Normally, when something makes a wave, the wave travels outward, gradually spreading out and losing strength, like the waves moving away from a pebble dropped into a pond.

But when the wave hits something, it can bounce (reflection) or be bent (refraction). In fact, you can "trap" waves by making them bounce back and forth between two or more surfaces. Musical instruments take advantage of this; every musical instrument is in some way a trap for sound waves.

Why are trapped waves useful for music? Any bunch of sound waves will produce some sort of noise. But to be a tone - a sound with a particular pitch - a group of sound waves has to be very regular, all exactly the same distance apart. That's why you can talk about the frequency and wavelength of tones.

Figure 1: A noise is a jumble of sound waves. A tone is a very regular set of waves, all the same size and same distance apart.

So how can you produce a tone? Let's say you have a sound wave trap (for now, don't worry about what it looks like), and you keep sending more sound waves into it. Picture a lot of pebbles being dropped into a very small pool. As the waves start reflecting off the edges of the pond, they interfere with the new waves, making a jumble of waves that partly cancel each other out and mostly just roils the pond - noise.

But what if you could arrange the waves so that reflecting waves, instead of cancelling out the new waves, would reinforce them? The high parts of the reflected waves would meet the high parts of the oncoming waves and make them even higher. The low parts of the reflected waves would meet the low parts of the oncoming waves and make them even lower. Instead of a roiled mess of waves cancelling each other out, you would have a pond of perfectly ordered waves, with high points and low points appearing regularly at the same spots again and again. To help you imagine this, here are animations of a single wave reflecting back and forth and standing waves. This sort of orderliness is actually hard to get from water waves, but relatively easy to get in sound waves.

You may have noticed an interesting thing in the animation of standing waves: there are spots where the water goes up and down a great deal, and other spots where the water level doesn't seem to move at all. All standing waves have places, called nodes, where nothing is happening, and antinodes, where the wave is largest.

Figure 2: As a standing wave waves back and forth (from the red to the blue position), there are some spots called nodes that do not move at all; basically there is no change, no waving back-and-forth, at these spots. The spots at the biggest part of the wave - where there is the most change back-and-forth during each wave - are called antinodes.

Nodes and Antinodes

Of course, to really trap the waves, the container would have to be the perfect size for their wavelength, so that waves bouncing back at each end would also reinforce each other instead of interfering with each other and cancelling each other out. And it really helps to keep the container thin, so that you don't have to worry about waves bouncing off the sides and complicating things. So you have a bunch of regularly-spaced waves that are trapped, bouncing back and forth in a container that fits their wavelength perfectly. If you could watch these waves, it would not even look as if they are traveling back and forth. Instead, waves would seem to be appearing and disappearing regularly at exactly the same spots, so these trapped waves are called standing waves.

For any thin "container" of a particular length, there are plenty of possible standing waves that don't fit. But there are also many standing waves that do. The longest wave that fits it is called the fundamental. It is also called the first harmonic. The next longest wave that fits is the second harmonic, or the first overtone. The next longest wave is the third harmonic, or second overtone, and so on.

Figure 3: There is a whole set of standing waves, called harmonics, that will fit into any "container" of a specific length.

Standing Wave Harmonics

Notice that the waves in the second harmonic have to be half the length of the first harmonic; that's the only way they'll both "fit". The waves of the third harmonic have to be a third the length of the first harmonic, and so on. This has a direct affect on the frequency and pitch of harmonics, and so it affects the basics of music tremendously. To find out more about these subjects, please see Frequency, Wavelength, and Pitch, Harmonic Series, or Musical Intervals, Frequency, and Ratio.

One "container" that works very well to produce standing waves is a thin, very taut string that is held tightly in place at both ends. Since the string is taut, it vibrates quickly, producing sound waves, if you pluck it (or rub it with a bow). Since it is held tightly at both ends, that means there has to be a node at each end of the string. Instruments that produce sound usisng strings are called chordophones.

Figure 4: A string that's held very tightly at both ends can only vibrate at very particular wavelengths. The whole string can vibrate back and forth. It can vibrate in halves, with a node at the middle of the string as well as each end, or in thirds, fourths, and so on. But there are many wavelengths that don't "fit" the string; any wavelength that would need one end of the string to move. To get any of those other wavelengths, you need to change the length of the vibrating string. That is what the player does when she holds the string down with a finger, changing the length of the string and changing where the nodes are.

