Point A which is at unwelded zone shows amplitude about 82% as in this point beam is far away from welded zone and as the probe moves towards the welded zone amplitude drops gradually and when the probe is at point B part of ultrasonic beam falls on the welded
(bonded) side and other part falls on the unwelded side and when probe completely moves from the unwelded zone to the welded zone then the full beam falls on the welded zone designated by point C and amplitude drops to around zero and again when probe moves from welded zone to unwelded zone amplitude rise gradually as shown by point D and reach to point E where amplitude again becomes 80% of FSH. The 80% FSH is taken as the reference
intensity in decibel-drop technique.