The behavior of a sound pulse when it strikes a boundary is determined by which of the following?

• A. The wave's frequency
• B. The tissue's density
• C. The angle at which the wave strikes the boundary
• D. The pulse amplitude

Answer: (C)

The angle at which the wave hits the boundary largely determines how the pulse behaves there. When a sound pulse meets a boundary, part of the energy reflects and part transmits into the next medium, and the directions of those resulting waves are set by the incidence angle. The reflected wave travels off at the same angle relative to the boundary’s normal, and the transmitted (refracted) wave bends according to the relative speeds of sound in the two media (Snell’s law). This directional behavior depends on the geometry of the encounter more than on frequency or amplitude. The amount of energy that reflects versus transmits is influenced by the impedance difference between tissues (related to density and sound speed), which determines echo strength, but it doesn’t change the fact that the directions are governed by how the wave strikes the boundary.