How to calculate the time delay created by a tube.

Hi,

I am trying to model a pressure tube of length 'L', where a fan is
attached at one end and the other end is open.
I know that Pressure  in the tube will vary w.r.to space and time when
there is some change in excitation at the input. I got the pressure
drop equation ,

[Pressure drop] dP = 4 f dx V^2 /(gD)

f - friction created by the walls of the tube.
dx - distance.
V - velocity of air flow.
D- diameter of the tube.
g- gravity

Using the above equation, I can calculate pressure drop at any distance
(dp/dx) directly. But how can I  get the time delay created by the
tube.  Please some one guide or teach me how to derive P(x,t).

Thanks
srinivas