Sound waves in solids or liquids are best viewed as propagating longitudinal disturbances in the density of the material. For sound in a gas, it is easiest to visualize a propagating longitudinal disturbance in the gas pressure.

The particle displacement function *s(x,t)*
and the pressure variation function *Δp(x,t)* are 180
degrees out of phase, so that a zero in one corresponds to a
maximum or minimum in the other. In doing Quest problems, note
carefully which function is given, or which function is
sketched!

If we write *s(x,t) = s _{m} cos[kx
-ωt],* then we get