Dual woof/isobaric question (1 Viewer)

[Chuck Bogie]

Okay - Have been _trying_ to work my way through The Book From Hell.

Someone please tell me if I have this right or explain it in plain English, and tell me if it's a good idea or not...

Okay - Let's say that I've got a pair of woofs... I stick both of 'em in a box that's vented, fire 'em both in-phase, and I get a decrease in distortion and a bit of added SPL?

Then I take that same pair of speaks, set 'em up pointing away from each other (like in a tube), fire one of 'em out of phase, so that they're both essentially moving in the same direction. I get the lowered distortion, but not the added SPL? And I can make the sealed box 1/2 the size that I otherwise would have to, with the tradeoff that the f3 won't be dropping significantly?

Have been sorta thinking on a "cheap sub project" and I suspect that another $30-40 for a driver might be better than making a larger enclosure, especially for just music.

 

[Dave Milne]

I believe you've got it essentially correct. The isobaric configuration allows you to cut enclosure volume in half and get essentially the same performance. Power handling is doubled, but since excursion isn't... you still end up with the same output limits at the lowest frequencies.

I've done this and it works. Enclosure volume isn't exactly halved, because the extra woofer and tube takes up some additional volume. I put both woofers in the same orientation and ran them in-phase. With this approach, you need to make sure the rear woofer cone doesn't smack the front-woofer magnet or input lead at max excursion. I used a router and circle jig to make a bunch of MDF rings that I glued together to form a "tube".

 

[Chuck Bogie]

So, basically here's what I'll do: Take my "nice" looking box (that'll likely be too large, since it'll be tall enough to basically act as a "speaker stand" for the BR-1 type two-way), then mount some sort of tube/box inside to lower the effective size, and mount a pair of 8 or 10 inch woofs inside, firing front/back - Or do you think that sides would work better, considering that they'll basically be working from 125hz down?

 

[MichaelAngelo]

FWIW, an isobaric design likes as little air between the drivers as possible... clamshell seems to work very well, ie., face to face on one side of the enclosure.

hope this helps

 

[Chuck Bogie]

So the enclosure is sealed, with one driver completely inside (very small box), and the other driver has the magnet sticking out, bolted "inside" the box, with the "inside" driver's faceplate touching the outside driver's faceplate?

I may be missing something, but doesn't the "cone" of the woofer have more to do here? Please bear with me - I'm in the throes of info overload...

 

[MichaelAngelo]

Exactly. Faceplate to faceplate. Remember that one woofer must be wired opposite the other, or they wil cancel each other.
One thing to check for is the height of the surround. Make sure the surrounds don't touch, or they will wear quickly. If they do touch, space them with a ring of MDF.

HTH

 

[ThomasW]

A clamshell isobaric looks like this

 

[Geoff L]

Chuck

There are a number of Iso configurations. They can even be Ported or PR'ed also. This might give you a better idea of some of the most common known Isobarik types.

Scroll down a little to Isobarik

Isobarik Info

Regards
Geoff

 

[Dave Milne]

Take my "nice" looking box (that'll likely be too large, since it'll be tall enough to basically act as a "speaker stand" for the BR-1 type two-way), then mount some sort of tube/box inside to lower the effective size

I guess I don't quite understand. Are you saying that your "nice looking box" would be large enough to support your two drivers in conventional configuration? If so, this would give better performance than an isobaric (6 dB improvement in output).

 

[Chuck Bogie]

Well, I'm thinking of a fairly thin "front cross section" - large enough for a 6.5" but I don't know if I'd want to put a pair of 10-12" speaks up front... Since the front speakers are basically going to be "towers" and about 3.5/4' high (for being next to my screen, I may have a little extra height and depth... Sorta thinking that a side-mounted isobaric (or a pair of side-mounted isobarics) might be an idea.

I've been listening to a few speakers, and the more I play, the more I'm thinking that I like the sealed boxes - just the tightness of the bass...

Also, if I understand correctly, I could mount a pair magnet/magnet, with a grill on each side? Not quite as nice as a clamshell, but I doubt if I'll be driving 'em hard enough for heat to really become an issue, especially if I pick a pair of woofs that will still require a bit of box volume? At this point, I'm figuring that "cheap is good" and may look for something that has a low f3 rolloff for a sealed box in the area of 6-8 cubic feet, assuming I can cut that in half by using two of 'em... I guess that basically I'm deciding what kinda box I want, and then hunting speakers for it, but hey...

Since I'm basically considering this to be a "cheap speaker project" but still come up with something that'll work for parties, etc., I'd really like to find a combination that I can power with a pair of plate amps (need to measure one of the amps, and figure that as the width, I guess), rather than springing for a larger amp to run "real" speakers.

Or maybe I could stick a cheap pro amp in a closet...

 

[ThomasW]

Also, if I understand correctly, I could mount a pair magnet/magnet, with a grill on each side?

Nope that's not isobaric.

These are the primary isobaric configurations. Note that one driver must fire into the box

 

[Jack Gilvey]

Also, if I understand correctly, I could mount a pair magnet/magnet, with a grill on each side?

? Not sure if I follow. Sounds like you're describing a rather wasteful dipole sub.

If both cones are exposed to the outside, you don't have isobaric. If given two drivers, true isobaric will approx. half the Vb required for a given Qtc. It may also "lessen distortion" some but, given that the one exposed cone will have to excurse twice as far as two drivers mounted conventionally for a given SPL, you may well up with more distorion.
The only "advantage" is less space at the cost of efficiency.