We live at the bottom of a sea of air.* As with the ocean, the farther down one goes, the greater the density of molecules and the greater the pressure it exerts on us. The average atmospheric pressure, as it is called, is measured in many different units, but the one most people are familiar with is barometric pressure (measured in millibars). Altitude, temperature and weather conditions affect barometric pressure (low barometric pressure, for example, is often associated with storm systems). If the micro-pressure level surrounding us remains steady or changes very slowly, we experience silence. Even so, individual air molecules themselves always exhibit at least a minimum rate of motion without causing perceptible pressure changes.
Sound is produced by a rapid variation in the average density or pressure of air molecules above and below the current atmospheric pressure. We perceive sound as these pressure fluctuations cause our eardrums to vibrate. When discussing sound, these usually minute changes in atmospheric pressure are referred to as sound pressure and the fluctuations in pressure as sound waves, which are a form of pressure waves. Sound waves are produced by a vibrating body, be it an oboe reed, loudspeaker cone or jet engine. The vibrating sound source causes a disturbance to the surrounding air molecules, causing them to bounce off each other with a force proportional to the disturbance.
Fantasy speaker cone cut-away with fantasy air molecules
The energy of their interaction creates ripples of more dense (higher pressure) to less dense (lower pressure) air molecules, that equates to pressures above and below the normal atmospheric pressure or state of equilibrium. When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound wave.
The average distance air molecules have to travel before striking one another is called the mean free path. As you will see later on, this distance is dependent on air pressure and temperature and therefore correlates to the speed of sound. The farther a molecule has to travel before striking another, the slower the speed at which sound propagates. It is estimated the mean free path of air at sea level and room temperature is between 34 and and 65 nanometers (a nanometer is a billionth of a meter), so these molecules don't have very far to travel before colliding with another.
The threshold of human hearing, or the softest perceptible sound, corresponds to a pressure variation of less than a billionth of the current atmospheric pressure (though the threshold of hearing varies with frequency).