A Quasi-stationary filter

This filter is also known as the first order series network and it consist of the circuit illustrated below.
Loudspeakers coupled in series
The operating principle of this circuit is that the audio signal must be transmitted either through the tweeter or through the bass/midrange driver, since these are the only resistive components in the signal path

Consider the case where a signal enters from the + terminal:

The signal will travel to the "- terminal" either via the capacitor and the tweeter or via the bass/midrange and the coil.
Low frequency signals choose the path with the coil and the bass in order to avoid the capacitor.
High frequency signals chooses the path through the tweeter and the capacitor in order to avoid the coil.
The same simple analogy can be used if the signal enters from the “– terminal”.

The filter is quasi-stationary because it depends strongly on the electrical impedance of the loudspeaker units, which is strongly frequency dependant. In that sense, this filtering scenario is not stationary but adaptive; since the gain will always be unity, although the filter may be overdamped/underdamped.

An elaborative explanation of quasi-stationary filters can be found at s_xovers.htm.