Building an air raid siren

When I experimented with building a blower for my small dust collector, I noticed some configurations made quite a howl. When building a blower, you try to minimize noise. But this inspired me to try building a blower that maximizes sound --- basically, a siren!

Building the impeller hub

The siren has a substantial size rotor, or impeller. To mount this hub on the motor shaft, I needed to figure out a precise way to make a hub, with a circular part that was exactly the diameter of my largest forstner bit.

The logical thing to do was to turn the hub right on the motor shaft, sort of like I did for my Bandsaw 3 motor pulley.

But with its rectangular outside, I wasn't keen on having my hands or chisels near it while it was spinning.

So I came up with an even more dangerous looking method. With the hub blank mounted on the shaft, I plugged in the motor, and passed it slowly over the spinning blade of my table saw.

I previously cut most of the waste away with my bandsaw, and I was only taking off two millimeters at a time on the table saw. It worked out well.

I wasn't sure if it would work though, so I kept way back from the action. I couldn't see if I was cutting, so I was listening to it to know if I was cutting. The sound changed once I got past the cylindrical part of the hub, which told me that I had pushed the motor far enough.

Building the impeller

Next, making the winglets for the rotors. I made a pattern out of plywood, and used that to mark the outline on a piece of spruce.

Cutting the winglets out on the bandsaw...

...and gluing them onto some 1/2" (12 mm) plywood.

I'm not using the good birch plywood here. This whole thing was very much an experiment, so I didn't want to use up any "good" materials. In fact, All the plywood for this project was trash picked.

I mounted the start of the rotor on the motor and clamped the motor to my workbench, ready to plug it in for a quick test.


It spun up ok the first time. But then I spun it up again to get video, and disaster struck. One of the winglets flew off. This totally threw the rotor off balance. The vibrations from that quickly loosened the bar clamps holding down the motor. With all the vibrations, it didn't take long for the motor to fall off the bench.

Fortunately, the power cord was caught behind the vise, and by pulling on the power cord, I was able to save the motor from crashing to the floor.

Other than the missing winglet, the rotor was undamaged.

Based on power measurements before that disaster, I also figured the layers of the rotor should be thinner, so I passed the rotor over my jointer a few times to plane the winglets down to 29 mm thick.

After gluing the second layer of plywood on, I used the table saw circle trick to make sure everything was perfectly round. Basically just rotating my work piece past the spinning blade.

I last used that same trick when I built a crokinole board

I removed the table saw insert for this step to try to suck more of the sawdust into the table saw.

The rotor has three layers of plywood sandwiching two layers of winglets. The first layer has six winglets, the second ten. The idea is that the siren will produce two notes simultaneously, just like WW2 air raid sirens in movies.

After gluing on the second layer of plywood, I realized --- oops, I forgot to lay out the geometry for the ten winglets. Without a center point, that's tricky to lay out.

So I wrapped a flexible tape measure around the perimeter, and it worked out to be exactly 95 cm around. So to do the layout for the ten winglets on the second layer, I just ran the tape around the edge, fixed it in place with some clamps, and then made a mark on the rotor for every 9.5 cm of circumference. You can barely see the yellow tape measure around the edge in the photo.

Next gluing on the winglets for the second layer...

... and another plywood disk on top of that.

Clamping it firmly to make sure the glue joints are good.

Balancing the impeller

My impeller turned out to be quite heavy by now. Considering the consequences of just one winglet flying off earlier, I knew it needed to be well balanced.

The "balancing it on a marble" method that I previously used for my dust collector impeller worked well enough, so I used it again to balance this rotor.

I jammed a marble, just slightly larger than the 5/8" hole into the middle of the hub...

...then balanced the rotor on this hub. I used a scrap of wood as a weight to figure out approximately how much the rotor was off balance.

I hadn't been very precise in how I made this rotor, so I had to drill out quite a bit of material to get it fully balanced.

It also turned out that the marble I was using wasn't completely round, so that made it very difficult to fine-tune the balance. Once I realized that and replaced the marble, it got easier, but by that time I had already drilled a few more holes than necessary!

The 5/8" hole that I drilled turned out to be a bit larger than the 5/8" motor shaft. But a shim, made from a pop can, nicely filled the gap for a tight fit.

This was a much easier shim job than when I shimmed some plumbing pipe up to fit into a 7/8" hole when I built my 14" bandsaws.

I ended up crushing part of the shim that stuck out when I tapped the rotor on with a rubber mallet.

I also stuck a large, cut-off nail into the shaft's keyway. I had filed a notch into the side of the hole of the hub, so the nail acts as a "key" to prevent the shaft from spinning inside the hub.

Ready for another spin-up test. This time on the floor, so there's no possibility of it crashing off the workbench.

I kept even further back this time. It worked ok, though the whole thing wandered on the floor a bit, so I had to tweak the balance some more.

I also checked the power consumption on the motor. For things like blowers, where the load is steady, it's too easy to overload the motor.

As it turned out, 1181 watts is about the rated power for that motor, but then again, I'm not planning on running this siren for long periods of time, so it should still be within reason.

On to part 2 of building an air raid siren