If you saw the second "Crocodile Dundee" movie, you may remember the scene in which the hero swings a flat stick around on the end of a string, which causes it to hum or buzz in a loud and curious way. I was surprised to find that this was not artistic license—these things exist in many early cultures, they are called Bullroarers, and they are quite easy to make with simple materials. All you need to do is to fasten a piece of string to a flat stick, like a tongue depressor. If you swing it rapidly in a circle, the wind will cause the stick to flutter and then spin rapidly around the long axis, which produces the strange sound.
The picture below shows two of my (successful) attempts to make a bullroarer. In the end of the tongue depressor pictured below, I whittled a couple of notches and then tied the string in a loop around the notches. In the other "craft stick", I burned a hole in the end, and tied the string through the hole. I can make both of these hum, although the tongue depressor is probably the easier of the two.
The third item in the picture is a plastic spoon. In the normal bullroarer, because it is spinning rapidly, the string tends to get all twisted and coiled and knotted up if you swing for very long. Apparently, with traditional ritual versions, a talented performer can use it continuously by coaxing it to wind it up in one direction, then reverse, unwind, and wind up in the opposite direction, thus alternating back and forth, but I could never get this to work. I just had to give up after awhile and allow it to unwind. A slightly more elaborate version of the bullroarer that doesn't twist up the string is a rubber band wrapped around a spoon. In a sense, however, this is cheating, because it is not a traditional bullroarer and it makes the sound by an entirely different mechanism. In this case, the sound comes from a rubber band vibrating like a guitar string, rather than a spinning stick. I also have a harder time getting this version to work. I think the best version of a bull-roarer is the tongue depressor with a thin string— you might just have to stop to let it unwind periodically.)
Both of the two flat sticks pictured above are commonly available in craft stores and hobby stores, and work quite well. I find that the shape doesn't make a great deal of difference. In the first stick above, I tried whittling various notches and grooves, thinking that it might make a more complex sound, but I couldn't detect any difference. The sticks do have to be sufficiently broad, however — I tried a thinner popsicle stick, and it didn't work nearly so well. Also, the string should be fairly thin and allow the stick to spin freely on its axis. Because the sound comes from a rapid spinning motion, if the string binds up and interferes with the motion, that will prevent it from making noise. I also tried a huge piece of cardboard in a similar shape, thinking I would make a louder noise with a bigger device, but the large cardboard didn't work. The flat noise-making bit has to be heavy enough so that you can work up some speed as you swing it in full fast circles, and not drag through the air like a falling leaf. Perhaps a piece of thin plywood?
I didn't know what to call these, and I'm not sure where they come from, so I just called them feather-sticks for their resemblance to feathers. These are very simple to make, too, although you have to be able to use a knife. You just whittle one end of a popsicle stick into a thin handle, like this:
Making a thin handle on one end allows you to roll it between your fingertips and make it spin around its axis, like the bullroarer. You can make it spin pretty fast through the air if you place the end between your finger and thumb and then snap your fingers. With a little practice, you can make these sticks fly through the air, spinning rapidly and emitting an odd kind of note as they go. (In this case, the narrower popsicle stick works better than the wider tongue depressor, probably because it is harder to make the tongue depressor spin fast enough to make noise, although I was able to make both kinds of stick work. The deformity in the pictured popsicle stick probably didn't help, either.)
Seebeck's Siren, and the Sound of Spinning
If you just want to goof around, that's what recess is for. If we are going to do a project in science class, there should be some kind of point to it. However, there are still some projects that I think are worth doing, but that don't directly illustrate any particular important scienctific principle. The two projects above fall into this category. They represent something fun to do and something interesting to observe, but they don't bear directly on any major scientific discovery or any big scientific idea. They give "food for thought", so to speak. So let's just chat a little about what we might be able to observe here, and then we can get back to throwing sticks around the room.
Do you notice anything interesting about the sound that the sticks are making? In both cases, the sound comes when the sticks spin rapidly, around their long axis. And in both cases, the faster the spin, the higher the note. With the bullroarer, you can speed up and slow down as you swing it around, and the sound should get higher or lower accordingly. The feather-sticks make an interesting "pew" sound—they start out loud and high-pitched, but they rapidly fade away, both in loudness and in pitch, as they fly and fall through the air. The note goes from high to low as it fades. Why do you suppose that is? Probably, it is slowing down as it flutters through the air, and as the motion gets weaker and slower, we can hear the sound get weaker and lower-pitched.
In both cases, what seems to be going on is that the flat stick is slapping the air really quickly as it spins, making a sort of note, and the faster it slaps the air, the higher the note. You have probably noticed a similar relationship in spinning machinery, or anything that spins fast enough to make a sound. The faster the spin, the higher-pitched the sound. There is a fundamental relationship between pitch of a sound, and the speed of the motion that makes it.
You can investigate this relationship quantitatively if you have access to a centrifuge. My school happened to have a table-clamped, hand-cranked centrifuge, and I decided to try to make a siren out of it. I cut a large disk from paper cardstock, used a hole punch to punch 36 evenly-spaced holes near the edge of the disk, then taped the disk to the top of the centrifuge. I (or a student) would then spin the disk very rapidly while blowing with a straw at the edge of the disk. The holes in the disk would act like gates at the end of the straw, alternately blocking the straw and letting a puff of air out through the hole in the disk. In this way, a very rapid series of puffs of air can be generated, which produces a rather loud scream, and the faster you crank, the higher-pitched the scream. This is the principle behind the mechanical air-siren, which is sometimes used for civil-defense purposes.
Besides capturing student's attention, and giving yet another example of the relationship between speed-of-spin and pitch-of-sound, this also provided a way to actually measure the relationship. The gear-ratio of the mechanism meant that for each turn of the crank handle, the disk would rotate 8 times, and thus 36x8 = 288 puffs of air would be emitted for each turn of the handle. At a cranking speed of roughly one turn per second, this means that the rate of the action would be a few hundred puffs per minute...maybe 200 puffs per minute for lower pitches, and maybe 400 or 500 for the highest pitch we could make. (We now officially measure the pitch of sound using "frequency", measured in units of "cycles per second" or "Hertz". Our centrifuge siren was capable of producing loud sounds from roughly 200-500 Hertz.) Once my students had heard the difference, it was then not too hard for some of them to estimate the speed of repetitive motions, based on the pitch of sound that they heard. For example, do you think you could estimate how fast a bee or a hummingbird flaps its wings, simply by hearing the sound of one flying by?
(If you want to try to make a similar siren yourself, there are a few handheld centrifuges available on Amazon, but they are probably a bit too expensive to buy just for one simple demonstration. I suppose you could use an electric drill, but that would introduce a great deal of noise itself, and you would have no way to measure the speed of rotation. If you have any suggestions for an alternative spinning device, please let me know in the comment section below.)