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Old 13th June 2008, 05:35 PM   #1
griffon is offline griffon  United Kingdom
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Default isobarically extending bass on fullrange driver

My theory to be ripped to pieces today is;
taking say a Lowther full range driver like the DX55 or PM4A which have possibly the straightest responses out of the Lowther drivers and placing them in a sealed enclosure which gives them a relative Qtc of 0.6.
Then putting a subwoofer with a matched air displacement to the Lowther (so possibly another lowther, dread the thought of the cost) in the chamber isobarically, and in close proximity to the first cone. the internal cone would have base above say 70hz reduced by possibly even first order crossover to stop internal midrange and hopefully achieving a fairly linear base response if figured out.

benefits
As far as i can tell the base in a horn or transmission design can't be in perfect phase as compared to a sealed or infinite baffle because of the extra travel needed before contact with the ears. infinite baffle bass would be in perfect phase with the mids and treble when using a single driver. Essentially using an isobaric driver set up turns the sealed enclosure into a simulated infinite baffle enclosure in respect to Qtc values in the bass spectrum.
Isobaric is supposed to produce a resonant frequency that is lower than that of the single driver by a factor of .707 in a similar sized enclosure space.
I also wonder whether the decibal rating of the sub base would be increased into perhaps a far more linear frequency response with correct tweaking.

Ideas?
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Old 14th June 2008, 04:08 PM   #2
mightym is offline mightym  United States
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Griffon,

I don't even pretend to Know enough to comment on your theory.

I have looked a a similar layout at this link:www.artec-france.com/etudes.php?lang=en

I don't know if this will help or not, but it seems like an interesting idea.

John
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Old 14th June 2008, 04:10 PM   #3
mightym is offline mightym  United States
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OOPS, It's the compact system at the aforementioned link

John
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Old 14th June 2008, 04:37 PM   #4
DaveCan is offline DaveCan  Canada
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I think in the pic you attached the drivers are to far away from each other and may not work like that??, I'm no expert though...
Dave
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Old 14th June 2008, 07:56 PM   #5
Henkjan is offline Henkjan  Netherlands
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Default Re: isobarically extending bass on fullrange driver

Quote:
Originally posted by griffon
Isobaric is supposed to produce a resonant frequency that is lower than that of the single driver by a factor of .707 in a similar sized enclosure space.
yes, but with a much lower Qb and an f3 that is probably higher or at best the same

for lower bass, isobaric is not the way... (for smaller cabinets it is)

but do I understand you correctly that you want to put another type of driver inside?
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Old 14th June 2008, 11:12 PM   #6
griffon is offline griffon  United Kingdom
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I am guessing that to put a different driver in would not really work because they would not match in the speed and x-max perfectly, especially given the very fast nature of the fostex.

now if it works better for a smaller cabinet why wouldn't putting an isobaric configuration in a large enclosure drop the base even further than without of the same size.

the aim for this setup I am suggesting would be to attempt to drop the resonant frequency of the full range cone making it even more full range. like closer to twenty something hertz.

the drawing was just to give an idea and is not to scale but you would probably want the internal cone directly behind the external one, cone to magnet. there have been isobaric setups similar to the one I drew though.

In all honesty the theory definitely needs testing because a single cone that could cover a frequency from <30 to 20,000hz would be incredible to hear.
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Old 15th June 2008, 02:35 AM   #7
GM is online now GM  United States
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Quote:
Originally posted by griffon

In all honesty the theory definitely needs testing because a single cone that could cover a frequency from <30 to 20,000hz would be incredible to hear.
Indeed, the late Babb Lorelei with its 1" (1.2"? I can't remember now) linear stroke could actually do 20 - 20 kHz in only a 1 ft^3 sealed cab at loud enough levels that in a side by side comparison in a fairly large room at a ~16 ft listening distance, a pair was still sounding composed when Jim Griffin's original Linus Arrays were in audible distress.

Supposedly it's being redesigned by the group that bought out Babb, but it's been several years now and no announcement that I'm aware of............

GM
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Old 15th June 2008, 02:59 AM   #8
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Isobaric loading does nothing to change the resonant frequency of the driver afaik. What it does allow you to do is make the box half the size of a box using the same two drivers in non-iso configuration but it won't go any louder than a single driver since the displacement is the same as a single driver. AFAIK, economy of space and possibly distortion reduction are the only benefits of an iso load.

Now you CAN use a box 2x larger than max flat alignment would be, and you can do that with a single driver OR an iso loaded pair and the results will be the same. The resonant frequency of the driver doesn't change so much as the tuning frequency of the box. You can make the box as large as you like and you can tune it as low as you want, but it won't be flat response, you will end up with an extended bass shelf type response. The lower you tune it the less power it will handle due to increasing excursion demands at lower frequencies.

All of this can be modelled very quickly with winisd and a number of other free programs.

OTOH, once you start playing with drivers that have 1 inch stroke, like GM mentioned (and maybe a touch of eq for good measure) you can force them to do all kinds of tricks.

Based on the questions you've asked in this thread and the other one, and the amount of industry buzzwords and you've been tossing about it sounds like you've been reading the forums I(or reviews) a lot. Some people here are like walking textbooks with built in calculators and some (like me sometimes) are just plain wrong (or purely subjective). It takes a lot of research to figure which is which and forums and reviews (with so many subjective opinions) can be a tricky tool to handle properly. I'd suggest a few good hours with some good software simulation programs and technical papers to gain a firm grasp of the fundamentals (which I have to admit I haven't even sorted out myself - so I should probably follow my own advice).

Anyway, good luck with this very ambitious project.
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Old 15th June 2008, 03:22 AM   #9
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If you make up a compound driver like you propose
out of two identical ones and wire them in parallel the
following happens:

Moved mass x2
Compliance /2 -> Vas /2
Power Efficiency /2
Impedance /2

Voltage Sensitivity stays the same.

Advantage: You can build the compound driver
into a cabinet of half size, for the price of double
current with same voltage.

Qb does not change, if cabinet is half of the size for
the single driver. No change in bass performance at all,
just a reduced size cabinet.

----------

DX 55 e.g. is a high fs, low Qts, low excursion driver.

The only way to get some bass out of it (if you don't want to
build a horn) is a properly designed bass reflex cabinet or
(better) crossing it with a dedicated subwoofer.

If you want to use 2 ones, a 1.5 way design might be
possible using a lowpass filter for one of them.
Both drivers mounted in a BR enclosure ontop
of each other, both facing the outside air.

This can increase sensitivity in Bass by 6dB without
changing excursion, because radiation impedance
of the drivers add up and you drive twice the power
through them in the low bass range.

The enclosure would have twice the volume as an
enclosure with same alignment for one driver.

Since the XO frequency has to be set fairly low, it is
difficult to manage this with a passive XO.

You need very low resistance inductors not to
increase the Q of the lowpassed driver excessively.

Maybe it is useful to tolerate some resistance in the
inductor and compensate that by adding a series
resistance to the unfiltered driver to tune both to
the same Q ...

An active XO would be recomended to circumvent these
problems ...

Cheers
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Old 17th June 2008, 04:20 PM   #10
badman is offline badman  United States
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Default Short Version

This won't work with a full range driver. You'll have reflections through the cone and and cancellations and all sorts of oddball effects. Basically, it'll sound like trash.
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