Table of Contents
What Is Acoustic Impedance?
Acoustic impedance measures how much resistance a medium offers to sound waves. It equals the product of density and sound speed. The SI unit is the Rayl (Pa·s/m), often expressed in MegaRayls. It determines how much sound reflects or transmits at material boundaries.
This property is critical in medical ultrasound imaging, audio speaker design, underwater acoustics, non-destructive testing, and architectural noise control. The impedance mismatch between materials creates the reflections that form ultrasound images.
Impedance Formula
Where Z is impedance in Rayls, rho is density in kg/m³, c is sound speed in m/s, and R is the intensity reflection coefficient at a boundary between two materials.
Acoustic Impedance of Common Materials
| Material | Density (kg/m³) | Speed (m/s) | Impedance (MRayl) |
|---|---|---|---|
| Air | 1.225 | 343 | 0.000420 |
| Water | 1,000 | 1,480 | 1.48 |
| Soft Tissue | 1,060 | 1,540 | 1.63 |
| Bone | 1,900 | 4,080 | 7.75 |
| Steel | 7,800 | 5,960 | 46.49 |
FAQ
Why is impedance important in ultrasound?
Ultrasound images are created by detecting reflections from tissue boundaries. The reflection amount depends on impedance mismatch. Large mismatches like tissue-bone or tissue-air produce strong reflections. Coupling gel eliminates the air gap between transducer and skin.
What is impedance matching?
Impedance matching designs systems so source and load impedance are equal, maximizing energy transfer. In audio, this means matching speakers to air impedance. In ultrasound transducers, matching layers transition between the high-impedance piezoelectric element and low-impedance tissue.
How does temperature affect impedance?
Temperature changes both density and sound speed. In gases, speed increases with temperature while density decreases, but speed dominates so impedance increases slightly. In liquids, sound speed in water increases from 1,402 m/s at 0C to 1,509 m/s at 35C.