1.0 What is sound, physically?

Sound is what happens when something in the world moves and disturbs the air around it. Pluck a guitar string, snap your fingers, or blow air through a reed, and you set the surrounding air molecules into motion. These disturbances propagate outward as alternating regions of higher and lower pressure (compressions and rarefactions) that we call sound waves.

Sound propagates in all directions in three dimensional space. However, a microphone, or your eardrum, sits at one fixed point in this traveling pressure field. If we measure the local air pressure at that point as a function of time, we get a one-dimensional signal. In this signal, pressure goes up, pressure goes down, and pressure passes through ambient atmospheric pressure on its way between the two. We call this measurement analog sound.

The animation below shows this in motion. A pressure pattern travels to the right, and a microphone at one fixed point reads the passing pressure over time, drawn at the bottom as the red signal \(x(t)\).