why is y less amp than x in this reading from an ultrasound's oscilloscope?

Looking at this exercise..

I can understand the question and answer of the exercise, so I get that with the ultrasound transducer in position 1, we get two readings, one marked T, which the device where it makes contact with the surface nearest to it - the top surface. And one marked X where it makes contact with the next surface(the bottom surface). And I get that the ultrasound wave travels there and back.

And I get that with the ultrasound transducer in position 2 , there are three readings. T,Y,X (So, that additional reading Y, in between T and X)

And I understand that the graph has time on the x axis and so I see the order there T,Y,X for position 2. And T,X for position 1.

But I don't understand the amplitudes/heights

I get that the amplitude is the strength of the signal.. and I can guess that all other things being equal, a further thing it hits is going to lead it to detect a weaker signal. I notice that Y is nearer. Why does hitting the crack "Y" lead to a weaker signal than hitting the bottom surface X?

Also I think the amplitude of X should not be the same in position 1 as in position 2. Because in the case of position 1, the ultrasound wave didn't have to travel through the crack to reach that bottom surface. Whereas in position 2 the ultrasound had to travel through the crack twice, (once to reach the bottom surface, and once to reflect back from the bottom surface), and i'd have thought that'd have some effect (strengthening or weakening i don't know), but some effect, on the strength of the signal

(edited 3 years ago)

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