Take into account that the amplitude of the wave (amplitude of the periodic curve) determines the energy of the wave.
Then, if some of the energy is aborved by the concrete, the amplitude of the reflected wave must be lower than the amplitud of the incident wave.
The previous situation is represented by the diagram C. There you can notice that the amplitude of the reflected wave is lower than the amplitude of the incident wave (related to the amplitud of the periodic function)
In the previous drawing the red line indicates the amplitude of a wave, to clarify the explanation above.