text: In the case that the fluid is irrotational, that is , we can then write  and thus write our equations of motion as
  The second equation tells us that
  And the use of this equation in the continuity equation tells us that
  This simplifies to
  Thus the velocity potential  obeys the wave equation in the limit of small disturbances. The boundary conditions required to solve for the potential come from the fact that the velocity of the fluid must be 0 normal to the fixed surfaces of the system.
  Taking the time derivative of this wave equation and multiplying all sides by the unperturbed density, and then using the fact that  tells us that
  Similarly, we saw that . Thus we can multiply the above equation appropriately and see that
  Thus, the velocity potential, pressure, and density all obey the wave equation. Moreover, we only need to solve one such equation to determine all other three. In particular, we have