The point of conventions
Symetrical conventions
It is used for the analysis of the energy conversion between different domains (electric
mechanic and mechanic
acoustic).
This convention shows that the total power injected into the quadripole is null.
Asymmetrical convention
For mechanical or acoustical systems, for which a source is used before the quadripole, and a receiver after, the asymmetrial convention is preferred to show the continuity of speed or flow.
Exemple : Acoustic transmission line
The schematic on the left shows a real acoustic system, and the one on the right the equivalent electrical circuit at low frequency. It uses three quadripoles under the asymmetrical convention.
