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Old 29th April 2002, 10:59 PM   #11
Bill F. is offline Bill F.  United States
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Chris,

Just to clarify my terms, I think of all subs whose passband is below Fb as ELF alignments, 4th-order resonant or not. Maybe the definition of ELF is strictly sealed, I don't know. Let me know if I need to correct my terminology.

There's plenty of attraction to Carver's PR design. What worries me is that the quasi-opposed motion of the two diaphragms doesn't cancel all the net force on the cabinet, as he admits in the FAQ. With the extreme excursion numbers I'd like to see, bad vibration could result unless there is balanced bipolar movement. I'm counting on the underhung switched coils to add efficiency and the huge back EMF and 1/3 duty cycle per coil to increase power handling.

Grey,

All I am pointing out is that for a given box volume, static counterforce on a low-frequency diaphragm at, say, 1" incursion will be higher than the counterforce on the same diaphragm at 1" excursion. This is because the compressibility graph of air is not a line, it is a curve tending toward vacuum at one end (1 atm. or 14.7 psi of return pressure on our hypothetical diaphragm) and infinite pressure at the other (infinite counterforce). The compressibility curve's rate of change for a given diaphragm displacement increases as Vb decreases.

It seems to me that this nonlinearity is inherent to any closed baffle alignment, except where room volume = enclosure volume, i.e, an infinite baffle. (It just occured to me that horns might be less effected as well.) Granted, it's manifestation would be small in alignments where Vd is minor compared to Vb (and at higher altitudes ). That's why I'm really just worried about it in connection with my extreme excursion ELF idea. Feedback would help, I'm sure. I was wondering if the dynamics were predictable enough to simply correct for it on the signal level.

Does it make sense to you as it does to me that the net effect of this nonlinearity would be a forward displacement of the diaphragm's excursion midpoint? Maybe such a phenomenon is already well known, I don't know.

By the way, my concept of a feedback-triggered coil-hopping amp isn't too far fetched, is it? It's something I've been thinking about for autosound SPL, etc. For slightly less extreme two-coil permutations, I also was thinking of a class B amp with two half-wave outputs.

Bill
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Old 30th April 2002, 12:22 AM   #12
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Warning: Unorthodox idea...
Use a small DC offset to push the voice coil off center, thus pre-loading the suspension.

Grey
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Old 30th April 2002, 03:01 AM   #13
CHRIS8 is offline CHRIS8  United States
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Bill:

Yes, an ELF is an alignment that operates below a resonant system. However, Carver's design is not an ELF, as I can tell from the white paper you specified. Accordingly, the passive driver would NOT work in an opposing direction to the active, if it was operated under resonace. Below resonance, behaviour changes on a spring and it moves with the source of energy, not in opposition. It appears this is simply a very low Q, low compliance woofer with extraordinary excursion capabality. Becuase of the low compliance, a very small box is possible, and with Carver's method for efficiently operating the woofer motor, it is now a practical design. Since you goal is to create a similar woofer operating principle for a woofer anyways, simply implement the same low compliance characteristics.

GRollins:

It seems that IF their is significant effect below resonance, as Bill suggests, then this would need to be accounted for in proportion to the input signal, and calibrated to the actual physical excursion of the woofer since the compression/rarefication effects are logarithmic, as actual incursion/excursion is increased. A static DC offest would not be effective, or at least it would seem from my perspective.

-Chris
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