Make your own bugle from a bull's horn, conch shell, or PVC pipe
There are a few different families of musical instrument, each producing notes according to their own principles. String instruments work by making a taut string vibrate, reed instruments work by making a flat flexible reed vibrate, flutes work by blowing across an opening, and horns work by making your lips flutter in the end of a tube.
If you want to demonstrate the different families of instruments and the different principles of sound-making in a classroom, horns are perhaps the easiest to make. All you need is a section of narrow pipe from the hardware store. I like PVC pipe the best, because it is cheap and lightweight and easy to find in a variety of sizes, but many kinds of pipes will work. Even mailing tubes or the cardboard tubes from paper towel rolls can work, although not as well as more solid material. (Paper towel rolls are also a bit too short to work well.) If you can find them, bona-fide bull horns (such as the one shown below, which my father gave to me long ago), or ram horns (such as a shofar from a Jewish supply shop), or conch shells with the tip broken off, are all capable of projecting a very impressive bellow.
(This is a video I made many years ago for an online science class, just in case you want to hear what the bullhorn sounds like:)
The trick with horns (or pipes, or shells) is to blow into them properly. You can't just exhale into the mouthpiece, or all you will produce will be a hissing sound. If you played a trumpet or a tuba in your school band, you already know the trick, and you should have no problem with sounding a PVC pipe or a bull horn. If you haven't: You have to press your lips together as you exhale in just the right way to make your lips flutter as you blow. This takes a little practice, but it isn't difficult once you get the hang of it — some of us do it already when we want to make an especially exuberant sigh. How you form your lips will also depend a little on the size of the opening — smaller openings require a tighter pucker, and larger openings require a looser embouchure, more like the exuberant sigh. If the opening is too tiny, making your lips flutter properly in the opening is impossible, which is why the bull horn pictured above has a trumpet mouthpiece stuck into the small end. It widens the opening.
With a tube the length of a paper towel tube or shorter, I have great difficulty making them sound, but I don't have much problem with any tube longer than that. With pipes around four feet long, I can even sound three or four different notes, like a bugle.
Various Shell Trumpets, the Alpine Horn, the Didgeridoo, and all the instruments in the horn section of a modern orchestra, all work by the same principle. (There's a species of particularly large sea snail which was even given the name "Triton's Trumpet".) These are all essentially tubes, straight or coiled, used to capture and resonate the sound of a person's fluttering lips.
So breathing through a tube does not make sound, or not a loud note anyway. The loud bellow comes from the fluttering of your lips, captured and resonated by the tube somehow. If one wanted to give a scientific name to this family of acoustic tools that all work on this principle, the proper name would be labrophone.
How Do You Make Different Notes?
If you have a few different pipes lying around, with different diameters and different lengths, and maybe a metal pipe or two in addition to plastic ones, you may want to explore what makes the difference between different notes. Why does a pipe produce the notes that it does, and only those notes?
First of all, you will probably notice that you can play different notes by blowing differently — but only a few. You can't play just any note you want. There is one basic note, the lowest note you can play, and then as you tighten your lips, the sound will jump to a higher note a few times, before you reach your upper limit. This is what enables buglers to play (limited) melodies without having any moving parts on their instrument. We call the lowest note that a tube can emit the fundamental note, and the “ladder” of ascending notes we call a harmonic series. A talented bugler can normally play five or six notes of a harmonic series. (If you listen carefully to the notes, you may notice that there does seem to be something curiously consonant or harmonious about those pitches. What's so special about these particular harmonic notes?)
Second of all, if you compare the fundamental notes of different pipes, you should be able to notice that the diameter of the pipe makes little or no difference, and the material of the pipe doesn't make much difference either. The most important factor that controls how high or low the note of a pipe is, is the length of the pipe. There is some vital property in the mechanical nature of a pipe that makes it produce the pitch that it does, and that property is length. Pitch goes with length. The diameter can make a difference in how easy it is to sound the pipe, and to play various notes of the harmonic series. This is the reason for mouthpieces. If the pipe is too wide or too narrow, you can't make your lips vibrate in the opening. A bell or a flare at the other end of the pipe can also make a difference. Brass instruments generally have a bell on the end because it makes them sound better, and this bell can affect the pitch. But if you stick with simple cylindrical pipes, then as long as you can flutter your lips in the opening and produce the fundamental note, you should discover that all pipes of the same length will produce the same note. And the shorter the pipe, the higher the note.
