By Zencalcy Editorial TeamLast reviewed About us
Characteristic acoustic impedance is the product of density and sound speed. It sets how much of a wave reflects at a boundary between two media: a large mismatch reflects strongly, while a close match transmits efficiently. Ultrasound gel exists to remove the air gap that would otherwise reflect almost everything.
Characteristic impedance from density and sound speed.
Z = ρ c
ρ is density in kg/m³, c the sound speed in m/s, and Z the characteristic impedance in Pa·s/m, also called the rayl. The reflection coefficient at a boundary is (Z₂ − Z₁)/(Z₂ + Z₁).
| Symbol | Meaning |
|---|---|
| Z | Characteristic acoustic impedance. |
| ρ | Density of the medium. |
| c | Speed of sound in the medium. |
With ρ = 1.204 kg/m³ and c = 343 m/s, Z = 413 rayl. Water is about 1.48 Mrayl, so the air-water reflection coefficient is close to 0.9995, which is why so little sound crosses the surface.
Continue with the Beat Frequency Calculator, Decibel Addition Calculator, or the Doppler Effect Calculator — or browse more acoustics and sound tools and all physics calculators.
One rayl is one pascal-second per metre, the SI unit of characteristic impedance. It is named after Lord Rayleigh and is numerically equal to kg/(m²·s).
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026