The refraction angle of longitudinal ultrasonic waves passing from water into a metallic material at angles other than normal to the interface is primarily a function of

• A. The relative velocities of sound in water and metal
• B. The impedance ratio of water to metal
• C. The density ratio of water to metal
• D. The frequency of the ultrasonic beam

Answer: (A)

Understand how acoustic refraction works at a boundary. When a longitudinal wave in water hits a metal at an angle, the direction of the transmitted wave in the metal is set by Snell’s law for acoustic waves. The key relationship uses the wave speeds in the two media: the transmitted angle inside the metal is determined by the ratio of the metal’s longitudinal sound speed to water’s sound speed. In practical terms, the angle of refraction is dictated by how fast sound travels in each medium, not by how stiff or dense the media are, nor by the wave’s frequency. The impedance or density influence how much energy is reflected versus transmitted, but they don’t set the refraction angle. Since the metal’s longitudinal speed is typically much greater than water’s, the refracted angle tends to be larger than the incident angle (the ray bends away from the normal), up to a critical angle beyond which refraction is not possible and total reflection occurs.