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Isobaric cab?

#21
The Ampeg IsoVent above has its two 15s clamped face-to-face with just the baffle sandwiched between them. It came out around 1992 during Ampeg's SLM era. At the same time Ampeg also offered the SVR-215 (which I bought in '92) with two 15s back-to-back in a sealed enclosure (there was also an SVR-212 doing the same thing with 12s), the theory being that they'd oppose each other's motion and reduce the possibility of excursion distortion. There was also a crossover directing the higher lows to the front 15 and the low lows to the rear; it was stated that the lowest frequencies were omnidirectional and not cause phase issues with the front. Of course, having a rear-firing speaker also means you can't put it up against the wall. The SVR-215 (and SVR-212) had rackmount sections built in on the top making a convenient place to put the SVT-III I bought with it. They also offered the same cabs without the rackmount section, but I forget the model name; might have been DD-215 and -212 or something like that. If I can find where I stashed the catalog I can look it up, if anyone is burning to know.
 
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#23
As a fairly insane tinkerer, I tried a weird experiment. I had read some comments about isobaric speakers by our esteemed Duke LeJuene. I have a Mesa Scout 15 cab, and a spare speaker for it. So I removed the grill and mounted it clamshell style to the existing speaker. All of the above comments were true; more, deeper bass, less sensitviy, more weight and looked like a hideous mistake. But, if it wasn't so fragile with the speaker hung out like that, I might have gone with it. I also tried it on my Coffeehouse DIY 10 using a non matching speaker (as per Duke's opinion that a matching speaker wasn't necessary) and got the same results.
 
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#24
pie_man_25 said:
so yeah, as has been outlined - isobaric loading in a design is a great way to increase bass and extension in any given design. It's drawback is that, like you said - it reduces your sensitivity by 3 db, basically halving the power input into it. Of course, you're also basically doubling your power handling anyways, so if you have the amp to spare what are you really harming?

As far as I know the volume doesn't include the space between the drivers, and if you wire them in parallel they'll be 4 ohms.
you seem to know what you're up to and what you're expecting, and you get the drawbacks involved. You also seem cool with the 4 ohm mid. If you're cool with lugging the thing around, and have a good design, I want to see this thing get finished.

Also, it should be noted, that if you're doing the drivers front to front or back to back, you'd need to wire them in reverse polarity to work together (so that they move inward and outward at the same time and don't damage each other or negate the effects of the design), if they're back to front, wiring them in "proper" polarity is good.
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I will probably do back to front, so I can still have a front firing woofer, and it can save me some space.

Yeah, I already found a good 8 ohm mid, now I just need to re find one for 4 ohms.

I think the weight will be fine, as size is smaller.
20lb per woofer, 10lb mid, some wood, around 60lbs. My current thing is a Yamaha pa cab that ways 120 and is massive.
 
#25
There's a lot of misinformation here. What you get with isobaric is halving of the net cab size. That's net, not gross, it doesn't include the space taken up by the second speaker or the space taken up by the chamber than connects the two speakers. It doesn't go any lower than with one speaker, and it doesn't go any louder than with one speaker. The thermal power rating is doubled, but the cone area that's exposed to the air isn't, so the maximum SPL is the same as with one speaker. You get a 2x cab that's maybe 30% smaller than a 1x cab, with the same response and output of a 1x cab, that weighs almost as much and costs just as much as a 2x cab. It's a configuration that just doesn't make sense.
Someone here has to have a better cabinet design for those JBLs.
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BFM Simplexx 15 or a fearful. The OP is asking way too many basic questions about speaker design that he wouldn't ask if he had the chops to design a cab, especially since getting the numbers out of WinISD is just the beginning. IMO he should stick to a proven design from a professional.
 
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#26
Rick James said:
There's a lot of misinformation here. What you get with isobaric is halving of the net cab size. That's net, not gross, it doesn't include the space taken up by the second speaker or the space taken up by the chamber than connects the two speakers. It doesn't go any lower than with one speaker, and it doesn't go any louder than with one speaker. The thermal power rating is doubled, but the cone area that's exposed to the air isn't, so the maximum SPL is the same as with one speaker. You get a 2x cab that's maybe 30% smaller than a 1x cab, with the same response and output of a 1x cab, that weighs almost as much and costs just as much as a 2x cab. It's a configuration that just doesn't make sense.
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It does make it a good deal smaller. I think it might be worth it for me, I allready have 2 woofers.
75l vs 150l is a big difference. The space between drivers can be minimized.
 
#39
Among experts, not everyone agrees that the output of an iso cab is equivalent to that of the single driver:
DukeLeJeune said:
Bill and I disagree on the subject. He's on record saying that "the output is the same as it would be with one driver in a larger box", and that's what the textbook theory predicts.

Year ago I built and carefully measured theoretically equivalent isobaric and conventional enclosures and got results that were significantly different from what the current theory predicts. My measurements indicated a parameter shift that isn't part of the current theory, which resulted in significantly deeper low-end extension than expected for the isobaric box.
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I've read that there are other caveats though, like the midrange of the inner cab interfering with that of the outer, so it seems to be quite complicated stuff to tackle.
I was wondering if @agedhorse feels like chiming in since IIRC he's commented about this driver (or the platform at least, not sure if there are more than one 2226 models) in the past.
 
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#40
HaphAsSard said:
Among experts, not everyone agrees that the output of an iso cab is equivalent to that of the single driver:

I've read that there are other caveats though, like the midrange of the inner cab interfering with that of the outer, so it seems to be quite complicated stuff to tackle.
I was wondering if @agedhorse feels like chiming in since IIRC he's commented about this driver (or the platform at least, not sure if there are more than one 2226 models) in the past.
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There is a 2226h j and k, all pretty similar
 

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