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Old 10th September 2011, 06:34 PM   #41
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I consider Isobarics ideal for applications where you'd be afraid of burning out the voice coil of a single woofer LOL.

The 13TW5 doesn't have the really extreme excursions of some of their deep speakers, but it's SO THIN! It doesn't have the lowest Fs for a 12" (or 13") and you'd need a lot of them and it would be very expensive, but you could make an amazing isobaric sub that's comparativley small. On the other hand, even one doesn't requre much of a box so there's not a huge savings of space gong isobaric, but I'd want to lower FS by adding some weight to the cones (we are sacrificing efficiency anyway, maybe brush on some epoxy or glue on a metal ring) and come up with an isobaric that goes much lower than stock 13TW5 's would.

Remember that clamshell mounting cancels (or at least attempts to equalize) any directional non-linearities (like magnetic structure behavior differences near + versus - extremes).

It is possible to make single drivers that behave much like the two in an isobaric. Recreating the TS parameters isn't impossible, just an odd request. But recreating the power handling of two voice coils using just one is difficult. I've taken two clam-shelled drivers, carved a connecting plug of light rigid foam, then used spray-in insulation foam to conform and adhere the foam plug to one driver's cone, then sprayed some foam on the other driver's cone and slapped them together clamshelled, wiping any excess. This makes a single driver that's similar to the isobaric, with a bit more cone mass and lower Fs, but the cone is incapable of the usual breakuip modes (can't flutter etc.). You could also lighten the foam plug several ways, or just use a foam ring. Gauss made drivers with two suspension spiders...they did not require the cone surround to keep the voice-coil motion linear. You can take such a Gauss driver and remove the cone, and glue something like a cardboard paper towel roll core from the voilce-coil ring at the dust cap of the coneless driver to a second clam-shelled driver with a cone, and make a dual-motor single-cone driver. You could certainly manufacture speakers with voice coils and magnets on both sides of the cone. There's all kinds of interesting possibilities.

RE: big amp. Comparing two drivers isobaric versus one driver its own box twice as big, the isobaric will require exactly twice the power and the driver cost is also doubled. Comparing two drivers isobaric to the same two drivers each in its own box twice as big, seperated non-iso they are more than twice as efficient at low frequencies as the cone areas couple and deliver more than double the perfomance (though perceived volume change may in DB may not be as great as the acoustic power change). Isobaric is always extremely inefficient and requires much more power than other larger equivalent boxes. Tapped horn designers and ported box designers struggle to improve efficiency to the max, while sealed box designers say the heck with efficiency and isobaric designs say "what's efficiency??"... But isobarics can be small and handle a lot of power, so their output can be startling.
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Old 11th September 2011, 10:44 AM   #42
dangus is offline dangus  Canada
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Why? Because you can burn out two voicecoils at a time with isobaric? You have to give each woofer the same power to equal the performance of the same single driver box. You don't get anything like a real 50% reduction in box volume, due to the space occupied by the tunnel, additional woofer, and wall thickness. Much better to build an extra box and get higher output levels, higher efficiency, and more even distribution of bass in the room. Or, if box volume really is an issue, mass-load a driver (like Kicker did to create the "Solobaric"), or choose a driver which is designed for that size box. Or use EQ to give the too-small box the response you want. (Like Bag End "subs")
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Old 11th September 2011, 01:33 PM   #43
ODougbo is offline ODougbo  United States
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Some good ideas, never thought about connecting the speakers mechanically.

I started off the thread with the thought, what can be down with the inexpensive MCM 8” long throw woofer. The history is that I bought 8 – 10 of them at really low price (around $25). I’ve put them in many cabinets, etc. and built at least 6 isobaric boxes.

This woofer is a great candidate for an isobaric sub woofer for a HT system; e.g. the one on the 1st page is very nice and I have compared it to others, manufactured and so on. Please remember, even if you were to start from scratch, this is a less than $100 project, but sounds so much more expensive.

Not to go on and on, but I had the little monster r hooked up to the stereo in the shop, It’s a little earthquake machine. I did roll it off low 50hz or so.

I finished up the 3rd tunnel config. Box yesterday, I used ½ MDF, but went over the box a box a second time, so the walls are one inch. It was more work/time than I thought it would be, but the results were good and super strong, since I lapped back over all the joints.
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Old 12th September 2011, 12:05 AM   #44
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Quote:
Originally Posted by dangus View Post
Why? Because you can burn out two voicecoils at a time with isobaric? You have to give each woofer the same power to equal the performance of the same single driver box.
You're absolutely right about that, as long as you don't care about box size; the smaller isobaric requires twice the power and handles twice the power. But for a given size box, I would be comparing 4 isobaric drivers against 1 non-isobaric, with the isobaric handling 4 times the power and making more than twice the output. The first time you hear one, it seems almost magical to get that much out of such a small box, until you see the size of the amp.

