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Old 26th February 2012, 08:55 AM   #1
kctess5 is offline kctess5  United States
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Default Isobaric theory

I was thinking about speaker design during physics a couple of days ago (as normal) and I had an idea that I thought might give near open baffleness without all of the drawbacks. After thinking about it a bit more I realized that it was basically just an isobaric enclosure

Here it is:
Picture a sealed box..

Now somewhere on that box add a second woofer so its a dipole type speaker, with the speakers out of phase so that the enclosure pressure stays constant.

Now put a very small sealed box over that second woofer on the outside. (primarily to close off the backwave when I first thought of the idea)

Here's what I was trying to get:
Effectively I wanted near open baffle performance without the problem of phase cancelation from the backwave

Near constant enclosure pressure to minimize box colorations (flexing) and maximize transient response. I realize that because the second woofer would be fighting the compliance of the sealed portion behind it the volume between the woofers would not be at perfectly constant pressure, but very close.


What I was not trying to get:
A smaller enclosure size which is the aim of normal isobarics

Here's how its different from a traditional isobaric (kind of):
The volume between the woofers is very large compared to a 'normal' isobaric arrangement and the sealed box on the second woofer is small compared to it (though the second part is not necessarily true, just what I originally thought of). This was the air between the woofer would have a compliance much larger than a traditional isobaric and therefore the driver could move more freely.

Now of course similar to an isobaric it will be 3db less efficient

What do you guys think? Is this just a totally standard isobaric in a slightly different configurations that has absolutely no difference from it?
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Old 26th February 2012, 11:21 AM   #2
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I like isobarics, and I am building another at the moment, as I believe there are benefits beyond smaller box size.
I think the large volume between will mean the two speakers will not be moving as one, especially as the small volume at the back would raise the resonant frequency of the rear speaker. Coupling the two units with a small gap (face to face or face to back, remember to switch the phase to suit) would, I believe, give the best results. The bigger the gap between them, the lower the upper limit. I intend to xover at about 100Hz to the next driver.
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Old 26th February 2012, 09:35 PM   #3
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I've been working on a similar idea, trying to put a full-range driver that normally requires a fairly large enclosure or open baffle into a small sealed enclosure by using an internal bass-only rear driver to "unload" the full-range driver. No good results so far.

With isobaric (2 identical) drivers, the pressure between the drivers is not zero - it is half that in the main enclosure behind the rear driver. You run into problems when the volume between the drivers becomes significant (or even larger than) the volume behind the "rear" driver. The tuning changes, and more significantly the driver excursions start to differ -the drivers no longer "move as one". You need to use AkAbak to simulate this properly.
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Old 27th February 2012, 12:44 AM   #4
kctess5 is offline kctess5  United States
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Ok I can see how the drivers would not be together, if the speed of sound was much higher than this would be more viable for high frequencies.

What if you did the same thing I described but for two subwoofers. Then the distance would be much less of an issue and the drivers would be apparently in phase (ie pressure near zero w/out the rear sealed chamber).

Also the phase cancelation from the backwave is only really a problem at the low frequencies. Maybe a sub like that could be paired with an open baffle mid treble with similar spatial awesomeness.
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Old 27th February 2012, 06:46 PM   #5
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The effect occurs at frequencies well below the resonances caused by the enclosures etc. In other words, you get the excursion problems at subwoofer frequencies.
I'll make you an offer: Pick a couple of drivers and provide their parameters, and provide some enclosure sizes. I'll build an AkAbak model and post the results.
Note that to provide a "near open baffle" environment for the front driver, the rear driver is going to have to either be driven harder or be larger in order to maintain close to zero pressure difference in the enclosure behind the front driver. Remember that in a true "isobaric" configuration, the rear driver only reduces the pressure behind the front driver to about half of what it would be for a single driver. Your setup, where the rear chamber is much smaller than the chamber between the drivers, will require the rear driver to be driven even harder.
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Old 27th February 2012, 08:20 PM   #6
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Your assumption is that open-baffle speakers sound so good primarily because of 'truly infinite baffle' big-box characteristics. So why not just use enormous boxes to achieve the same goal and eliminate most of the undesirable characteristics? As it gets bigger and bigger, after some huge point of diminishing returns, you will have acheived about the same thing. On the other hand, some believe the back-wave is an integral asset of the open-baffle sound.

On the other hand, if the rear driver of a isobaric operates into an extremely small space, it will only operate at high frequencies.

For more normal isobarics:

If you really want the drivers to move together, try facing them toward each other and actually physically connecting them. There's several ways, with cardboard or cf tubes over the dust caps, or with careful construction of forms from tape you can fill the entire space between with lightweight expanding foam molded in-place. Or variations of the two, with a rolled paper tube anchored in each cone with some spray foam to glue it, or a spiralled paper "tube". Many variations possible. You could have a styrofoam plug made to the dual-cone shape to fit into opposed clamshelled cones, perhaps with varous voids to save weight, and glue it in with expanding foam. You could even leave the surround off of the inner (or outer) driver. There's more interesting possibilities if you are manufacturing the drivers. If you work with the kind of precision needed for interchangeable parts and field-repaceable diaphragms, you could manuafacture a foam "cone" (shaped like two cones) with a voice coil sitcking out either end, with a single surround and one or two spiders glued pre-centered onto a frame, to which two magnet assemblies would bolt. How about using just one paper cone, but two voice-coil/magnet "motors"...using a very long voice coil former and wind a coil on each end. There's construction complications, but nothing than can't be done. No problems hearing the back driver thru the cone of the front driver if you get rid of the back-driver's cone. What is cool about an isobaric is that you have two motors (a good thing), not that you have two cones (source of a lot of problems).

There's also interesting unexplored possibilities via various manifolds...making "isobariks" with small "front" drivers (capable of higher frequencies) mounted to the outer air, with a complex manifold to a single larger rear driver inside the box. If you design the manifolds carefully, you can balance volume of air between the drivers versus air velocity, and with care you might end up with an isobaric with very little air between the cones, and unexpected bandwidth, extending higher than usual for an isobaric.

Might even experiment with crossovers between the front and back drivers, and unmatched drivers.

There's also trisobarics etc. etc.

Plenty there to keep you thinking during that physics class, and all night after.

Last edited by cyclecamper; 27th February 2012 at 08:26 PM.
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