Shrinking an air-raid siren to fit into your pocket
Quoted as being ideally suited for those looking to be really annoying, this laser cut project by Mark Langford on Instructables might catch your attention. Taking the same principles that give air-raid sirens such an impressive audio impact, he has condensed them down into a neat little package that can fit on a key ring.
After several iterations, the mechanics of the three-layer design were perfected and (as you can hear in the following video) it really does work. Extra points of course go to the fancy eyebrow acrobatics!
Here is how it works:
The air you blow in blows out through the pattern of holes, and at the same time, it makes the turbine spin.
If there was no turbine, the air would just hiss out of the holes, but the holes and blades are designed so that the spinning turbine alternately covers and uncovers the holes, rapidly blocking and releasing the air in a series of pulses that make the noise you hear.
See the Turbine Whistle on Instructables where you can learn from Mark’s thorough project walkthrough. There are plenty of step-by-step photos and of course you can download the files to make a pocket siren of your own.