What is sound?

Sound can be defined as an undulatory perturbation of any type of elastic matter (gas, solid or liquid). This means that sound does not exist in the vacuum.

The first useful example is wind blowing on a wheat field: the single spikes actually swing back and fourth around a single still point since their roots are well planted in the ground. This is what happens to the single particles of matter: when they are set in motion by the arrival of a sonic vibration they swing around a still point in the direction of propagation. The medium elastic forces tend to bring them back to their starting position, to quiet. This happens because they actually pass most of their energy to the nearby particles, perturbing their state of quiet and hence propagating the oscillation.

Undulatory (Wave-like) motion can be encountered in other natural phenomena, think about seismic waves and all kinds of electromagnetic waves. It is important to remember that it always regards a movement of a perturbation, this carries energy but doesn’t move matter along its propagation!
If we throw a rock in a pond we’ll perturbate the surface quiet state and we’ll see circling propagating waves: when a floating leaf is hit by the waves it will swing up and down but it will always remain in the same position respect to the plane of the surface as it was before.

Sonic waves are longitudinal, this means that the perturbation swings along the wave direction of propagation; The vibrational waves in a pulled string are trasversal, they vibrate perpendicularly to the direction of propagation. Sea waves have both directions of oscillation, their movements are called spherical.