This page is designed to illustrate how particle displacement in a sine wave sound relates to the sound pressure. The blue dots illustrate the air molecules (particles) with a single dot highlighted in red. The red arrow shows the displacement of this particle (both distance and direction from the starting point). As the particles move back and forwards they bunch up (condensation – higher pressure) and then spread out (rarefaction – lower pressure).
Observe the behaviour with the default values and then increase the amplitude to maximum and speed to minimum to see what is happening in slow motion. If you observe the relationship between the displacement of the red dot and the pressure wave you will notice that the maximum pressure occurs when the displacement is zero. [hint: use the pause/play button to freeze the display at different points in the cycle]
Now click the checkbox to show the displacement wave. Positive values of this wave indicate displacement to the right, negative values indicate displacement to the left. Notice how the displacement wave is 1/4 cycle (90 degrees) behind the pressure wave.