Magnet-to-magnet isobaric design question

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If one were contemplating a magnet-to-magnet configuration in an isobaric subwoofer, what adverse effects might arise if a custom "hourglass" isobaric chamber were used? It could be cast or molded or even 3D printed, depending on material. In design cartoons one typically sees a straight cylinder spanning the distance between the mounting rings of the two drivers, but it seems that the entrained volume of air could be reduced below what it is with that concept, tapering in and back out in close proximity to the drivers, thus reducing air compliance as well as freeing up internal volume in the main box.

Would there be issues with the air that sloshes around in the isobaric chamber, perhaps related to higher velocity?

For what it's worth, I am aware of the general disdain for isobaric designs given modern driver performance standards, and in particular for any isobaric layout that's not a cone-to-cone ("clamshell") configuration. Indulge me. :D
 
If one were contemplating a magnet-to-magnet configuration in an isobaric subwoofer, what adverse effects might arise if a custom "hourglass" isobaric chamber were used?
Would there be issues with the air that sloshes around in the isobaric chamber, perhaps related to higher velocity?
A custom "hourglass" isobaric chamber would reduce the air volume between the drivers compared to a cylinder of the same depth, which is a positive feature. The reduction in the air volume between the drivers will increase voice coil heating, a negative feature especially if the drivers are used for heavily compressed low frequency program material.

The air velocity would be reduced with a custom "hourglass" isobaric chamber compared to a cylinder of the same depth, less "sloshing".

Curious as to why you don't want to use a cone-to-cone isobaric chamber?
 
Thanks. Why would you say the air velocity would be reduced? It would have the same volume displacement (= 2*Vd of each driver) but that air would be moving around in a smaller entrained volume. Maybe I'm thinking about it wrong.

I'm not hard sold against a clamshell arrangement, but I've been led to believe that the back side of a driver tends to be noisier than the front side -- chuffing noises, wires moving around, etc. Plus, I would need this to get up into at least the low midrange (maybe 300 Hz) and I'm worried about diffraction around the basket and magnet structure. Maybe that's not an issue.
 
Thanks. Why would you say the air velocity would be reduced?

I'm not hard sold against a clamshell arrangement, but I've been led to believe that the back side of a driver tends to be noisier than the front side -- chuffing noises, wires moving around, etc. Plus, I would need this to get up into at least the low midrange (maybe 300 Hz) and I'm worried about diffraction around the basket and magnet structure. Maybe that's not an issue.
Assuming the air is all moving with the drivers, there is no relative velocity. The larger the enclosed volume, the more pressure variation potential between the two drivers. In either case, the volume is enclosed, and won't be creating chuffing noises.
Chuffing noises can be a problem with the magnet side on some vented pole piece or spider designs, but easy to determine that with a free-air test.

The wavelength of 300 Hz is over a meter long, wouldn't worry about diffraction around the basket and magnet structure as an issue to not use a cone to cone arrangement.
 
Well, there may be no relative velocity between the air and the cones, but there will be between the air and the fixed structures (magnets, baskets, chamber walls, etc.). I don't know what might occur if local velocities get high (decoupling of the cones, etc.?).

Just to be clear, when I referred to "chuffing" I was speaking only about noise emanating from the exposed magnet side of the "heard" driver in a cone-to-cone isobaric pair. I'm not too worried about such noise within the isobaric chamber.
 
Hmm, shouldn't since the goal is a small enough coupling chamber to ensure a ~uniform particle density, so no velocity and a bad plan for cooling the motors except through the dust caps.

I'm not entirely sure I follow. In a cone-to-cone configuration, as I see it, the entire "boundary" is moving so to first order there should be no air motion relative to anything. It's a slug of air moving back and forth with the cones. In the other two common configurations, there are at least some stationary boundaries, and fluid mechanics tells us the air must also be stationary at those boundaries; i.e., there will be a velocity profile developed within the chamber.

Now, whether that MATTERS is another question entirely! ;)
 
Right, the trapped air mass volume is moving with the diaphragms as a ~ uniform particle density air mass ['slug'] and it's my understanding that at worst, the motor frame structures add an insignificant bit of friction, though whether there's a proper velocity profile as a result of this 'whipsawing' I leave to the better educated.

Regardless, the object is to minimize this volume to best damp this system and of course have adequate cooling, but if performance desired isn't too extreme, don't see any reason not to try it.

FWIW, someone long ago did a tiny sealed bi-pole with a clam shell driver on the rear that was pretty impressive considering it was only ~ [1] 4" driver square and [3] deep + material thickness.