Standing Waves on a String

The wave that we hear giving the pitch of the string is from the whole string vibrating back and forth (the fundamental. But the string is making all those other possible vibrations, too, all at the same time, so that the actual vibration of the string is pretty complicated. The other vibrations (the ones that basically divide the string into halves, thirds and so on) produce a whole series of harmonics. We don't hear the harmonics as separate notes, but we do hear them. They are what gives the string its rich, musical, string-like sound - its timbre. To find out more about harmonics and how they affect a musical sound, see Harmonic Series.

Exercise 1

When the string player puts a finger down tightly on the string,

1. How has the part of the string that vibrates changed?
2. How does this change the sound waves that the string makes?
3. How does this change the sound that is heard?

Solution

1. The part of the string that can vibrate is shorter. The finger becomes the new "end" of the string.
2. The new sound wave is shorter, so its frequency is higher.
3. It sounds higher; it has a higher pitch.

Figure 5: When a finger holds the string down, the finger becomes the new end of the vibrating part of the string. The vibrating part of the string is shorter, and the whole set of sound waves it makes is shorter.

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The string disturbs the air molecules around it as it vibrates, producing sound waves in the air. But another great container for standing waves actually holds standing waves of air inside a long, thin pipe. This type of instrument is called an aerophone, and the most well-known of this type of instrument are often called wind instruments because, although the instrument itself does vibrate a little, most of the sound is produced by standing waves in the column of air inside the instrument.

If it is possible, have a reed player and a brass player demonstrate to you the sounds that their mouthpieces make without the instrument. This will be a sound much more like "white noise", with lots of extra frequencies in it that don't sound very musical. But, when you put the mouthpiece on an instrument shaped like a tube, only some of the sounds the mouthpiece makes are the right length for the tube. The only sound waves that the mouthpiece can produce now are the ones that are just the right length to become standing waves, and the noise becomes a musical tone.

Figure 6: Standing waves are waves that can keep on vibrating in the same place or in an object, because they are just exactly the right size for that space or object. The only sounds you hear coming from a wind instrument are the ones that could set up standing waves in the instrument.

Waves in Wind Instruments

The standing waves in a wind instrument are a little different from a vibrating string. Each end of the tube can be either closed or open. A closed end acts like the held end of a string; the wave must have a node at that end. But a standing wave will have an antinode at the open end of a tube.

The wave on a string is a transverse wave, moving the string back and forth, rather than moving up and down along the string. But the wave inside a tube, since it is a sound wave already, is a longitudinal wave; the waves do not go from side to side in the tube. Instead, they form along the length of the tube. Another module will be published soon with more details on Standing Waves in Wind Instruments.

So far we have looked at two of the four main groups of musical instruments: chordophones and aerophones. That leaves membranophones and idiophones. Membranophones are instruments in which the sound is produced by making a membrane vibrate; drums are the most familiar example. Most drums do not produce tones; they produce rhythmic "noise" (bursts of irregular waves). Some drums do have pitch, due to complex-patterned standing waves on the membrane that are reinforced in the space inside the drum. This works a little bit like the waves in tubes, above, but the waves produced on membranes, though very interesting, are too complex to be discussed here.

Idiophones are instruments in which the instrument itself, or a part of it, does the original vibrating. Some of these instruments (cymbals, for example) produce simple noise-like sounds when struck. But in some, the shape of the instrument - usually a tube, block, circle, or bell shape - allows the instrument to ring with a standing-wave vibration when you strike it. The standing waves in these carefully-shaped-and-sized idiophones - for example, the blocks on a xylophone - produce pitched tones, but again, the patterns of standing waves in these instruments are a little too complicated for this discussion.

Here is a handout on Standing Waves that you can copy for your class. If you have trouble with the PDF file, the handout is reproduced as a figure below.

Try to arrange for some demonstrations: by a string player to demonstrate open and held strings; by a brass and/or woodwind player to demonstrate mouthpiece sound vs. whole-instrument sound; by a percussionist to demonstrate tuned versus untuned drums, and that size affects the sound even of untuned drums.

You may want to have your class try some of the activities in Sound and Music.

Figure 7