The data at the left was collected with the lab stand and the tube maximally separated. This results in two strong echoes: one starting at around 150 cm and one starting at around 250 cm. The data at the right was collected with the tube much closer to the lab stand. The tube no longer returns a separated echo. The echoes from the lab stand and the tube have fused into a single echo.

The minimal distance at which the echoes are separable lies somewhere between the two extremes shown below.