My friend is working on his thesis which is to measure concrete density by using ultrasonic wave. The wave's frequency is 50 kHz. He calculate the density of the concrete by measuring how fast the wave propagate through the concrete. So, he emmits the wave on one side of the concrete and a receiver receive the wave on the other side of the concrete. The problem is the wave cannot penetrate the concrete. Can really a ultrasonic wave penetrate a concrete? How strong (in voltage) should the wave be to penetrate?