The factor determining the amount of reflection at the interface of two dissimilar materials is:

• A. The acoustic impedance
• B. The index of rarefaction
• C. The frequency of the ultrasonic wave
• D. Young's modulus

Answer: (A)

The amount of reflection at a boundary between two different materials is governed by the acoustic impedance mismatch. Acoustic impedance is Z = ρc, the product of the material’s density (ρ) and the wave speed (c) in that material. When an ultrasonic wave encounters the interface, part of its energy reflects and part transmits, and the proportion reflected depends on how different the impedances are. The amplitude reflection coefficient is (Z2 − Z1)/(Z2 + Z1); a larger difference yields a stronger reflected signal, while equal impedances produce almost no reflection. The frequency of the wave doesn’t determine the reflection amount at a simple planar interface; it mainly affects wavelength and resolution. Young’s modulus shapes wave speed, but the key factor is the impedance difference, which already combines density and speed. The index of rarefaction isn’t a factor in this context.