Measurement of the frequency dependence of the ultrasonic reflection coefficient from thin interface layers and partially contacting interfaces

Reflection coefficient measurements are an important method of characterising thin interface layers and partially contacting interfaces. Often such measurements are simply derived from time domain measurements in which the amplitude of the reflection from the interface is monitored. More information is available in the frequency domain although this is often not utilised. In this paper, examples are presented where apparently reasonable time domain results did not show the expected behaviour in the frequency domain. If the frequency domain information had not been considered, incorrect conclusions would have been drawn from the experiments. In each case, the problem has been traced to subtle flaws in the design of the experiments. These problems are discussed and ways of overcoming them and obtaining high quality results in both the time and frequency domains are proposed. (C) 1997 Elsevier Science B.V.