08 Ultrasound
Acoustic impedance
Z = ρ c of the medium. 1 Rayl = kg/(m²·s).
Listen
Listen · English
Simulation
Acoustic impedance — Change the numbers; the scene follows.
Where it works
Ultrasound
Tissue path
Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus.
Open this machineFormula
Typical values
Variables
Results
Z
Acoustic impedance
1.632e+6kg/(m²·s)
Z
Impedance
1.6324MRayl
Explanation
What it means
Acoustic impedance Z = ρ c is the ‘stiffness × inertia’ of a tissue for sound. Reflection at an interface is set by the Z mismatch, not by density or speed alone. Soft tissue ~1.63 MRayl, fat ~1.38, bone ~7.8, air ~0.0004 — hence bright bone echoes and total reflection at air. This is a working relation in Ultrasound.
Where it is used
Clinically it sits on the Ultrasound — Tissue path. Along the beam in tissue — impedance mismatch, Doppler shift, MI/TI at the focus. Ultrasound equations sit on the probe face and along the beam: impedance, Snell, Doppler, MI and TI. They explain why gel matters, why aliasing appears, and why a mechanical index is on the screen.
Ultrasound · Open this machineHow to use it
Presets load typical ρ and c. Pair two Z values in the reflection calculator. Matching gel exists to couple the probe (Z ~ tissue) and exclude air. Change one input and watch the curve and the simulation follow.
Symbols
- ρDensity1,060 kg/m³
- cSpeed of sound1,540 m/s
Worked example
A typical case from the default values: ρ = 1,060 kg/m³ (Density); c = 1,540 m/s (Speed of sound). Substituting into the relation gives Z = 1.632e+6 kg/(m²·s); Z = 1.6324 MRayl. These are teaching numbers — align them with your machine.
Typical values give
- Z = 1.632e+6kg/(m²·s)
- Z = 1.6324MRayl
Where it comes from
The displayed formula is the working relation. Z = ρ c of the medium. 1 Rayl = kg/(m²·s). Usual reference: Bushberg / Edelman. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Bushberg / Edelman
Assumptions & limits
Longitudinal wave, non-absorbing medium. Z is weakly frequency-dependent in real tissue; we treat it as constant.
Pitfalls
Soft-tissue 1540 m/s is an assumption — not a measurement in that patient. Doppler angle 90° gives no shift. MI and TI are on-screen estimates, not absorbed dose. Longitudinal wave, non-absorbing medium. Z is weakly frequency-dependent in real tissue; we treat it as constant.
Keep this
Gel, angle, and assumed speed of sound — get those three right before you trust a centimetre. Longitudinal wave, non-absorbing medium. Z is weakly frequency-dependent in real tissue; we treat it as constant.
In this specialty