Acoustic Impedance

The concept of acoustic impedance is useful for a materials for calculating reflection and transmission coefficients for sound or pressure waves at material interfaces. It can be defined macroscopically for a given medium geometry as Z=p/U where p=acoustic pressure and U = acoustic volume flow, or defined as an intensive property of a medium, where z = specific acoustic impedance:

This treatment will be limited to the specific acoustic impedance. See the references below for a more general treatment. For a sound wave, the particle displacement y as a function of time is of the form

and the associated velocity v and pressure p may be written

where B is the adiabatic bulk modulus of the material. Substituting into the defining relationship for z gives

which reduces to

This is a convenient form for calculating reflection and transmission coefficients for sound waves.

References:

Acoustic impedance, intensity and power

The wave equation for sound