Hi Kijanski,
Well, I guess I disagree with JBL! That doesn't seem to put me on very solid ground, does it??
In listing the changes that take place in system characteristics when using two woofers in either series or parallel connection, Vance Dickason (Loudspeaker Design Cookbook) makes no mention of changes in electrical Q nor does he offer any warnings about dropping the amplifier's effective damping factor to less than 1.
I could name several high-end loudspeaker systems that use either series connection or series-parallel connection for two or more woofers. I think that the reason most dual-woofer systems are wired in parallel is in anticipation of the increased wattage output from a voltage-source (solid stae) amp into a lower impedance load.
I have measured loudspeaker systems with series-connected woofers, and my measurements do not indicate that the damping factor goes to 1 with series driver hookup. What I did was this: I measured the same system with a low output impedance solid state amp, and again with a high output impedance specialty tube amp. If the damping factor goes to 1 with series driver connection, there should be negligible change in the bass response between the two amplifier types, as the effective damping factor would be the same in both cases. Instead, I measured approximately 4 dB greater output at system resonance with the high output impedance tube amp (relative to the solid state amp), indicating that the solid state amplifier's high damping factor was in effect. Unfortunately for my position, claimed observations posted in an internet forum are seldom accepted as proof of anything.
Let me present this thought exercise: Think of an 8-ohm woofer's voice coil as consisting of two halves, the first half presenting the amplifier with a 4-ohm load, and the second half also presenting a 4-ohm load, in series with the first. The first half of the voice coil does not isolate the second half of the voice coil from the amplifier. Instead, the amplifier sees them together as a single 8-ohm coil.
Now suppose that, instead of those two halves of the voice coil being continuous, we connect them with a short piece of wire that comes off the voice coil former, and then returns to it. Would this make any significant difference in the load that the amplifier sees? No, neither the impedance nor the inductance of the load is changed.
So let's suppose that we use a separate voice coil former for the second half of that voice coil, along with a separate motor and frame and spider and cone (in other words, we now move to two 4-ohm drivers connected in series). Would that make any significant difference in the load that the amplifier sees? Again, the answer is no.
Duke
