At normal incidence, is the reflection strong or weak when the incident medium impedance is 1.5 MRayl and the second medium impedance is 1.6 MRayl?

• A. Weak
• B. Strong
• C. None
• D. Moderate

Answer: (A)

At normal incidence, how much of the ultrasound is reflected depends on how closely the two media’ impedances match. A small difference in impedance means most of the wave’s energy continues into the next medium, so the reflection is weak.

Here, Z1 = 1.5 MRayl and Z2 = 1.6 MRayl, a difference of 0.1 MRayl. The amplitude reflection coefficient is (Z2 − Z1)/(Z2 + Z1) = 0.1/3.1 ≈ 0.032, so about 3% of the incident pressure is reflected. The reflected energy is the square of that, roughly 0.001 (about 0.1%). So the boundary is nearly matched and the reflection is weak.