In this technique two pulse are sent into the sample. The first signal is delayed such that the first echo wave of the first signal over laps the second echo wave of the second signal. The time difference between the two waves tells how long it took for the both waves to travel through the sample. The interference minima is the lower peeks of the sinewaves that are overlapped. These points are the data that are used to calculate the roundtrip travel time.
Good data is obtained through the sharpness of the interference maxima or minima.
Predetermining how good your signal will be before you run and experiment is virtually impossible. The best one could do is make sure that the buffer rod, sample and backing do not chemically react with one another. This applet allows you to visually predetermine which combination of materials will give you a good signal for overlapping. Obviously, the bigger the signal the better quality of data you will receive. By entering the impedance of each material this applet will calculate and apply the amplitude information to a sinwave.