Re: Ultrasonic Testing : Could someone please explain in detail the 6 dB drop method? . The 6dB drop method has been used for flaw sizing for many years. However it has been proven that it is not very accurate for the sizing of reflectors, which are smaller than the width of the beam at the sound path where the reflector is located, unless beam spread measurements are taken and compensated for.
For example, if you have a reflector response at 80% full screen height (FSH) and you move the search unit until the reflector drops to 40% FSH, you have reduced the signal by 50% or 6dB. There is a formula to calculate this. ((A1/A2 Log) x 20).
Theoretically, if a reflector is larger than the beam width and you move the probe until the signal is reduced by half (-6dB), that will give you the end points of the reflector.