This is how all brass instruments (other than bugles) make melodies: They produce a full scale of notes by having moving parts that change their length. The working of a slide trombone is obvious. You make the tube longer to produce a lower note, and make it shorter to produce a higher note. But how do you know exactly where to put the slide to produce exactly the right note? This is why most brass instruments have valves. They have a set of “detour tubes” of exact lengths, and when you press the valves, they divert your breath through one or more of these side-tunnels, making the total length of the tube longer by the right amount.
Now let's come back to the harmonic series for a moment. Why in the world would a single tube of a single length be able to produce a few different notes, and only those notes? What's so special about this harmonic series? Depending on the mathematical sophistication of your students, and depending on the pipes you have available, it might be interesting to see if you can explore this question...by comparing pipe lengths. Being able to relate audible pitch to measurable length gives us a way to compare notes numerically and mathematically. So let's try an experiment: If we play, not the fundamental note of a pipe, but one note up, then how much shorter would a second pipe have to be to play that same note as the fundamental? In other words, if we had two pipes playing the same note, one as the fundamental and one as the first note up, how would their lengths compare? I never actually tried this as a classroom project, but in retrospect, it doesn't seem like it would be that hard to do. If nothing else, just try comparing two pipes, one twice as long as the first. You should discover that the lowest note from the shorter pipe is the same as the second note on the longer pipe. Or in other words, the first note in the harmonic series is somehow “twice” or “half” of the fundamental. There's some very special relationship between the notes of a harmonic series, and simple whole numbers! A similar relationship is also the basis of the pan flute playing the Do-Re-Mi scale, or the Diatonic Scale.
Demonstrating that the Air is Vibrating
If you just listen to a person flutter his lips without a musical instrument, it doesn't sound very pretty. It certainly doesn't sound “clean” or “pure” or musical. What you hear is not a musical “note.” The pipe is apparently doing more than simply amplifying the sound of the person's lips and making it louder. It is also cleaning it, it is re-creating and purifying the sound somehow. It is re-sounding or resonating. But what does this mean?
If you want to demonstrate in an unforgettable way that the air itself inside the horn is actually vibrating or ringing, try dipping the end of a length of PVC pipe in soap-bubble solution. If you blow into the other end simply by exhaling, without sounding the pipe, you can blow a normal bubble at the other end. Then if you repeat the demonstration, but this time flutter your lips and sound the horn, the bubble will provide you with quite a spectacle, and will illustrate quite vividly that there is a lot of vibrating going on — not just in the solid walls of the tube, but in the very air itself. The lips only provoke the air, and the loud sound actually comes from violent vibration of the air itself in the tube. That is what is meant by re-sounding or resonation. The demonstration takes a little practice and a good thick soap-bubble solution, but it's worth it.
If you want to try this demonstration yourself, I can offer a few suggestions. You can buy bubble solution from Wal-Mart or many other home supply stores, but you can just as well make your own, from liquid detergent and water. I've been told by a science performer that Dawn and distilled water give the best results, in a ratio of about 1:10, and my experience agrees with that, although I prefer a bit more soap. Apparently, antibacterial agents or other additives to the detergent make the bubbles more fragile and less stable. So does hard water. It is often suggested that you use a thickener, like corn syrup or glycerin, but I haven't found that it makes enough difference to be worth the trouble. (Every year at science fair time, I would suggest to students that someone do a research project to find the best bubble recipe, one that gives the most durable bubbles, but no one ever took me up on my suggestion.) Also, the end of a PVC pipe purchased from a hardware store can be a bit rough and raw, which is not good for bubbles. You may want to clean it and smooth it with sandpaper or steel wool, or just be sure to find a pipe with a clean end when you purchase it.