In ultrasonic testing, the acoustic impedance of a material primarily determines which of the following?

• A. The velocity of sound in the material
• B. The density of the material
• C. The ratio of transmitted to reflected energy at an interface
• D. The attenuation of the wave within the material

Answer: (C)

The ability of energy to split at an interface is governed by impedance mismatch. Acoustic impedance, Z, is ρc (density times wave speed), and when a wave hits the boundary between two materials, the boundary conditions require pressure and particle velocity to be continuous. The result is that the reflection and transmission of energy depend on the impedances of both media, Z1 and Z2. The larger the difference between Z1 and Z2, the more energy is reflected and less is transmitted; if the impedances match, almost all energy transmits with minimal reflection. This is why the ratio of transmitted to reflected energy is determined by impedance contrast.

Density or velocity alone don’t directly set that energy split, since they are only pieces of the impedance; attenuation within a material is a separate effect due to absorption and scattering, not the boundary energy division.