Quote:
Originally Posted by dangus View Post
You don't get anything like a real 50% reduction in box volume, due to the space occupied by the tunnel, additional woofer, and wall thicknes.
I don't usually use tunnels and prefer to clamshell. The additional driver space comes out of my room, not the box. The internal pressures are greater (maybe 4 times greater if you put 4 times the drivers into the same size box) in the isobaric box, but a smaller box is inherently stiffer and stronger, so it depends on your design...indeed a crappy box won't do, if you're still using large boxes. The driver weight gets to be a lot, so I tend to go for smaller boxes with isobaric.

Quote:
Originally Posted by dangus View Post
Much better to build an extra box and get higher output levels, higher efficiency, and more even distribution of bass in the room.
Depends on your priorities, there's no free lunch and engineering is often about where you want your compromises. Like I said, if you don't care about driver cost and power required, isobaric techniques can make extremely impressively small subs. If you don't care about box size or number of boxes, or efficiency or total output is critical, then isobaric is a very poor choice. I'd never choose isobarics for a large outdoor venue; tapped horns are so incredibly efficient they can easily achieve levels that are just not practical with sealed box isobarics.

Quote:
Originally Posted by dangus View Post
Or, if box volume really is an issue, mass-load a driver (like Kicker did to create the "Solobaric"), or choose a driver which is designed for that size box.
The Kicker is like an isobaric that doesn't handle twice the power, so it is missing a critical attribute. Yes, doubling the cone-to-air surface area gives you even more than twice the output, and the same 2 drivers in an equivalent-Q larger non-isobaric system will give you more than twice the output from twice the cone area to the air. BUT 4 isobaric drivers in the same-size cabinet as 1 non-isobaric will give you twice the cone-to-air surface area as 1 driver in a same-size equivalent-Q non-isobaric system. Isobaric meets a special need, but the masses are definitely rejecting large cabinets. If you mass-load you lose some efficiency and the Q gets sharper in the small box and as you go low the power handling of one voice coil can be limiting; or when you can't put as many drivers in the cabinet excursion becomes a limit. You can indeed use a single driver with appropriate TS parameters to get the same performance as two isobaric drivers in one single driver, but then the power-handling spec for the one driver can be limiting...the two isobaric'ed is like a dual-motor driver which handles twice the power, which I'd actually like to see them manufacture.

Quote:
Originally Posted by dangus View Post
Or use EQ to give the too-small box the response you want. (Like Bag End "subs")
In which case boost makes your power requirement REALLY go WAY up AND your power handling REALLY becomes a severe lmiit, and you just can't get enough excursion. The original Bag-end processed subs used pretty good drivers (nice roadworthy boxes too) but when they operate way below resonance their electronics make the best of it but their electronics severely limit the power down low giving the driver all it can take but protecting it. Isobaric and EQ make a far better combination in a given box size, as the additional drivers handle 4 times the power and have twice the cone area to the air, requiring less EQ and less excursion and handling 4 times the power. Non-isobaric EQ'd subs operating below resonance require drivers with really large voice coils and excursions.

Your points are all perfectly valid choices, isobaric is not for all applications, and there are genuine reasons it's not more popular. There are some jobs they are good for and others where they are just not a good choice at all. It's wonderfully suited for situations where box size and power handling are big concerns but efficiency and driver cost are not. The availability of cheap very powerful switching-mode power amps has forever altered the economic tradeoffs for some applications. I had 2 extra Crown CE-4000 amps (which I had to sell when the job market tightened up and I had to start working contracts instead of regular-hire). Staring at an extra 3,600 watts, suddenly inefficient isobaric starts to look reasonable, as long as you can find two 220v high-amp outlets. Isobaric options also provide a way to have a greater selection of production drivers for a small box.
At one time I had 8 JBL 18" woofers from a PA app, and 4 sealed boxes with 18" cut-outs, and wanted to use them in my living room. The math said the 18's should be used in huge bass-reflex boxes or truly enormous infinite baffles. I doped up the cones with epoxy to add some mass and clam-shell isobaric'd them into the smaller cube cabinets. Sometimes adding mass is still not enough, and it still needs isobaric and EQ. Instead of adding mass, perhaps I should have clam-shelled them isobaric and added ports to make half-size bass-reflexes. But the sealed weighted EQ'd isobaric'd 18's with large voice coils did have usable output incredibly low, down where the reflex would need electronic protection against excursion-limiting. Not that there's much material down there, but the occasional enormous orchestral bass drum was astounding to feel...an entire instrument missing from the orchestra on most systems.

