Figure 1. Reflection and transmission of an ultrasonic wave from a boundary between two materials.

Solids usually have larger ultrasonic velocities and acoustic impedances, than liquids, which have larger values than gasses. Air has a very low acoustic impedance compared to liquids or solids which means that ultrasound is almost completely reflected from an interface between air and a condensed medium. This can be a problem when ultrasound is used to test dry materials, e.g., biscuits or egg shells. A small gap of air between an ultrasonic transducer and a sample can prevent ultrasound from being transmitted into the material. For this reason coupling materials (often aqueous or oil based) can be placed between the transducer and sample to eliminate the effects of the air gap, or alternatively ultrasonic transducers with soft-polymer faces can be used.

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