Specific acoustic input impedance

The property of the interface which controls the relative amounts reflected and transmitted is termed the specific acoustic input impedance Z which is defined as the ratio of the acoustic pressure to the particle velocity associated with the wave motion at the boundary. Note that this definition is similar to that used previously in defining the specific impedance of the media z. The use of the term impedance for both input impedance at the boundary and specific impedance of the media occasionally leads to some confusion if care is not taken. 

Considering perpendicular sound incidence, the specific acoustic input impedance Z2 at the boundary is simply... 

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