Although it seemed to you that the lower crests were closer together than the upper ones, in reality they were the same distance. But the lower crests likely seem more compact than the upper ones, which may have led to the illusion you experienced. If you look at the illustration below, you will probably see more motion taking place in the lower sound — the molecules of air are moving faster in this sound than in the upper one. This sound has a greater amplitude and would sound louder than the upper sound.

Next we will introduce you to a device that can help us to visualize sounds, known as an oscilloscope.