Sound Waves
Sound waves are an example of longitudinal waves. In the photo to the left, you can see the compressions where air particles are bunched close together and the rarefactions where air particles are further apart.
Here is an animation of a single sound wave. The red bar on the left vibrates to create the disturbance in air particles (represented by black dots). The disturbance is passed along to the right.
Here is an animation of continuous sound waves. If you focus on one of the red dots, you can see that each particles moves right and left, but does not travel along the whole length with the wave.
Here is a "pure tone generator" website that plays sounds at various frequencies: http://www.fearofphysics.com/Sound/sounds.html
For sound waves, the frequency can also be referred to as pitch.
Here is an animation of standing sound waves. The first panel shows the model particles moving. If you focus on the red dots, you can see that at a node a particle is stationary, and at an antinode a particle has a large amount of displacement.
The second panel (black line graph) shows how much displacement occurs along the tube graphically, so you can clearly see where the nodes and antinodes occur.
The third panel (red line graph) shows how the pressure changes along the tube as the standing waves go by. Notice that at the nodes the pressure increases and decreases the most, and that at the antinodes there is no pressure change.
The second panel (black line graph) shows how much displacement occurs along the tube graphically, so you can clearly see where the nodes and antinodes occur.
The third panel (red line graph) shows how the pressure changes along the tube as the standing waves go by. Notice that at the nodes the pressure increases and decreases the most, and that at the antinodes there is no pressure change.