Why curve is non-linear

A problem of defects propagation along the full vertical extent of the object is considered. The time intervals of echo-signal involute run with the 6 dB level are measured experimentally depending on depth of position of the cylindrical reflector for some inclined transducers. The experimental measurements are executed on the carbon steel with the fine-grained structure. The 'hump' is being observed in zone with the possible unaccounted effect of damping. It is assumed that the deviation from linearity is induced by insufficient value of arrow of the inclined transducer in object of control. The assumption is checked experimentally.