Acoustic Impedance
Definition
How much a material resists the passage of sound — the reason echoes form where two materials meet.
Symbol: · Unit: rayl (Pa·s/m)
Technical definition
Defined as Z = ρc (density × sound speed), in rayls (Pa·s/m). When sound crosses a boundary between different impedances, part reflects and part transmits — the greater the mismatch, the stronger the echo.
Example
Ultrasound gel is used because air's impedance mismatches skin so badly that almost all the sound would reflect — the gel bridges the gap so waves enter the body.
Common confusion
Acoustic impedance has nothing to do with electrical impedance despite the shared name — it is a mechanical property (density × sound speed), not resistance to electric current.