slots of any number along the wall, and in your case, I recommend 8 or 10. Within the chopper are flat scoops (vanes) which grab the air and throw it out the slots in the side. The chopper needs to be balanced and centered, as rotating speeds of 2800RPM+ are desired.

Accompanying this spinning chopper, is a stator. It follows a similar design, using matching slots in the walls to grab and split the air coming out of the slots of the chopper. The slots on both assemblies need to be blunt or sharp. Between the chopper and stator is an air gap. This gap is critical, and needs to be as thin as possible, while reducing risk of collision between the chopper and stator. For a chopper that is 18" (~45cm) in diameter, you would need a 7.5HP (5.5kw) motor.

Topography plays a major role in sound coverage. If you live in an open area with even elevation, the siren specified above should cover fine. If that is not the case, or you desire coverage indoors, you could expect to need an additional siren or two. You could also just build a bigger siren. Horns do help with range, however their affect may be negligible, depending on construction, and topography.



https://www.youtube.com/watch?v=rA6FhuYEHxE

[sebastian.salmhofer]

That is very helpful, thank you.
Do you have some advice about the amount and exact shape of the vanes?
2800RPM+ means 3000RPM would be good or is more more better? I guess then I we would need a belt drive since most motors are either 1500 or 3000RPM with a 50Hz grid.
The siren would be mounted horizontally like in the Video right? So it has 360° coverage. And a cover on top with a gap along the side with wire mesh would probably be good against direct rain and birds or other animals, or does that have some negative effects?

The are is very flat. So if we go through with this I think I would just build one and test if it's enough. I can always make another one. So you would recommend 18" diameter for our desired range?

[Synther]

That is very helpful, thank you.
Do you have some advice about the amount and exact shape of the vanes?
2800RPM+ means 3000RPM would be good or is more more better? I guess then I we would need a belt drive since most motors are either 1500 or 3000RPM with a 50Hz grid.
The siren would be mounted horizontally like in the Video right? So it has 360° coverage. And a cover on top with a gap along the side with wire mesh would probably be good against direct rain and birds or other animals, or does that have some negative effects?

The are is very flat. So if we go through with this I think I would just build one and test if it's enough. I can always make another one. So you would recommend 18" diameter for our desired range?

The shape of the vanes is not super important, you just want something that can pick up the air and push it towards the inner walls of the chopper. You can choose to orient the vanes toward a particular direction, increasing air pressure when spun said direction. Otherwise, a flat vane going directly towards the center from each slot is adequate. RPM again is not critical, but you do want to get the siren spinning fast enough. 3000RPM is good.
Mounting the siren like it is in the video will spread the sound evenly through the area. You can fabricate an intake cap and screen to protect the siren from weather and debris with no negative effects, as long as the cap does not obstruct the flow of air and sound coming in or out of the siren.

If your area is flat and you have an ideal elevated spot for the siren, 18" is certainly be enough. You could also start with 15" or even 12" and possibly yield good results, but 18" should definitely be loud. It's good to keep in mind the forces at play in a drum spinning so fast, things can fly apart. Safety is critical and I wish you luck.

[TheVintageLouisiana]

There isn't a real budget yet because we all don't know anything about the topic. It should be as inexpensive as possible while doing it's job and being reliable. My time is free if I am going to build something, so it would just be materials in this case.

I hope this project turns out well for you guys, hoping for the best!

[sebastian.salmhofer]

Thank you very much for your help. If we end up building something I will show you the result here.