Air raid sirens have been used for decades as a way to warn people of everything from an enemy bomber to natural disasters. They have a unique sound that has become synonymous with danger, and everyone reacts to the sound the second they hear it. The sound is created by chopping air with a rotor, and you can make a simple hand-crank version of this siren yourself with a little determination and the right tools.
Cut a metal pipe to about 6 inches long using a mini saw and then cut a small circular hole on the bottom of the pipe large enough for the bolt to slip snugly through. Cut several rectangular holes about 1 inch high and 2 inches wide above the circular hole around the pipe equidistant apart until they go all around the pipe. Put the cut rectangles off to the side because you will need them again later in the project.
Place the bolt through the circular hole so that it can turn, but remains securely in place. Attach a gear in a vertical position. Grind the fan blade down until it can fit snugly into the pipe, leaving just enough room for the rectangles to pass through along with the fan.
Attach a bolt to the fan and to the other end attach a gear horizontally. Weld the flat side of the rectangles to the edge of the fan blades and place them into the metal pipe. The rectangles should line up with the rectangular holes cut earlier so that when the fan spins, the rectangles cover the holes completely. The gears should fit together so that when the handle turns, so does the fan.
Glue the plastic cone around the end of the pipe to direct the sound and then test the siren out by turning the crank. The faster you turn the crank, the faster the fan will spin and chop the air flow. Each time the rectangle covers it forces the air out through the cone, creating the sound.