Acoustic Impedance
Also known as: Characteristic Acoustic Impedance · Specific Acoustic Impedance
Impedance measures how hard a medium pushes back when sound tries to move it: dense, stiff media (high rho, high c) resist more. When a wave meets a different impedance, part of it reflects — the bigger the mismatch, the more bounces back.
A pressure wave hits a boundary between two media; the reflected and transmitted amplitudes update live from the impedance mismatch R = ((Z2-Z1)/(Z2+Z1))^2.
Equivalent forms
Two bulk properties — density and sound speed — multiply into the one number that decides whether sound passes a boundary or bounces off it.
Unit systems
- SI:
- Z in (rayl); rho in kg/m^3, c in m/s
- CGS:
- Z in rayl
- Imperial:
- rarely used
Where it holds
Rayleigh's 'Theory of Sound' introduced the impedance concept to acoustics by analogy with electrical and mechanical systems, giving a single number that predicts transmission and reflection at every interface.
Why does ultrasound need gel on your skin to see inside you?
Sound reflects at boundaries where the acoustic impedance Z = rho*c jumps. Air-to-skin is a huge mismatch — almost all the ultrasound would bounce off — so gel bridges the gap and lets the wave in.