Acoustic impedance measures how much a material resists sound passing through it. When sound meets a boundary between materials with very different impedances – air and skin, water and steel – most of it reflects. That is why ultrasound scans need gel between the probe and the skin.
How to use the Acoustic Impedance Calculator
- Material 1
- Density ρ₁
- Speed of sound c₁
- Material 2
- Density ρ₂
- Speed of sound c₂
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose both materials from the list or enter a custom density and speed of sound. Impedance is shown in MRayl (10⁶ kg/(m²·s)).
Inputs explained
| Field | What to enter |
|---|---|
| Material 1 | Choose one: Air, Fresh water, Seawater, Blood … |
| Density ρ₁ | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
| Speed of sound c₁ | Units: m/s, km/h, mph, ft/s, knots |
| Material 2 | Choose one: Air, Fresh water, Seawater, Blood … |
| Density ρ₂ | Units: kg/m³, g/cm³, g/mL, lb/ft³, lb/in³ |
| Speed of sound c₂ | Units: m/s, km/h, mph, ft/s, knots |
What each result means
- Intensity reflection coefficient = (([Impedance of material 2] – [Impedance of material 1])÷([Impedance of material 2] + [Impedance of material 1]))^2×100
- Intensity transmission coefficient = 4×[Impedance of material 1]×[Impedance of material 2]÷([Impedance of material 1] + [Impedance of material 2])^2×100
- Pressure reflection coefficient = ([Impedance of material 2] – [Impedance of material 1])÷([Impedance of material 2] + [Impedance of material 1])
Example values
How the main result changes when Density ρ₁ changes and everything else stays at the example values above:
| Density ρ₁ (kg/m³) | Intensity reflection coefficient (%) |
|---|---|
| 500 | 0.4786 |
| 750 | 0.4786 |
| 1,000 | 0.4786 |
| 1,250 | 0.4786 |
| 1,500 | 0.4786 |
| 2,000 | 0.4786 |
Formula
Z = ρ·c, R = ((Z₂ − Z₁)/(Z₂ + Z₁))², T = 1 − R = 4Z₁Z₂/(Z₁ + Z₂)²
Worked example
Inputs — Material 1: Fresh water; Material 2: Muscle.
Results — Impedance of material 1 = 1.48 MRayl; Impedance of material 2 = 1.7 MRayl; Intensity reflection coefficient = 0.4786 %; Intensity transmission coefficient = 99.5214 %; Pressure reflection coefficient = 0.06918.
Key terms
- Characteristic impedance
- Product of density and sound speed.
Frequently asked questions
What is a rayl?
The unit of acoustic impedance, 1 kg/(m²·s).
Why is ultrasound gel needed?
Air between the probe and skin would reflect almost all of the sound.
Why can't you hear people underwater well?
Air and water impedances differ by a factor of about 3,600, so 99.9% of sound reflects.
What does a negative pressure reflection mean?
The reflected wave is inverted (going to a lower impedance).