Introduction: 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:

Siren Sewing Machine Bottom by Downunder35m on Sketchfab

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.

Siren Sewing Machine Fan by Downunder35m on Sketchfab

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.

Siren Sewing Machine Top by Downunder35m on Sketchfab

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.

Comments

author
tomatoskins (author)2015-09-02

This is awesome! I'd love to hear how it sounds! Maybe a video?

author
Downunder35m (author)tomatoskins2015-09-02

Can't try tonight as the neighbours would most likely feel a bit annoyed.
Tried the recording earlier but it was totally distored, maybe because of the sound level...
Will give it another try tomorrow and if in doubt cover the microphone so at least the frequency can be judged.
Only other way would be to record from far away LOL

author
Kiteman (author)Downunder35m2015-09-02

I'm looking forward to the video as well - that impeller is rather interesting.

author
Downunder35m (author)Kiteman2015-09-02

Why settle for 100 yera old designs that never changed? ;)

author
Kiteman (author)Downunder35m2015-09-03

I'm tempted to say, if it ain't broke, don't fix it.

I look forward to you convincing me...

author
rafununu (author)2015-09-03

Did you make measures regarding the air pressure or the sound strength ?

author
Downunder35m (author)rafununu2015-09-03

I could only use an app on my phone, but it never goes higher than 100DB - not even if you fire a shotgun next to it.
Since I neither have the equippment to measure the air pressure, nor a suitable simulation software I went from my findings from other projects.
There is no real science behind the design, just a combination of different designs.
Basically it is an impeller with the wrong output direction ;)

My main goal was to create something of usable and printable size.
The dimensions are small enough that even the budget printers are big enough for it.
But even in this scale the classic siren design causes far too much drag for a small motor, so it had to be tweaked for a lower power consumption.
Still thinking of making another one with just three outlets for a slightly lower frequency....

author
Downunder35m (author)2015-09-02

As requested, a little video showing the siren spinning.
Still not the sound you would get from a real one but the best I could do on such a small scale.

author
unclelar (author)2015-09-02

Did you 3-D print these parts,other than the motor ? Can't find anything like that on my sewing machine. lol

author
Downunder35m (author)unclelar2015-09-02

I tired printing the motor but my hotend always gave up when I tried feeding it the steel ;)
Not every sewing machine motor might be suitable, but the older models with external motor should all be fine for this (not saying it is a 100% fit).
But plenty of these motors out there, even on Fleabuy:
Link1
Link2
And even my motor!

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Bio: I like to improve myself and things I find :) Learning new things every day is next to impossible but I still try - only a working ... More »
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