You can make really impressive little isobaric boxes that need a lot of power but handle a lot of power and have surprising output and bandwidth and go low.

You can make really impressive tapped horns that are larger, incredibly efficient, have a lot of bandwidth, go low, and make a LOT LOT LOT more output.

Between those extremes are many alternatives and compromises.

If you want to get creative, an isobaric does not have to be sealed like we've been discussing. You can also make great half-size bass-reflex enclosures with more output (but don't try to drive them too exremely low). Or put clamshelled drivers into various bandpass boxes. Just halve the Vas you give your calculator.

Whenever your calculation shows an ideal box size that sounds ridiculously huge, consider isobaric (or isobaric and EQ, as the additional drivers will handle the extra power required by the EQ). And remember that in a given size box, 4 isobaric drivers instead of one means twice the cone area to the room; more, not less. If you think of the number of drivers as the constraint (what's the best box I can put these 8 drivers into) then you will never choose an isobaric. If you like a sleeper car with a powerful motor hidden in a mundane shell, you'll love a "dense pack" isobaric.

I just got 8 used 12" drivers free, and bought a bunch more used and new ones which match for a really good price, and I'm about to make 6 cabinets with 2 pairs of clamshelled drivers (4 total) in each cabinet. That's going to be 12 drivers on each channel, with 6 visible and 6 hidden; 24 drivers in use and 2 spares on the shelf. I'll be able to stack them or distribute them as I please. Each cabinet will be heavy, yet manageable. My total cost was incredibly cheap, mostly wood and paint (damn good thing, as I just start working tomorrow, finally, so I get to keep my house but I"m going to be starving waiting for my first check). I'm just lucky I already have a LOT of power amps available to drive them. If I didn't isobaric, 24 12"s would be difficult to fit in my room (for comaprison, three Marshall 12AX double stacks would be way too small), and my wood costs would double. The problem I have now is that the sealed-box isobaric/EQ combination is inefficient, and I have to plug a secondary fusebox into a nearby spare 220v. electric clothes dryer outlet to make enough 110V 20A circuits to run the power amps. When you scale up isobarics, the power requirements do become ridiculous, and I'm using FET amps rather than more efficient class-D. And with so many fans in the amps, they have to go in an adjacent room.
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Old 12th September 2011, 01:30 AM   #45
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Just food for thought (wishing we had an isobaric forum...I'll try not to steal your thread):

Trisobaric. Quadsobaric. Might actually have some value when the single-driver box size would be truly enormous.

What happens if you isobaric-mount your passive radiator? Might actually have some use in extreme cases where the weight of one passive cone gets too high for the suspension to control?

What are other interesting ways of connecting the voice coils of two clamshelled drivers, making a dual-motor driver? Maybe bolt them together, cut a hole in one cone, insert and glue in a carbon-fiber tube, then cut out the rest of the cone, leaving one cone with two motors.

How about many small flatish drivers in front of a large manifold to a single very large isobaric driver behind them? Might be able to drive the front drivers to higher frequencies than the rear, getting nearly full-range yet extreme extension. Might be interesting mis-matches of different TS parms to some advantage. Might be interesting to experiment with absorbing some frequency ranges between the drivers.

How about adding a time-delay between the two drivers, to let the rear driver start propagating earlier and the front one to join in-phase...that ought to also allow you run to higher frequencies.

Ultimate Minimus-7 mod might be to bolt on another clamshelled drivers.

Might be a nice mod for any sealed D'Appolito MTM systems to bolt on two more clamshelled mid-bass, if you put a spacer behind the tweeter to keep time-aligned.

Last edited by cyclecamper; 12th September 2011 at 01:34 AM.
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Old 2nd October 2012, 12:28 PM   #46
ODougbo is offline ODougbo  United States
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I gave up on these for while...but revisiting them. I have a friends Adcom 7300 (5 channel) using 2 channels, one per woofer.

They do sound better with the Adcom, getting very nice flowing bass. Yes they do take some extra power; might be why they are dipping down so low (the amplifier is working harder)

It's a nice, easy to build, inexpensive box/woofers in a small package.

I did have them measured, they were flat to 35hz (sitting on sawhorse in middle of room when testing).

Last edited by ODougbo; 2nd October 2012 at 12:35 PM.
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