Similar to displacements of the particles, the pressure variations in the medium
travel to the right. Sound waves can be represented either by the
displacement of the particles or by the pressure variations in the medium.
Pressure variation of sound waves
At this region, the sound wave separates the air particles apart. The
density is lower than the equilibrium density and, consequently, the
pressure is lower than the equilibrium pressure. Such a low-pressure
region is called a rarefaction.
At this region, the sound wave brings the air particles closer. The
density and the pressure are more than their equilibrium values. Such a
compressed region is called a condensation.
The pressure variation is the pressure measured relative to the pressure of
undisturbed air. For example, Dp(x,t) = 0.1 Pa
indicates that the pressure is 0.1 Pa above the equilibrium pressure and
Dp(x,t) = 0 means that the pressure is equal to the
equilibrium pressure. Usually, the equilibrium pressure is equal to the