As the regions travel through the tube, any small element of the medium moves with simple harmonic motion parallel to the direction of the wave. The harmonic position function is s (x, t) = smax cos (kx wt) smax is the maximum position of the element relative to equilibrium. This is also called the displacement amplitude of the wave. k is the wave number. Is the angular frequency of the wave. Note the displacement of the element is along x, in the direction of the sound wave. The variation in gas pressure, dp, is also periodic. It is the maximum change in pressure from the equilibrium value. k is the wave number. w is the angular frequency. The pressure can be related to the displacement. This relationship is given by dpmax = b smax k. B is the bulk modulus of the material. A sound wave may be considered either a displacement wave or a pressure wave.
