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4 Sound Waves in Air

Sound waves travel through air in the form of waves called longitudinal waves. Sound waves transport energy as they travel through a medium. Sound waves are produced when an object, such as a plucked guitar string, vibrates. On one side of the string, the air molecules are compressed, or squeezed together, creating a high-pressure area. Areas of high pressure are called compression. At the same time on the opposite side of the string, a low-pressure area is produced allowing the air molecules to spread out, which is called rarefaction. Figure 1 models the compression and rarefaction of air surrounding a vibrating string that is represented by the red line. It is important to note that the air molecules around the string are moving back and forth but they never move far. Instead, each time the air molecules are compressed, they hit against other air molecules pushing them farther away from the string. The sound wave continues to move away from the source of the sound, the vibrating string, in all directions. Sound waves continue to move and bounce off surfaces until there is no more energy. The compression and rarefaction of longitudinal waves can be demonstrated with a Slinky.

FIG 1

See for Yourself

Materials

 Slinky

What to Do

1 Lay the Slinky on a table.

2 Hold the ends of the Slinky with your hands and stretch the Slinky so that it is about 2 feet (60 cm) long.

3 Holding one end of the Slinky stationary, push the opposite end inward about 4 inches (10 cm) and then pull outward again (Figure 2). Observe the movement of the coils in the Slinky.

FIG 2

What Happened?

Compressing the coils models the compression of air molecules when a vibrating object pushes on air molecules. As the object is pushing air together on one side, at the same time, on the opposite side, the air molecules are spreading apart. Stretching the coils models the rarefaction of air.

The compression of the coils sends a visible wave down the Slinky, modeling the way the air molecules behave when bumping into the next air molecules, and the next, and so on. At the end of the Slinky, the waves are bounced back. This back and forth movement continues but it will slow down over time until stopping, due to a loss of energy. The coils compress and stretch apart as the wave passes through the coils. At the secured end, the wave bounces back, sending the wave in the opposite direction. The person holding the end of the Slinky can feel the energy of the modeled sound wave because the last coil pushes against their hand. This back and forth movement occurs several times. Likewise, sound waves also bounce off of objects. Sound waves are absorbed by soft surfaces and reflected by hard surfaces. Figure 3 compares compression and rarefaction in the Slinky to a sound wave in air.

FIG 3