After seeing Mathias' video on his home-built "air raid siren" (http://woodgears.ca/siren/) I've thought it might be a good idea to use a downscaled version of it for my ongoing alarm clock project. Key factors for me are small size - so it can fit on top of the enclosure, aswell as loudness.
I'm planning to use a Spindle from an old HDD (which essentially is a brushless DC motor) - mainly because it's small and strong enough for this project and I happen to have some of these laying around.
After doing some research on the internet, and not finding much info about the rotor design I've decided to take the experimental path. I quickly made a stator / rotor model in CAD Software and printed it. It seemed to work, although the sound it produced wasn't loud - about 75 dB at 10cm distance (Measured with a phone app, so the measured values may be off).
During the print, I've found one paper suggesting that the ratio between rotor blanks and stater holes widths effectively affects the waveform the siren produces (1:1 ratio - triangle wave, smaller stator holes - square wave). So I've quickly designed and printed a new stator, so the stator/rotor ratio is 25°/30°. As a result I've ended up with a siren producing ca. 83 dB! (same measurement method as above). Unfortunately I don't have a video using this setup.
Next I've decided to try experimenting with different rotor "fin" shapes aswell as producing two tones instead of one, to make it sound more authentic (Old WWII Air Raid Sirens seem to produce two tones). This is done by adding a second row to the stator and rotor, with a different amount of fins / holes. I went with the 6 / 10 ratio and changed the shape of the fins (new ones are on the right):
The siren did produce the dual-tone, however, it was not nearly as loud as the one to the left.
I will have to continue to experiment with different shapes and opening ratios, aswell as try to find some more info on the net.
P.S. I've found another HDD spindle at home, which turned out to be much more powerful. I've managed to reach 89dB with the first siren design.