Vet Detective · Ultrasound physics teaching guide

Acoustic Impedance at an Interface

At normal incidence, a larger impedance mismatch reflects a larger fraction of the incident intensity.

Animation paused. Liver and kidney selected.

CLINICAL SCENARIOS:
Incident packet
Reflected packet
Transmitted packet
Hollow gold rings mark reflections too small to draw to scale.
Acoustic impedance: Z = ρc. It combines a medium's density (ρ) and sound speed (c). At an ideal perpendicular boundary, the relative difference between Z₁ and Z₂ determines the calculated fraction of incident intensity reflected from that boundary.
⬚ Two-media model · Normal incidence Intensity reflection coefficient: R = ((Z₂ − Z₁) ÷ (Z₂ + Z₁))²
Your browser does not support this teaching diagram. The written explanation below describes the calculated reflected and transmitted intensity fractions.
Calculated intensity split at one ideal interface
Z₁ (top layer)
1.65 MRayl
Liver
Z₂ (bottom layer)
1.62 MRayl
Kidney
Reflected (R)
0.01%
Intensity ratio: −40 dB
Calculated reflection
VERY SMALL
Most intensity crosses this boundary
Compare the two media. Choose a pair and watch the incident packet reach the interface. In this ideal normal-incidence model, a larger difference between Z₁ and Z₂ produces a larger reflected intensity fraction and a smaller transmitted fraction. The percentage panels show the calculated values; hollow gold rings keep very small reflections visible without claiming extra energy.
Clinical interpretation. Coupling gel displaces air between the transducer and skin, allowing sound to enter the patient. Gas can strongly limit deeper imaging and may produce reverberation or dirty shadowing. Bone and mineral can form an echogenic surface with posterior shadowing. Do not read this single-interface bar as total posterior attenuation: absorption, scattering, repeated reflection, frequency and path length also affect the returning signal.