Variables Affecting Observed Backwall Signal
Reduced available energy to strike tube outside energy.
Reduced by amount returned as the frontwall signal
Reduced by amount returned by the sleeve tube interface signal
(weld aperture is less than focal spot).
Reduced by the dispersion expected after the sound passes
through its focal spot.
Minimal energy returned
The reflected sound (energy) must be perfectly perpendicular
to the tube outside diameter to return back through the weld aperture.
The backwalls signal's additional travel distance causes
additional energy dispersion.
The differing weld grain orientation may reflect a portion
of the returning signal back into the tube rather than letting it travel
back through the weld.
Tube outside diameter deposits
Absorbs the UT sound
Rougher outside diameter surface deflects more of the return
energy away from the weld aperture.
Increases the travel path with associated additional energy