The acoustic impedance of a material is used to determine which of the following?

• A. Angle of refraction at an interface
• B. Attenuation within the material
• C. Beam spread within the material
• D. Relative amounts of sound energy coupled through and reflected at an interface

Answer: (D)

Acoustic impedance tells you how a boundary will split the energy of an ultrasound pulse between reflection and transmission. Impedance, Z, is the product of density and sound speed (Z = ρc). When the wave hits an interface, the amount of energy that reflects versus transmits depends on how different the two impedances are. A big mismatch causes most of the energy to reflect; a small mismatch allows most to transmit. This is why impedance is used to determine the relative amounts of energy coupled through and reflected at an interface.

The other options don’t directly describe energy partition at a boundary: the angle of refraction is set by the speeds in the two media (Snell’s law), attenuation is about absorption within the material, and beam spread is influenced by focusing and diffraction rather than impedance mismatch.