3. (a) What must be the thickness of, and the speed of sound in, a plastic layer having a density of 1500 kg/m3 if it is to transmit plane waves at 20 kHz from water into steel with no reflection? (b) What would be the intensity reflection coefficient back into water for normally incident waves impinging on an infinitely thick layer of this plastic? The density of steel is 7850 kg/m3, and sound speed in steel is 3150 m/s. The density of water is 1000 kg/m3, and sound speed in water is 1500 m/s