A study of the transmission of ultrasound across solid–rubber interfaces

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Abstract
The reflection coefficient at the solid–rubber interface of a rubber coupled ultrasonic transducer may be used as a measure of its coupling performance; the lower the solid–rubber reflection coefficient, the better the coupling. An experimental and theoretical study of the transmission of ultrasound across dry coupled solid–rubber interfaces, and in particular the effect of surface roughness and particulate contaminants on solid coupling, is described. The effect of surface roughness is modeled using a numerical contact model of the solid–rubber interface from which the static stiffness is calculated. The calculated interfacial stiffness is then used in a spring model of the interface to predict the solid–rubber reflection coefficient behavior. By repeating this process for different loads applied to the solid–rubber interface the variation of reflection coefficient with interfacial pressure is found. The agreement between these predictions and the experiments is shown to be reasonable in general and good at higher pressures. The numerical contact model is then used to predict the pressure required to achieve a given level of coupling on a variety of surfaces. This modeling procedure can now be used to provide the operator of a dry coupled system with an estimate of the pressure which must be applied to a dry coupled transducer on a given system to achieve the required coupling. The effect of dust and dirt on the transmission of ultrasound across solid–rubber interfaces has been considered experimentally. These experiments showed that particulate contaminants at a solid coupled interface significantly reduce the degree of coupling and this effect is quantified.
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