Intro: Air Raid Siren From a Sewing Machine
I looked around for a usable siren that is neither based on a speaker nor just a tiny "whistle".
Sadly there was nothing good around.
What was there not to like on the many available siren models?
1. Most are just tiny and createnothing but an annoying noise.
2. Classic designs, scaled to around 10cm diameter require massive motor power.
3. Nothing I tested was really loud enough.
What is so different on my model?
1. It combines modern fan designs with a complete different way of interrupting the air flow.
2. Far less motor power is required.
3. Even though it is a light design it holds up to the tested 5500RPM.
Check the models:
This is the bottom part that mounts onto the motor.
Due to the speed of the motor only 4 air vents are used as otherwise the frequency would be far too high.
This is the part that makes the difference, the siren fan.
Unlike other designs it uses fan blades similar to what you find on a PC fan for example.
Also the blocked off parts are not located at the end of the fan blades like you would expect but about half way through.
This design uses far less motor power as there is vertical walls to spin in the air but also provides much higher air flow.
As the result the sound pressure level is about 30% higher than conventional designs while using over 40% less energy to spin.
The top cone for the siren.
It helps to give more stability but most of all even more noise :)
The sound is also quite directional when using it.
As you can see on the models I kept it as simple as possible.
No smoothing has been applied accept for the top cone.
It keep the size down and makes silicing a bit easier but the main reason is to keep it in a form that can be modified.
For a different motor you can remove the existing holes and create new ones to fit.
On perfectly cleaned and smoothed models this would be a pain to do.
The big hole in the bottom part is to still aloow for some air flow from the motor as the exit vents are located there, so no prblem closing this big hole if you don't need it.
The sound meter on my phone maxed out at 100db - but that is also the loudest it can react to as even a shotgun fired next to it only reads 100db.
When I measured it the full 10db showed till I was about 3m away from the siren before slowly dropping down.
Use ear protection when testing this thing!
Here is a video of the siren in action.
Please note that I wrapped the phone in a towel but still the mic is distorting a bit.
You don't want to have this siren running in the same room with you for long LOL
Step 1: The Parts and Assembling Them
I used a motor from an old sewing machine - Janome type, driven by this external motor and a short belt.
If you use a different motor please adjust the models to your needs before printing them.
The pulley is secured with a set screw that goes completely through the drive shaft, one of the reasons I choose this motor fom my scrap bin.
A 1.5mm Allen key (hex) is required to get the screw out but as they used a good glue I had to heat it all up.
For this I used an old allan key and a small blow torch to apply heat directly on the screw.
Brute force might work too but I was not in the mood to risk the screw or my tools.
The bottom part is mounted after removing the original bracket and with the original screws.
Don't get the screws all tight already!
Tolerances are quite small and you might want to adjust the placement a little bit before fastening all up.
The fan is mounted loose to allow for minor adjustments of the bottom part.
Once all is aligned and the screws tight the fan is hold in pace with the original set screw, it will sit flush in the fan from both sides.
The top cone is pushed on and you are done, well almost.
I did not include a locking system in the design as I planned on glue for this step.
But if you want to avoid this drill 2 or 4 small holes where the side walls are located and put some screws it.
Maybe I change the design a bit and include holes but such tiny holes are hard to print and would rquire drilling in most cases.