A boundary with large impedance mismatch will cause most of the ultrasonic energy to be

A boundary with large impedance mismatch will cause most of the ultrasonic energy to be

• A. Most energy transmitted
• B. No energy reflected
• C. Most energy absorbed
• D. Most energy reflected

Answer: (D)

When an ultrasonic wave meets a boundary where the two materials have very different acoustic impedances, most of the energy is reflected back into the original medium. Acoustic impedance Z is density times wave speed, and the amount of energy reflected at normal incidence depends on the reflection coefficient R = (Z2 − Z1)/(Z2 + Z1). If the mismatch is large, Z2 differs greatly from Z1, so R is near ±1 and the reflected power (proportional to R^2) is nearly all of the energy. The transmitted portion into the second material becomes very small, though some energy can still be absorbed or attenuated within the second medium. A boundary like steel–air, with a huge impedance difference, is a classic case where almost all energy is reflected.

When an ultrasonic wave meets a boundary where the two materials have very different acoustic impedances, most of the energy is reflected back into the original medium. Acoustic impedance Z is density times wave speed, and the amount of energy reflected at normal incidence depends on the reflection coefficient R = (Z2 − Z1)/(Z2 + Z1). If the mismatch is large, Z2 differs greatly from Z1, so R is near ±1 and the reflected power (proportional to R^2) is nearly all of the energy. The transmitted portion into the second material becomes very small, though some energy can still be absorbed or attenuated within the second medium. A boundary like steel–air, with a huge impedance difference, is a classic case where almost all energy is reflected.