GM
 
It depends. Have you already selected your drivers and decided which box alignment (sealed, bass-reflex etc.) to use?

Well, I'm hardly "settled" but the small Tang Band drivers are the best I've found in terms of low F3 in a small enclosure. I'm not concerned about SPL (which is obviously a good thing, because I'm not gonna get it). My favorite candidate at this point is the 5" TB W5-2053, which is part of their RBM series. They come with considerable cost and weight, but for this exercise, I'm not concerned.

I'm targeting a 6 L enclosure. I believe I can get away with a long vent. In my mind, the right way to compare isobaric and conventional alignments is to let the conventional enclosure be slightly larger by roughly the volume of that second driver I'd need in the isobaric design. So, I'll compare a 7.5 L conventional single-driver configuration to a 6 L (rear chamber) isobaric configuration.

It looks like the isobaric F3 is a little lower than that of the conventional design (25.7 Hz vs 26.9 Hz). In addition, the isobaric frequency response has a bigger peak in the 29-31 Hz range. I don't know how that would sound, but I suspect it could help mitigate some of the lower output of the isobaric design. The vent in the isobaric design is longer (approximately 30" vs 24"), both of which are going to be difficult to achieve and take up considerable enclosure volume. Driving at max power, SPL on the isobaric design is 3 dB down in the asymptotic midbass region, but the differences near F3 are very small (about 1 dB). Cone excursion is lower in the isobaric design, and in fact slightly exceeds maximum in the conventional design at ~ 38 Hz. Both options would have to be rolled off a great deal below F3 to keep the drivers happy. Max port velocity is high at F3 in both designs, and nearly equal at 35-39 m/sec. I'm willing to live with that, because I don't expect to be pushing this thing at max volume on real music. Group delay is lower near F3 in the conventional design (36 ms vs 47 ms). That seems important.

So, I get the point. Given the practical concerns (weight, cost, cooling, power) and varying performance tradeoffs (lower F3, higher group delay, reduced output, lower cone excursion, lower port velocity) there is no clear winner.

Thanks for getting me to go through that exercise!
 
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I like the using 2 sets of clamshells- you get some distortion cancellation, you can mount them on opposite box walls for force cancellation, you double your displacement and reduce power through any given coil. All in the same size box that a single driver of the same model would require (or slightly larger if you're accounting for extra magnet structures). 4 drivers acting as one. With some subs as cheap as they are nowadays, I'd rather use multiple cheaper drivers (within reason) that aren't being pressed as hard as one big mutha. The advantages of more cone area and force cancellation are substantial, and not having to build as thick-walled a box offsets some of the additional driver weight and cost (again, though, there are quality inexpensive drivers around, 2 clamshells of which would compete well with all but the most extreme supersubs).

Now, if box size isn't an issue you can have all that and more (much higher efficiency and more effective distortion cancellation) with a SLPP arrangement (slot loaded push pull). The best sub I've built and used is a dual SLPP (4 12"s), followed by a dual clamshell (same 4 12"s).

Magnet to magnet is a waste of time IMO. I can see some advantages in it functioning as a lowpass, and having less heat dissipation per-coil, but those are pretty big stretches for adding another driver and a bunch of box space lost.
 
Isobaric Enclosure Types – JL Audio Help Center - Search Articles

"Back-To-Back Isobaric
This design was thought up by someone who wanted to reap the advantages of canceling driver non-linearities without having to resort to the “clamshell” loading and its inherent cosmetic problem (namely that of hiding an exposed subwoofer basket). This design, like its cousin, the “piggy-back” tunnel-loaded isobaric, also has several issues that make it an undesirable choice:

  1. It shares the same problems with the added springy mass of air that couples the two drivers. This problem is made even worse by the fact that the coupling chamber is now even larger, adding more moving mass and springiness over the “piggy-back” design, which makes frequency response predictions even more difficult.
  2. The increased coupling chamber (yellow volume) means that the blue volume and the entire enclosure must be even larger, even more closely approaching the volume of a conventionally loaded single subwoofer. In a home this might not be a problem, but in a vehicle where space is at a premium, this is a definite disadvantage!
  3. Now that both magnet structures are in identical cooling environments, they will more closely track each other’s performance, but, unfortunately, now we have two heat dissipating structures in the same tiny enclosure, which will greatly reduce the thermal power handling of both drivers, not to mention the fact that as the air heats up, it expands, thus pushing each of the subs outward and further limiting output by reducing each driver’s potential excursion!
While the original creator of this design should be given a pat on the back for creativity, he might also merit a kick in the behind for the reasons stated above. This is definitely not a design that we recommend under any circumstances. "
 
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