When you slap the open end of a pipe onto the palm of your hand, a compression pulse travels upward (a), reflects from the open end as an expansion, and moves back down to your hand (b). The expansion reflects from your hand, travels back up the pipe a second time (c), and comes back to your hand as a compression (d). The process then repeats (e). Click to enlarge the diagram below.
The molecules that have been squeezed together, in turn, squeeze the molecules next to them. Those molecules, in turn, squeeze the molecules next to them, and so on. In a sort of domino effect, the pulse of compression (high-pressure air) travels up the tube.
When the pulse of compression reaches the top of the tube (see b in the diagram above), it expands outward into the air around the tube. In the process, some air molecules overshoot the end of the tube, producing a region of expansion (low-pressure air) at the top. Air molecules just below the area of expansion rush upward to fill it, creating a pulse of expansion that travels back down the tube. When this pulse reaches the bottom, it reflects off your palm and travels back up the tube as another pulse of expansion (see c in the diagram above). When it reaches the top, some air from outside the tube rushes into the low-pressure area, creating an area of compression, which travels as another pulse back down the tube (see d in the diagram above).
When this pulse of compression reaches the palm of your hand (see e in the diagram above), it reflects, and at this point, the whole process repeats itself.
A pulse that starts at your palm as a compression makes four complete transits of the tube, traveling up as a compression, down as an expansion, up as an expansion, and down as a compression before one whole cycle is completed. This four-part cycle corresponds to one wavelength of a sound, or a single vibration. A series of these repeated cycles is the source of the sound you hear when you “play” one of the pipes.
The length of the tube affects the note the tube produces. Because the speed of sound is the same in all the tubes, the length of the tube has a direct effect on the time it takes for a compression/expansion pulse to make its four transits of the tube. The longer it takes for a pulse to complete its cycle and start over again, the fewer the cycles, or vibrations, per second. The fewer the vibrations per second, the lower the frequency of the sound, and the lower the musical note. Thus, long tubes produce lower notes, and short tubes produce higher notes.
Instruments with long tubes, such as bass saxophones, produce lower-frequency notes than instruments with shorter tubes. Although modern symphonic instruments may seem quite complex, the basic relationship between long and short tubes is the same as exists for the simple Palm Pipes in this Snack.
Palm Pipes are part of a larger family of instruments known as idiophones, which includes rattles, bells, gongs, and xylophones, among others.