Caption: An animation dynamically illustrating the 2nd harmonic or 1st overtone of standing waves (specifically, standing sound waves) in air in an simplified musical pipe.

One full wavelength is shown. You can tell this because there are 3 nodes and 2 anti-nodes with oscillations going in opposite directions.

Features:

1. Sound waves in fluids are a longitudinal wave phenomenon since the oscillation direction of the transmission medium is parallel to the propagation direction when the waves propagate---which they are NOT doing in the animation since the displayed waves are standing waves.

   Transverse sound waves as well as longitudinal sound waves occur in solids.

   Transverse sound waves CANNOT exist in fluids when they are sufficiently ideal since shearing forces (i.e., "sideways" forces) do NOT exist in ideal fluids.

2. The points in the animation are oscillating cartoon air molecules.

3. Three macroscopic-scale things are oscillating: velocity, density, and pressure.

4. The velocity has nodes (fixed zeros) at the ends and middle of the pipe.

   That there are 3 nodes shows that one full wavelength is shown: one complete up-and-down cycle for the oscillation. The "up" is motion to the right; the "down" is motion to the left.

5. Pressure and density oscillate in their own abstract spaces, NOT in space space, of course.

   But we notice their oscillations in the obvious rarefactions and compressions of the displayed cartoon air molecules.

6. The density and pressure (caused by the collective action of the molecules) are 90° out of phase from velocity: their anti-nodes (points of maximum amplitude) at the ends and middle of the pipe.

   Saying 90° out of phase is equivalent saying 1/4 wavelength out of phase.

   Density oscillates in phase with pressure. Thus, the two thermodynamic variables have their nodes and anti-nodes in the same places.

7. The wave motion in pressure is sound itself.

8. The standing sound wave in the animation is an ideal limit of what happens in a pipe closed at both ends. For example, a closed-end organ pipe. The molecules can't move at the ends, but the pressure oscillates between its maximum and minimum values there.

   In real wind instruments, there are holes somewhere to let sound (i.e., sound waves) escape.

9. Usually, in musical instruments of all sorts, there are multiple harmonics (i.e., multiple superimposed standing wave). The particular mixture of harmonics of a particular musical instrument is a main cause of that musical instrument's characteristic timbre (see Wikipedia: Timbre: Harmonics).