Sound is propagated in air, much like blowing up a large balloon, which expands equally in all directions.
For sound to be generated and heard it must have a source, a medium through which to pass and a receiver.
For purposes of this discussion we will assume that we are talking about normal speech communications. The source is the speaker’s voice, the medium through which it is transmitted is air and the receiver is the listeners ear.
As sound is generated by the speaker’s speech, the speakers voice acts like a diaphragm which causes the molecules in the air to pulsate back and forth while moving in all directions, at a speed of 1130 ft per second (770 mph).
A single segment of a sound wave may be characterized as pressure compressions and rarefactions.