Ultrasonic transmission coefficient

Fig. 25.17 Calculated phase of the transmitted wave of fundamental frequency as a function of the ultrasonic transmission coefficient of the interface in the range from 0 (complete delamination) to 1 (perfect bond, i.e., complete transfer of ultrasound) ...

The impedance is practically important because it determines the proportion of an ultrasonic wave which is reflected from a boundary between materials. When a plane ultrasonic wave is incident on a plane interface between two materials of different acoustic impedance it is partly reflected and partly transmitted (Figure 3). The ratios of the amplitudes of the transmitted (At) and reflected (Ar) waves to that of the incident wave (Aj) are called the transmission (T) and reflection coefficients (R), respectively. 

The greater the difference in acoustic impedance between the two materials the greater the fraction of ultrasound reflected. This has important consequences for the design and interpretation of ultrasonic experiments. For example, to optimize the transmission of ultrasound from one material to another it is necessary to chose two materials with similar acoustic impedance. To optimize the reflection coefficient materials with very different acoustic impedance should be used. The acoustic impedance of a material is often determined by measuring the fraction of ultrasound reflected from its surface. 

The subscripts 1 and 2 refer to the material the wave travels in and the material that is reflected by or transmitted into, respectively. These equations show that the maximum transmission of ultrasound occurs when the impedances and Z2 of the two materials are identical. The materials are then said to be acoustically matched. If the materials have very different impedances, then most of the US is reflected. The reflection and transmission of ultrasound at boundaries has important implications on the design of ultrasonic experiments and the interpretation of their results. In addition, measurements of the reflection coefficient are often used to calculate the impedance of a material. 

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