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Lesson: Chapter 4 — Waves and their applications

4.3. Sound and how it travels 2 of 4

Compressions in the air

To see how sound travels through air, follow the wave sent out by a loudspeaker as its cone vibrates.

Figure to be added

Figure 4.23 — six snapshots, (a) to (f), of the air in front of a vibrating loudspeaker cone: first evenly spread air particles, then bands where the particles are crowded (compressions) and bands where they are spread out (rarefactions), moving away from the cone.
  1. Before the cone moves, the air particles in front of it are spread out at random.
  2. As the cone moves right, it pushes the air in front of it forward, making a compression. The particles pass the push on, so the compression travels forward.
  3. As the cone moves back left, a rarefaction forms next to it.
  4. Each time the cone moves right again, another compression forms and follows the first.

The cone keeps making compressions and rarefactions one after another, and they all travel forward at the same speed. These travelling compressions and rarefactions are the sound wave in air. In a compression the number of particles rises, so the pressure there rises for a moment; in a rarefaction it falls for a moment.

The air stays where it is

The compressions and rarefactions move forward, but each air particle only vibrates about one position. Because the particles move along the direction the wave travels, sound is a longitudinal wave. In dry air at room temperature it travels at about 330 metres per second.