It is variations of pressure which is what affects our ears - i.e. what our ears physically respond to. A sound wave consists of pressures above and below the normal undisturbed pressure in the gas.
Static pressure is the normal atmospheric pressure in the absence of sound.
The unit is the dyne per square centimetre. The maximum variation of pressure above or below its normal value is sometimes called the pressure amplitude . The sound pressure in a spherical sound wave falls off inversely as the distance from the sound source. The passage of a sound wave passing through a gas medium produces a displacement of the particles or molecules of gas from their normal positions, i.e. their positions in the absence of the wave in question. The particle displacement of the medium through which the sound waves of speech and music pass is a very small fraction of a millimetre.