Yea the rings are scaled in dB, but the plot does not show at WHICH places we have those 6dB reduction and so on, What's the reason of that? If we do not have any info about the locations where the output decrases by every 6dB then showing those dB's in the plot seems to be useless, unless every 6dB is for doubling the distance and so?!
the second point which makes me confused is that I think the ultrasonic sensor should be placed at the center of the plot, so it has the max of power at the center and the minimum at the outer ring, but What I see in the plot is completely in oppose to what I think, i.e the outer ring has 0dB and the iner ring has -30dB???!! So does the imaginary sensor is placed at the outer ring??