When an object moves back and forth, it produces bands of compressed and expanded particles that move through the medium (bands of compression travel, but particles do not).

The density of particle compression gets greater when the amplitude of vibration at the sound source increases.

The distance between compression bands appears to change when we change the frequency of vibration.

Collisions between the particles in the medium result in these compression bands moving away from a sound source.

The collisions transfer energy across the medium.