Acoustic impedance

Acoustic impedance (also known as characteristic acoustic impedance) is the property of a medium that describes how easily an acoustic wave can form within the medium.

Different media have different acoustic impedances.

Acoustic impedance is a measurable characteristic of a medium and is determined by the product of the medium's density and the sound's propagation speed.

The unit for acoustic impedance is the rayl

The symbol for acoustic impedance is Z

The important thing about acoustic impedance for sonographers is that it is the mismatch in acoustic impedance at the interface between two media that results in the formation of an echo.  

The adjacent video describes acoustic impedance and the importance of a mismatch in acoustic impedance for the formation of echoes.  

The related topics of impedance matching and matching layers can be found here.