12:56 May 23, 2010 

Although the phrase you have posted has no correspondence in the associated text, the idea is that the reflection, or echo, of the ultrasonic pulse from the bottom (or presumably the inside diameter of the pipe) can be considered equivalent to a flat plate or boundary (reflector) only if the diameter of curvature (more usually referred to as radius of curvature, with the appropriate factor of 2) exceeds 3.7N, where N is the length of the near field.
The equation is rather difficult to follow, especially as the "-" sign does not here mean minus but is simply a punctuation mark. I suspect D2 is meant to be D^2, and the ":" is meant to be "/". This then gives a unit of length, as it should be, ie. D^2.F/(4C).
This is a very challenging piece for a non-native, non-technical translator!