Impedance, Reflection & Gel
A scanner needs some reflection to reveal an internal boundary, but it also needs enough of the pulse to enter the body and continue to deeper structures. Acoustic impedance controls this balance.
Impedance belongs to the medium
Definition
2 marksWhat is meant by specific acoustic impedance?
Model answer: The product of the density of a medium and the speed of sound in that medium.
Symbols
- = specific acoustic impedance (kg m⁻² s⁻¹)
- = density of the medium (kg m⁻³)
- = sound speed in the medium (m s⁻¹)
The definition must include both quantities. “Resistance to sound” is not the mark-scheme definition (9702/42/M/J/24 Q10(b)(i)).
A larger mismatch gives a stronger echo
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At normal incidence, compare the two impedances using the intensity reflection coefficient.
Symbols
- = fraction of incident intensity reflected (—)
- = specific acoustic impedances (kg m⁻² s⁻¹)
- Equal impedances give zero reflection in this model.
- A larger difference between the impedances gives more reflection.
- Air and skin have a very large mismatch. Coupling gel replaces the air gap, so more ultrasound enters the body (9702/41/M/J/23 Q8(c)).
Worked example
Water-glass boundary
The acoustic impedances are 1.42 × 106 and 11.4 × 106 kg m−2 s−1.
This is the verified result in the 2024 paper (9702/42/M/J/24 Q10(b)(ii)).
Two tissues have almost equal acoustic impedances. Predict the echo intensity at their boundary and explain your answer.
Show worked answer
The echo is weak because the impedance mismatch is small, so only a small fraction of the incident intensity is reflected.
Echo delay answers “how deep?”. Impedance mismatch answers “how strong is the reflection?”. Do not treat them as the same measurement.