Choice of an analogy
The usual representations are dependent on the available measurement facilities: it is easy to estimate a mechanical displacement or an electrical voltage, without modification of the systems.
Because of this, the common representations associate a degree of freedom with an easy to measure quantity :
The speed of a mass for a mechanical system.
The voltage drop across a conductor in an electrical system.
The most natural analogy of a mechanical system is therefore the indirect analogy. However, it is more common to see equivalent networks represented using the direct analogy, mostly to avoid mixing the two analogies when coupling it to an acoustical system (which is naturally represented using the direct approach).
