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Magnet-to-magnet isobaric design question
Magnet-to-magnet isobaric design question
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Old 18th February 2019, 11:56 PM   #11
jimmyjazz is offline jimmyjazz  United States
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Originally Posted by GM View Post
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!
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Old 19th February 2019, 01:59 AM   #12
GM is offline GM  United States
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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
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Old 19th February 2019, 12:02 PM   #13
scottjoplin is offline scottjoplin  Wales
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Originally Posted by jimmyjazz View Post
I'm not sure it's pointless if a small box is paramount. Thoughts?
The trend these days is to but a large excursion driver in a small box and EQ it. The increase in suitable drivers and cheap amplification has made this a more attractive option.
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Old 19th February 2019, 12:03 PM   #14
scottjoplin is offline scottjoplin  Wales
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Originally Posted by jimmyjazz View Post
I'm not sure it's pointless if a small box is paramount. Thoughts?
The trend these days is to put a large excursion driver in a small box and EQ it. The increase in suitable drivers and cheap amplification has made this a more attractive option.
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Old 19th February 2019, 12:32 PM   #15
Windforce85 is offline Windforce85  Poland
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Originally Posted by jimmyjazz View Post
I'm not sure it's pointless if a small box is paramount. Thoughts?

It depends. Have you already selected your drivers and decided which box alignment (sealed, bass-reflex etc.) to use?
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Old 19th February 2019, 01:49 PM   #16
jimmyjazz is offline jimmyjazz  United States
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Originally Posted by Windforce85 View Post
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!

Last edited by jimmyjazz; 19th February 2019 at 01:55 PM.
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Old 19th February 2019, 02:09 PM   #17
badman is offline badman  United States
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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.
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Old 21st February 2019, 12:13 PM   #18
Think is offline Think  Netherlands
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Magnet-to-magnet isobaric design question
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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Old 21st February 2019, 10:06 PM   #19
jimmyjazz is offline jimmyjazz  United States
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I read that article last week. Suffice it to say I think it's not completely thought out and a bit presumptuous. Engineering is all about tradeoffs -- it's rare that there are true absolutes once you get into the process of optimization.
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