Isobaric sub - bipole? Dipole?

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mcgsxr

Isobaric sub - bipole? Dipole?
« on: 9 Apr 2007, 01:41 pm »
I have been using OB 12's for the better part of 6-8 months now, and I do enjoy the natural bass sound.  I miss the air movement capabilities of my old ported 12 though, so I am starting to get that bass itch that always comes back to me!

I have a quick question, about what this configuration would model like?

If I build an isobaric sub - ie an active driver on each end of a tube - since it operates 100% in both directions (since the woofers are out of phase), does it model like a dipole OB 12?

Anybody have any ideas on this, or push push type subs?

JohninCR

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Re: Isobaric sub - bipole? Dipole?
« Reply #1 on: 9 Apr 2007, 04:53 pm »
Assuming cones are facing out and wired out of phase, it's a dipole with path differential, D, being the tube length.  I doubt it will satisfy your bass thirst, unless the tube is long with one end facing you.  Also, that's actually push/pull, but I'm not sure you get the full distortion benefit since the baskets are enclosed.

Cones out and wired in phase is a push/push sealed bipole, but in the bass region the only difference from a monopole is the vibration cancellation.  It requires double the box volume of a single driver in a sealed box.

You might want to try a U before putting the second driver on.  Just stuff some pillows or something soft in the tube.  To my ear, U's have a sound somewhere between dipole and sealed.  Same natural quality as open, but more umph.  That may be because of the added extension, or according to JohnK, they pick up room gain.

How high do you need them to play, and what size pipe are you thinking?

mcgsxr

Re: Isobaric sub - bipole? Dipole?
« Reply #2 on: 9 Apr 2007, 05:54 pm »
In terms of how to configure the subs, I was thinking piggy-back - so cone out, then cone to magnet, and magnet out in rear.  Out of phase, so push, push.

The recommended box volume for a single MAW-12 is 1 cubic foot.  As I recall, when you isobarically mount subs, you can 1/2 the volume of the box, so very short box - 12 by 12 by 7 or so.

Since the local HD carries 12.5 inch sonotube, I would simply create several end pieces to mount the subs in.

In terms of how high to play them, well, not higher than 150Hz, that is the upper limit of my sub Xover.  I currently run those 2 MAW-12 in OB, Xover at 75Hz, 12db/octave.

My other option would be to buy 2 more MAW-12 and do a Quasar type baffle.  Of course, I could always eBay the current MAW-12 and pick up a quartet of the MAW-15...

http://www.mach5audio.com/product_info.php?products_id=53&osCsid=6afc1

tubamark

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Re: Isobaric sub - bipole? Dipole?
« Reply #3 on: 9 Apr 2007, 07:47 pm »
mcgsxr,

You won't get any more (excursion limited) output than a singe driver in middle of same tube, as an H-frame.  For this reason, the alignment has been unpopular.

However,  I've tried this alignment anyway to test something.  For my tests I've done 'em faces-out, totally symmetrical.
The following is important:
IF you size the tube such that the rear cone-to-cone distance inside is HALF that of the cone-to-cone distance around the outside of the enclosure,  you effectively cancel most of the F-peak *** [credit for this discovery goes to Larry Selmer].  For my testers, the peak measured only about 2db, compared to the 6-9db on H/U alignments.  That's it! - No pipe resonance(s) to deal with at all.   Pressure inside the enclosure is effectively zero, even when driving like mad.   I've popped off the terminal cup and put my ear in there just for fun.   

Additionally, the rolloff on the low end is less steep.  If aligned as above, you will mitigate much of the Lambda/2 effects that dipoles must always address.

I'm currently enjoying w/inexpensive 10's.  Works very well, VERY clean.  'Just not as loud as the same 4 woofers would be in, say, double-stacked U-frames.  Now I'm thinking of making my enclosures convertible . . .

-- Mark

JohninCR

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Re: Isobaric sub - bipole? Dipole?
« Reply #4 on: 9 Apr 2007, 08:11 pm »
Cones facing same literal direction at the ends of your pipe wired out of phase, is a sealed box, not isobarik, requiring twice the enclosed volume.  It would also be push/pull (one driver "pushes" out of its frame while one "pulls" into it's frame), so your gains the distortion cancellation of push/pull and mechanical cancellation.  Isobarik to achieve half the box size is two drivers working together to push 1 cone against the airspring of the box, which cuts the Vas in half so it requires only half the box.  Clamshelled is typical if you can handle an exposed magnet, and they are wired out of phase, and the benefit is smaller box and push/pull, but requires more power for same output.

Wire yours on the ends of the pipe in phase and you have a dipole, and Tuba Mark is exactly correct, no pipe resonance, but same size and output as an H or a properly damped U of half the size.

mcgsxr

Re: Isobaric sub - bipole? Dipole?
« Reply #5 on: 9 Apr 2007, 08:36 pm »
Sounds like tail chase to me then, vs an open alignment.

Now, if I could only justify selling the 12's, and buying 4 15's!

JohninCR

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Re: Isobaric sub - bipole? Dipole?
« Reply #6 on: 9 Apr 2007, 08:57 pm »
Try just a 12-18" length of sonotube behind your baffle surrounding your bass drivers.  Cut the terminus at an angle, say 3" longer on the bottom than the top, and stuff the cavity.  Your B200's play plenty low enough to keep the woof's out of resonance range.  Best guess from looking at the pics of your baffle in your gallery, is that you'd gain close to a full octave at the bottom (equivalent to doubling your baffle), even if you lopped off a big portion of your wings.  You can test it cheaply, just a piece of sonotube, without any permanent baffle mods.