Meaning of a -6dB drop

-6 dB drop

In ultrasound, the -6 dB drop is widely used to measure the length or width of an indication. A -6 dB drop correspond to half the original amplitude.

1. A probe is swept from left to right
2. When the edge of the beam encounters the indication, a signal appears on the A-Scan
3. When the beam is completely over the indication, the echo is at its maximum amplitude
4. When half the beam is outside the indication and half is over, that is when the -6 dB drop or half the maximum amplitude is obtained
5. The distance between the two -6 dB positions gives the width of the indication

This method for measuring the width or length of a defect implies that the indication is planar. Since an indication is rarely perfectly planar, the measurement is thus an approximation.