Length -6 dB Drop

Length -6 dB Drop

To measure the length of a defect, the -6 dB drop can be used.

1. The part is scanned for indications
2. When an indication is encountered, its signal is peaked and its amplitude recorded
3. The probe is moved sideways until the first -6 dB point is encountered. Its position is marked on the part and recorded
4. The probe is moved sideways in the opposite direction until the second -6 dB point is encountered. The position is marked on the part and recorded
5. The length of the indication is the measured distance between the two -6 dB points

Since the indication can be of irregular shape, it is important to measure the length where it is at its widest.