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Old 21st October 2004, 05:38 PM   #1
rathek is offline rathek  Poland
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Default Isobarik sub

I`m looking for a basic nad advanced information about
Isobarik ( compound ) subwoofer system
Theories, models, samples, links…. . . . . .

Thanks
Rathek…. . . . . .
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Old 21st October 2004, 06:37 PM   #2
AndrewT is online now AndrewT  Scotland
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Isobaric is very useful for reducing the size of the speaker.
The internal volume is reduced exactly by half compared to the single speaker design. I suppose you already knew this, oops. It is a long while since I did it, but I recall that a few of the Thiele Small parameters are changed to half or twice there single speaker values then plug these new numbers into your calculations. Be carefull that the port is designed using the real size of the box. One thing to point out which I didn't know at the time;- the +-X displacement doubles when you have two motors driving the speaker box. So at high volume or low frequency you may run into excessive distortion if the speaker cannot cope. I tried it on a Tannoy 385 HPD into a 100litre box. 2 speakers mounted face to face completely transformed the bass behaviour for the good, strong undistorted bass that seemed to extend much further down. A successfull experiment that looked really ugly!!!! & sounded wonderfull. By the way couldn't use the Tannoys full range, instead they supplemented Acoustic Energy AE1s
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Old 22nd October 2004, 02:10 AM   #3
mike.e is offline mike.e  New Zealand
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Old 22nd October 2004, 11:11 PM   #4
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Default IsoBaric Bliss

If I had the money, I'd make every sub an Isobaric. Piggyback is indeed the best configuration minimizing the air volume between the speakers and making them a better "single driver." The Isobaric design cancels out some order of harmonic distortion using piggyback design and typically increases linear excursion. Now most of you would think it wouldnt increase linear excursion, but you consider most subwoofers to be too ideal. The fact is, most subwoofers have a higher linear excursion in one direction than another. In some the differences are only slight, In others the differences are greater. For instance, The Adire Audio Brahma 12" Mark I has a dumaz tested BL curve that is +or- 1 N/A from -19mm to + 22.5mm. Therefore the BL curve is only + or - 1N/A from + or - 19mm excursion. If you used two brahma subs isobaric you would get slightly more excursion due to the fact that one of the subs would have an extremely falt BL curve to 22.5mm excursion in or out. The peak-peak excursion would probably increase by 2-4mm in a with the subs mounted vertically (cone facing sideways) and by EVEN MORE with horizontal mounting due to cone sag. Isobaric design also increases the power handling due to the combined power handling of two motors. Isobaric design is very similar to JBL's differential design for motors, but it also balances suspension nonlinearitys. Who really cares if you have to look at the basket and motor of the subwoofer... Just get a pretty one like a Tumult or a Brahma.

Isobaric design, I love it, if only it didnt cost so damn much!

Perhaps a driver manufacture should develop a driver that uses two motors (on oposite sides of the cone) with a single cone, basically isobaric, just subtracting the center volume of air. Using a single cylinder with voice coils at either end as a former, dual spiders, It would be ultra linear.
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Old 23rd October 2004, 04:42 AM   #5
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Bassandy, I must admit you have a good point about driver linearity!

In general I often wonder why people would want to do isobarik, considering that you need twice as much power, spend twice as much on drivers, and the only advantage is a smaller box. It is smaller by half the volume less the volume occupied by the second driver.
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Old 23rd October 2004, 06:31 AM   #6
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Default Re: IsoBaric Bliss

Quote:
Originally posted by BassAwdyO
Perhaps a driver manufacture should develop a driver that uses two motors (on oposite sides of the cone) with a single cone, basically isobaric, just subtracting the center volume of air. Using a single cylinder with voice coils at either end as a former, dual spiders, It would be ultra linear.
There is a commercial speaker out there with just such a beast (at least similar).

