How to Build a Siren

Understand how a siren works. The heart of a mechanical siren is a spinning cylinder, or rotor, inside a stationary object, or stator. Both the rotor and the stator have holes cut at regular intervals, so air is alternately blocked and forced through. This results in a wave of pressure through the air, or in other words, a sound wave. Because this design involves heavy materials moving at high speeds, a homemade siren can be difficult to make and potentially dangerous to operate. Starting with a siren rotor no more than 6 inches (15 centimeters) across is recommended. This can still be quite loud if attached to a motor.

• Alternatively, you can purchase a kit or download a plan for building an electronic siren. These use circuitry to send out a sound wave instead of mechanical air flow, and building them does not require any carpentry.
• While these instructions assumes you are using wood, it is possible to build a small siren out of almost any material. Try using thick cardboard, or a wide, shallow cookie tin.

Cut out two or three plywood circles. To make a simple, one-pitch siren, you'll only need two circles. If you would like a siren that has the distinct, two-tone sound of an air raid siren, cut out three circles instead. Trace each circle to the same size using a compass or circular object, then cut them out using a jigsaw or router. Optionally, for greater accuracy, attach a circle-cutting jig to your jigsaw before cutting.^[1]

• Circles no more than 6 in. (15 cm) across are recommended for your first siren. A larger circle can produce a louder noise, but this will require a more powerful motor and more careful building to prevent the siren from breaking during use.
• Eye protection is recommended while handling high-speed saws.

Make holes in the center of the plywood circles. The first plywood circle will need a hole for attaching the shaft of a motor, in order to set it spinning. Drill this precisely at the center to keep it balanced, and choose a size that will tightly fit the shaft so it does not slip off. The second (and third) plywood circle should have a much larger circle removed using a jigsaw, 1/3 or 1/2 the radius of the full circle. This turns the second (and third) plywood circle into a disc shape.

Trace out a "shark fin" shape. Trace one of your circles onto a new piece of plywood. On this trace, use a flexible tape measure to measure 1/12 of the circle's circumference and mark it off at either end. Draw a concave line from one end and a convex one from the other, tapering toward each other to meet at a point, which will just touch the inside edge of your plywood discs. This should look similar to the stereotypical shape of a shark's dorsal fin, pointing toward the circle's center.

Cut out six shark fin shapes out of wood. Cut out your "shark fin" piece of plywood, and use it to trace the same shape onto a length of 2 x 4 (38 x 89 mm), six times. Use your jigsaw or router to cut out these shapes.

• If making a dual-tone siren with three plywood circles, ^[2]

Glue the shark fins onto the first plywood circle. Arrange the "shark fins" around the circumference of the plywood circle, so their bases are even with the circle's edge, and the tips tapers in pointing toward the center. Space these evenly, so you can see curved channels between them where the air will be pushed through from the center outward. Use a strong carpenter's glue to attach these to the surface, and clamp the plywood and the "shark fins" firmly together while the glue dries.

• Check the label of your adhesive to find out how long it takes to dry.
• If, after the glue has dried, you can pull off or move the wooden fins, repeat with a stronger glue or a stronger clamp.

Complete the rotor. Glue the plywood disc on top of the shark fins, and clamp until dry. If you are building a two-tone siren, cut out more shark fin shapes of a different size to form the second cylinder. For instance, trace shark fins with a base covering 1/20 of the circle's edge, and cut out 10 of them to glue on top of the plywood disc. Glue the last plywood disc on top of these.

Balance the rotor. Find a marble or other perfectly round object, and fit it into the small hole on the plywood circle, where the shaft will eventually go. Place the entire rotor on a flat surface, resting on top of this marble, and attempt to balance it. When the rotor falls over onto one side, mark which side that is and remove some weight from that side by drilling holes through the plywood circle, near the circumference, or through the ends of the shark fin shapes.^[3] Repeat until the rotor is as balanced as possible, to minimize stress on the siren during operation.