Which of the following can occur when an ultrasonic beam reaches the interface of two dissimilar materials?

• A. Reflection
• B. Refraction
• C. Mode conversion
• D. All of the above

Answer: (D)

When an ultrasonic beam hits the boundary between two different materials, the mismatch in their acoustic properties (speed of sound and impedance) forces the energy to split. Part of the energy reflects back into the first material, creating a reflected wave. Part of it transmits into the second material but changes direction and speed, giving a refracted (transmitted) wave. If the incidence is not perpendicular to the interface, the boundary can also couple different wave types, so a P-wave (longitudinal) can generate an S-wave (shear) in the other material, or a reflected P-wave can convert to an S-wave, and vice versa. Because all of these effects can occur under the right conditions, you can observe reflection, refraction, and mode conversion at an interface between dissimilar materials.