Essentially by doubling the motor & the mass one gets a halving of the Vas (at twice the price with no gain in the ability to move air over a single driver). One benefit of the isobarik that is oft forgotten is that the back woofer isolates the front woofer from the air/resonances of the main enclosure, typically yielding less box sound.

dave
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Old 23rd October 2004, 07:15 AM   #7
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Hi,
My speakers consist of satellites in mtm config and two isobarik subs, build with vifa 10" drivers - vented. I run them for the last 8 year. According to my measurements those units go down to 20 Hz at - 6db (measured with behringer deq 2496). The advantage is beside the smaller footprint decreased distortion. Those speakers have slam without any muddiness and the decay projected is that of the instrument and not that of the driver. They are set up to cover the range from 2oHz to 80 Hz via a behringer dcx 2496. I only can recommend the principle. The drivers at the time cost a total of 300$ cancan (4 x 75$). Just try it. Literature is also available from audioexpress/ ex speaker builder. Consult their archive.
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Old 23rd October 2004, 07:32 AM   #8
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Default downfiring isobaric

I love using subs Iso, but it is very costly. You do get significantly more linear excursion with isobaric designs, but the volume displacement will not match using the woofers seperately. Another way to cancel the even order harmonic distortion is to use two subs, one mounted facing in, one mounted facing out. This will not give you any more linear excursion than conventional design, but I've heard it helps with some harmonic distortion. Drivers move in and out differently, everyone knows that. Whether it be from the suspension or from a non uniform magnetic feild it will occur even with the best drivers out there to some degree. Get some cheap subs with high Cms and Mms and mount them clamshell isobaric down firing, and you should get WAY more excursion than you paid for.

"One benefit of the isobarik that is oft forgotten is that the back woofer isolates the front woofer from the air/resonances of the main enclosure, typically yielding less box sound."
-dave-

I have noticed that effect myself, I'm not so sure if its a benefit myself, but it does add a different sound. Its kinda perspective I suppose.
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Old 23rd October 2004, 08:56 AM   #9
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I once experimented with an isobarik setup a few years ago,nothing special just some "junk" I had around..
I used a pair of cheap-o Thump/Anaba 12" woofers (super cheap,and crummy car speakers.) face-to-face (top-polarity reversed) in a large cardboard tub with an open top (upside down,open side on the floor) with a hole in the new top for the speakers. The cardboard tub was about 2 feet in diameter,and maybe 16 inches tall,I think it was some kind of food shipping bin?
It would "walk" across the floor,if you turned up the volume a bit..Made the entire house buzz,and rattle at a higher volume..and it would go down pretty low..I used it for quite a while.. I was mighty impressed for a simple slapped together sub! (The neighbors weren't so impressed however..)
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Old 9th December 2004, 07:29 AM   #10
hclarkx is offline hclarkx  United States
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I can only second all positive comments about isobaric subwoofers. I've never owned a high quality commercial conventional subwoofer, but my guess is that one would have to pay $1000 or more for something that could compete with $500 spent on a home made isobaric subwoofer. My investment was about $200 for a pair of JBL 15" 5 ohm drivers on a Crutchfield close-out and about $200 for a pair of Apex Junior sub amps (about 120 Watts RMS into 5 ohms). The box was a commercial office end-table (the kind you might find in a waiting room) for $35 at a used office furniture place. This "end table" has 1.25" thick MDF walls and top and I put two 3/4" MDF panels in the bottom. It provides 11.5 cubic feet which was perfect for the two JBL speakers according to the free WinISD speaker design program. I put the speakers clamshell in the side of the "box" along with two 4" x 10" ports. Each amplfier is connected to one of the drivers. The two Apex Junior amps seemed to provide plenty of power for the relatively efficient ported design.

The bass sound is incredibly solid. With the Stryke BassZone test CD I can rattle every picture and window and loose item in the room, indeed in other rooms as well. I've seen the Blue Man Group live including their huge drum (about 8' in diameter I would guess) and you can feel it in your chest when it comes through this subwoofer at 100 db++.

I'm currently driving this subwoofer with an Adcom 555II. This amp is supposed to do 300+ watts into 4 ohms (per channel). Hence with one channel connected to each driver, I'm able to put about 600 watts into the subwoofer. I'm happy with it, but can't say it's better than it was with the Apex Junior amps. The windows rattle about the same when the Blue men hit that 8' drum.

I guess what I'm trying to say is that while a home made isobaric subwoofer will cost more than a run-of-the-mill several hundred dollar subwoofer, what you get is the equivalent something that might cost $1000 or $1500.

Incidentally, my son built an isobaric subwoofer using a pair of 12" Polk/Momo drivers from Circuit City ($160/pair on sale) and using inexpensive 100 Watt sub amplifiers. These particular speakers need less than 3 cubic feet in the isobaric design, so his subwoofer is not much bigger than a good sized bookshelf speaker. He used 3/4" MDF braced internally. The sound is truly impressive.

There are lots of subwoofer designs on the internet. Give it a try.

hclarkx
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