I've found it always useful to look at a behavioral physical model of speaker rather than at its direct electrical network equivalent.
The Force -- Velocity Speaker model:
- The speaker driver's motion is steered (but not controlled) by the force the voice coils exerts on the cone, and the force is proportional to electrical current. This is the actuator. There are other forces at work, too (spring force).
- The drivers voice coil always produces an (open load) voltage proportional to cone velocity, no matter what caused the movement. Some call this Back-EMF, I prefer to call it microphonic velocity voltage as the more general and physical term. This is the sensor.
The Voice Coil, as seen from the driver terminals, is the interface for this "black box". Let's see what happens when we short the terminals (which is what our ideal amp does, apply a voltage -- including zero -- with zero output impedance).
View attachment 179920
Consider the voice coil as a
sensor produces +1V of velocity voltage at the moment. This voltage is applied across the transfer impedance of the driver, Re+Le and forces a current to flow in Re and Le and the same current (though negative) must flow through the VC as a
motor, -0.25A in this case.
The applied force opposes the velocity, reducing it. The relationship of velocity signal vs force injection is complex (introducing phase angles vs frequency etc) but for the given purpose we may neglect this here and use it as a black box which we know at least is passive/dissipative.
By this, as long as any movement is present the short produces a corrective signal to reduce the movement. The better the short, the better driver opposes its own "parasitic" movement (degenerative local feedback). But we cannot get any better than Re (and Le to some part) allows, unless we start trickery with negative amplifier output resistance and such...
That's all what this Damping and Back-EMF stuff is about, in the end.