There is a concern with putting dynamic drivers in series and that occurs at resonance. If you look at a typical impedance curve, two things happen at resonance. First, the phase goes from leading to lagging. Second, the impedance reaches a peak that can be ten times or more than the impedance at other frequencies. If the drivers in series have different resonant frequencies (Fs) they will get out of phase. That is the movement of the cones will not be in sync, with one leading the other. I have used a stroboscope to view this situation and it is real. The result is a drop in output at that frequency.
The second problem is that the high impedance of the drivers at resonance interferes with the ability of the amplifier to damp motion of the cones. Unless you have a servo system with only one driver, the amount of current the amplifier can provide into the driver decreases dramatically with the impedances that occur at resonance. You then effectively have two or more generators in series that are not in phase.
I have used drivers in series very effectively as long as they are used higher than the range of resonance. They are then both lagging in phase and have much lower impedance. Looking at them with a stroboscope, they are then in phase and there is no change in response beyond what a single driver would have.
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