Building a wooden air raid siren

siren

A while back, [Matthias] was working on a dust collector for his shop. Being the master woodsmith he is, he decided to build a dust collection system out of wood. Everything worked out in the end, but in creating wooden impellers and blowers, he discovered his creations made a lot of noise. For this project, instead of trying to quiet his blower, he decided to make one as loud as possible in the form of an air raid siren.

The basic idea behind [Matthias]’ air raid siren is to make two impellers that force air through two stators along the perimeter of the rotor. As the siren spins, the air coming from the impellers is either blocked or passes through the stators, creating an alternating high and low pressure; to be more accurate, it creates a ton of noise. Stack two of these impellers together and you’ve got a two-tone air raid siren made out of wood.

For something that’s spinning very fast, we’re surprised [Matthias] didn’t have more problems with balancing his siren than he did. There are a few useful tricks to be picked up from his tutorial, though: balancing everything on a marble really seemed to help with the build.

As for how loud the siren is, [Matthias] can’t give us a decibel volume. From the video after the break, though, we can tell you it’s really, really loud.

Thanks [Dimitar] for sending this in. [Matthias], it’s alright if you send projects in yourself. You’ve earned the right to say, “I am so cool!”

44 thoughts on “Building a wooden air raid siren

    1. Agreed. That table saw/motor mount combo for turning the hub might be the most dangerous(and possibly most hilarious) thing I have ever seen on HAD.

      1. He would be more likely to lose a finger if he tried that trick with the bandsaw imho. Fingers tend to bounce off a table saw whereas a band will chew right through before you realize.

    2. He has a lathe; if you watch his shop tour video he mentions that its not used very much, and acts as storage for other under-utilized equipment.

      in the video he mentions that he used the table saw method so he can get the center of the block exactly aligned with the motor shaft quickest. He also mentions that the setup is the most dangerous thing he has done with his table saw.

    3. Come on! Cant you see that this guy knows exactly what he is doing? His fingers are nowhere near any moving parts. He is showing us how to manage without a lathe which requires serious skills.

      1. “Knowing what you’re doing” often leads to complacency and unnecessary risks, and that’s when you screw up and hurt yourself or someone else. I have seen it happen to many people, in all sorts of semi-dangerous professions(carpentry, forklift drivers, crane operators).

        A kid in my high school shop class was using a table saw to shoot 2×4 chunks across the room and then lost two fingers when he didn’t let go fast enough. My father was a carpenter for 15 years and looked away while cutting – bam, lost a finger. These things are mad dangerous.

  1. I wonder how loud you could make one a them. Like, REALLY loud? Does the volume scale linearly with size or what?

    I’m thinking one that was powered by a drill motor, or if you wanna go crazy, a motorbike engine. Or even a 4-cylinder car engine, though you’d need a hell of a long wire to the ignition switch. I’m thinking something that could annoy people for at least a mile around.

    Perhaps if money was no object I’d go looking round a helicopter scrapyard. If they have them. An old helicopter’s got 90% of the difficult bits implemented anyway! And they’re loud even when they’re supposed to be quiet!

    1. Many of the old municipal air-raid sirens were powered by industrial Chrysler Hemi V8 engines.

      A modern 4-cylinder engine is probably reasonably comparable power, actually; might be a little lower.

        1. On victory siren dot com you download real wave files to listen to several sounds.
          I have said for more than 30 years about electric cars how fun it would be to have the Hemi^R sound for Drivetones^R!

      1. Sound output is not linear with power, it’s logarithmic…Bel is a logarithmic unit (3 dB up means nearly double the power, 3 dB down means half the power…)

        1. Which is why I suggested that 1/1000th the motor power would produce a 30dB drop in sound level, yes. :-)

          What I am assuming is that output power is linear with input power, which may not be a correct assumption.

    2. Could you convert a playground merry-go-round into a giant siren?

      I once saw a video where some kids had a motor scooter tipped on it’s side, wheel against the MGR, spinning it crazy fast. So I’d think that, with some baffles and big noise. Or, just attach a pulse jet.

  2. The noise is radiating in all directions. Add a focus/amplification cone around it and it will be like a directed sound cannon.

  3. I once disassembled a vacuum cleaner and it had the same style and almost size rotor in it, just made from aluminium instead of wood.
    I guess this explains why vacuum cleaners aren’t so silent then :)
    Anyway if you got an old vacuum cleaner you could make a siren too I guess, all you need is a motor and rim with holes.

    In fact if you strategically made holes in the housing of a working one… instant siren.

  4. If you want to make a siren out of a vac just hook up swimming pool drain hose instead of the regular hose. I did such once thinking I had found a source of cheap vacuum hose. I stepped on the switch and almost lost my balance trying to turn it off again, as the harmonic wailing went on. Yesterday we had an hour of intermittent sirens to promote tornado awareness. They have horns and rotate. I usually don’t hear the one 3 blocks away. Maybe if the horn rotated like a Leslie on fast speed, instead of slow.

    1. Makes me wonder if you can simply place some sort of reed into the hose. If it’s tuned right, it will vibrate like mad.

      1. Small puffs of air would come out of the much larger hose from the reed, making a untuned reed organ pipe. In the case of the rippled surface hose above the whole mass of air is modulated to move at higher and higher harmonic series, gushing out (or in the vac in) in a much louder manner.
        In the 70-80’s they had a toy on TV adverts, a piece of corrugated hose of thin wall molded plastic. The whirly tone or something like that. Whirling the hose above your head you could look and SOUND like a UFO. Centripetal force pushed air thru the tube enough to do the same thing. Try blowing thru a crinkly hose, gooseneck lamp or mike holder or gas pipe flex coupling.
        Early 60’s Atlas series NASA found out the hard way (boom) that main fuel flex hoses with wavy insides produced anything but smooth flow.

    1. If you do 12 rings, it *might* be possible to make a musical instrument out of this.

      Just stack 12 rings and 12 stators, and add 12 ‘baffle rings’ around the stators that open and close.

      Of course this would require some crazy machining to get all the rings in tune with each other, but the speed of the whole confabulation shouldn’t matter too much (unless you’re tuning against another instrument)

      Someone with a welder get on this.

  5. Seems like you could build one faster if you used hot wire cut foam for the “winglets” and the stators. Of course this gentleman is a wood worker so his goal was to make it out of wood.

  6. Getting to be the wrong time of the year to be playing with sirens outdoors. Everyone will be running out side to see the tornado. That or nearby volunteer fire fighters will be running to their cars wondering why their @#$ pager didn’t go off.

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