Regarding my proposed subwoofer design, I would have prefered to have a better driver made to retrofit existing systems using the LAB12. I asked Eminence to work on such a design, called the "B12." It would have been easier for some people to implement, because people that already have LABhorns could have upgraded to the improved driver. It was sort of a way to have cake and eat it too.
But there are other ways to accomplish the same thing. The proposed design is a way to improve symmetry using existing drivers. And I am realizing that there are probably some benefits to this push-pull plenum design, particularly since two drivers are used anyway. If only one driver is used, then a shorting ring is the only way to improve symmetry. But when two drivers are used, the push-pull plenum becomes an attractive possibility. And it looks like it will work at a much lower frequency, which is indeed important for a bass system.
One thing is certain, and that is Eminence made a good driver at a good price. They are able to make the existing LAB12 driver very inexpensively. That is good, and is one of its most attractive points. It does provide a lot for the money. What surprised me is that it must have taken them a lot of engineering time, and I would have expected some of these other things wouldn't have been more than pennies added to the cost. But now that the ball has been rolling for quite some time, maybe the momentum keeps it rolling.
After asking Eminence to work on an improved driver, I realized that maybe I was looking at this the wrong way. The reduction in distortion from a push-pull plenum might very well be better than using an improved driver with flux control, and occur much lower in frequency. That's the problem with shorting rings; It's hard to get reduction of distortion down low. So it might be a better deal to design a horn with a push-pull plenum.
The acid test will be in the measurements.
As for your concerns about pneumatic asymmetry, there again, the tests will show one way or another. But putting pencil to paper, it appears that both sides are equal. The vent of one driver will face the throat and the other will face the rear chamber.
The rear chamber vents with restriction from one driver and the throat plenum vents with restriction from the other driver. Both sides have the same size vent, so I don't expect asymmetry from that.
Both rear areas will be connected to form a common rear chamber, so the pressure in that common chamber will be the same. That is the place where sealed tuning is most relevant.
Horns can be configured with vented or open rear chambers. The larger the rear chamber is, the lower the motor system resonant frequency is. Making the rear chamber very small is a way to raise this resonance to boost output near horn cutoff.
Excursion is reduced in the passband of the horn. Below that, the main thing that limits excursion is the motor and the rear chamber. At frequencies below horn cutoff, the system acts very much like a direct radiator. So at low frequencies under horn cutoff, the horn and front chamber can be likened to a psuedo-bandpass system and the rear chamber can be seen as a sealed or vented loudspeaker system, depending on its configuration.
Tests will show for sure, but I suspect that the ducted cooling vent will have little effect on tuning. What little effect there is can be compensated for with slight resizing, if necessary. I think the cooling vent will prove to be a benefit, with no real losses. We will see.
Concerning the loudspeaker vent's ability to cool, it can be likened to a reciprocating compressor pump without valves. The air in the vent is pumped back and forth rather than valved so that it will cause flow. So naturally, if power levels are high and the cone is held rigid, the cooling effect is greatly reduced. Horns reduce excursion, so this tends to make the cooling vent less effective. If excursion is high enough to purge a large slug of air each half-cycle, it stands a better chance to draw in a blast of cool air. But even this is stymied if the chamber it vents into is small and easily heated.
The proposed solution relies on two things. One is ducting the vent to cool outside air and the other is to valve the pump so that it actually works as a pump. We aren't looking at moving valves or vanes, so the valves will not "seal." But they should impede flow in one direction more than the other enough to induce unidirectional flow. Using fixed flow valves, we hope to cause the pump to actually move cool air through the system rather than simply vibrating a slug of hot air back and forth within the loudspeaker vent. If it can be made even marginally more efficient as an air pump, it will probably go a long way towards keeping the voice coil cool. What makes me think it will probably work well is that it shouldn't be too hard to beat the performance of an unvalved reciprocating pump operating in a small sealed chamber.
As for overall symmetry, the biggest offender is magnetic, not pneumatic or mechanical. The push-pull plenum will correct for this. Since the arrangement is designed specifically to address the fact that one driver is stronger than the other on each half-cycle, I think it will work well to counteract asymmetry using complementary pairs. This will counteract any asymmetry, whether the cause be magnetic, mechanical or pneumatic, and it should work very well at low frequency up to and beyond the upper limit of the horn.
Don't forget that each driver is already asymmetrical. The LAB12 driver does not have any mechanism to reduce flux modulation. Most drivers don't. Any speaker with a ferrite magnet that doesn't contain a shorting ring will suffer from this effect to some degree. The fact that both drivers are connected to pressurize from the face means that whatever asymmetry is present from each driver is doubled. Turning one around is a way to address this.
The push-pull plenum is pretty straightforward, and really should be expected to work. The venting is a little more esoteric, I suppose, but I don't think anyone should be scared off by the fact. I think it is important to remember that horns can be configured to run with open rear chambers, and that the closed back chamber is there primarily to boost output near the flare frequency.
Most users that push 'em hard use high-pass filters to protect the system below the flare frequency, so I don't think that unloading at very low single digit frequencies will be a deal-breaker. If we find that the ducted vent provides significantly improved power handling in the passband at the expense of reduced ability below say 5Hz, I think that will be excellent. I hope this is what we find. We'll see.
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Topic: B12 Subwoofer (Read 10421 times)