-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.
- A probe is swept from left to right
- When the edge of the beam encounters the indication, a signal appears on the A-Scan
- When the beam is completely over the indication, the echo is at its maximum amplitude
- 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
- 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.