The experimental setup for pulse-echo mode is illustrated in Figure 2. Pulse-echo mode can transmit and receive signals with an ultrasonic transducer. A JSR PR-002 pulse generator was used to produce the electrical impulses to drive the ultrasonic transducers. In our study, since the echo signals received from the concrete are located within the transmitting signals, back wall echo signals could be observed by subtracting air by water samples. In addition, two different sets of water sample data were acquired and the difference was recorded for later comparison.
T is the total time duration of the ultrasonic signal.
The spectrum of the received echo was obtained by performing Discrete Fast Fourier Transform. Zero padding of the signal in the time domain was used before the SSP to increase frequency resolution. Filter Figure 3. Filtering scheme of split spectrum processing The procedure of SSP splits the spectrum into different frequency bands as shown in Fig.3 . Theoretically, a time limited signal produces an infinite bandwidth.