This method is used for sizing large reflectors. If the probe is moved until the signal amplitude from a reflector drops to half its original screen height, then it can be said that the sound beam is half on and half off the reflector. So by moving the probe until the signal from the end of a large reflector halves in height, we can thus estimate that the edge of the reflector is immediately below the center of the probe.

This method is called the 6dB drop sizing method because the amplitude of the signal drops by half which corresponds to 6dB when the probe is moved to the edge of a large reflector.

Note : The peak of the reflector is normally taken as being the last peak on the screen before the probe goes off the end of the reflector, not necessarily the maximum signal from the reflector.