Sound is diffused in the air through multiple collisions between particles. Let's take a loudspeaker as our sound source. The magnet moves backwards and forwards following the amplitude of the electric signal applied to the inductor it rests upon (for a detailed description see the section on loudspeakers [Sound diffusion systems ] ). This makes the air particles move, compressing them and dilating them successively.
Let's now follow the diffusion of sound coming from its source (the loudspeaker). Let's presume to start off with, that a compression takes place towards the right, then a dilation towards the left.
Movement of a loudspeaker
The loudspeaker moves and pushes the air particles on its right (phase a) causing compression to take place. These compressions push the adjacent particles and transfer the energy they have received from the loudspeaker to these particles. The loudspeaker then moves back again and carries out a compression in the opposite direction, in other words a dilation towards the left takes place (phase b) and in doing so forms a dip that gets filled up by the air particles in its immediate proximity. These moving particles themselves create other dips to their right, and so on. This procedure allows the particles to pass on their energy by oscillating without physically moving in the direction the sound is diffused in. An example that illustrates this could be a cork floating in a pool of water that a stone has been flung into. The cork will oscillate up and down as the wave generated by the stone diffuses itself in the water, but it will remain immobile in respect to the direction of the wave's diffusion. If the loudspeaker is piloted by a sinusoidal signal, the atmospheric pressure in its immediate proximity shall flow as described in the following figure:
Sinusoidal flow of the atmospheric pressure
Related topics on Wikipedia
Compression (C= compression) and dilation (R= rarefaction) of particles in the air