Making an Isobaric Subwoofer from 15" Dayton Tianic MK III drivers?

I've been intrigued by this design for a while, but James Loudspeaker and Pinnacle are about the only ones actively pursuing it. That's not exactly a ringing endorsement for the design. Other then the lack of efficiency, I'd love to know if it's a viable alternative to the traditional front or down firing options.

Unless you have the drivers close coupled as in face-face, you probably introduce more problems than you solve, and waste a lot of space. For the most part this was of interest back in the day when there was less ability to have a woofer built for a specific need. In most cases you are better off spending 2x the money on a better quality driver. In the end it simply gives you a driver with the same Xmax, 2x the Mms, 2x the motor strength, 1/2 the Vas (2x Cms) and the same Sd. In most cases you are better off using a second driver to increase headroom and apply some EQ rather than the attempt to make the box smaller. If you want a smaller box, look for a driver with 2x the mass and more motor strength.

James uses a 4th order bandpass using a PR in their EMB designs, not an isobaric design unless there are other models you have in mind.

I made an isobaric subwoofer a long time ago. I think they're not very popular because in general they don't make any sense. The main advantage is you can halve the box volume for the same response, but you need two drivers. The simplest design methodology means you have to mount one of the driver inside out which 1) isn't very pretty and 2) makes audible air noise from the vent in the pole piece a serious concern.

IMHO, generally speaking you'd be better off choosing a driver more suited for the response you want in the size box you want since you've got 2x the money to play with on a "better" driver.

Submersive uses isobaric designs, and it has to be one of the best subs of its price range period.

It most certainly is not an isobaric design. It's a dual opposed design, but that's a far cry from an isobaric subwoofer.

I certainly agree with the second part of your statement -- that the SubM is a great subwoofer -- but from the sound of Mark's post it doesn't seem as though he's a fan of the isobaric design, so I'm not so sure he employed that in any of his products. Unless I misunderstood what you meant...

Hmm, my understanding of isobaric subwoofer design is certainly lacking.

These are the most common isobaric driver setups. The key is that the two drivers move in the direction at the same time. Meaning the air trapped between them isn't pressurized.

The common definitions have changed over time for an isobaric sub. A typical isobaric design will be cone to cone, push pull, wired out of phase. Less typical is cone to magnet, in phase. Magnet to magnet in the SubM is considered dual opposed, as the cabinet design isn't a typical magnet to magnet isobaric driver configuration.

Not really. Placed face to face or magnet to magnet reverse polarity or facing the same direction with like polarity, the function is the same, halving of Vas compared to one driver. What is often seen are other dual driver alignments incorrectly labeled as isobaric. They do not. Their only advantage is reduced cab size, but output and response is the same as with one driver. They made some sense 30 years ago when very high Vas/Qts drivers could require 12 to 20 cubic foot cabs. With today's driver choices they make no sense at all.

Aren't there (at least claimed) reductions in harmonic distortion in these designs? I thought I remembered those claims from back in the day.

You know let's say you do want to make one. I'd use a piece of 14" PVC bolting frame to frame. Cone to magnet with excursion room of course, and mimic Blues Audio ISO kits Ok so I'm an old skool linear power fan

That's for reverse mounted reverse wired dual driver alignments, not isobaric.

The clamshell isobaric designs made some claims like that because one driver was moving in while the other was moving out so any inconsistencies in the motor were averaged out between the two.

An isobaric only exposes one cone to the air. Using dual reverse mounted drivers to reduce THD requires that both cones be exposed to the air. So you can have an isobaric or you can have a dual driver that reduces THD, but it can't be both.

I'm certainly not an expert but will claim to have built both.

Below is a shameless plug for my own thread.... Sorry don't mean to thread jack.

http://www.avsforum.com/t/1438061/transmission-line-isobaric-subwoofer-for-home

Today's subs with huge magnets make it harder to realize isobaric designs as much volume is consumed buy the rear portion of the speaker.
For the Dayton's, I would favor a baffle made from two sheets of MDF as it looks like they are very good excursion.

Your design won't have lowered THD by dint of the isobaric configuration. That would only be realized if both drivers shared the same rear chamber, one reverse mounted with reverse polarity, with both firing into the front chamber. And it's not a transmission line, which requires the front wave be direct radiating, the rear wave energizing the line. As configured it's a 4th order bandpass.

I don't agree but that's okay. Front and rear wave do combine as the exposed cone is open and direct radiating into the room. See the finished pictures. I debated doing a demo with the front woofer enclosed just for kicks but knew the resulting chamber would be too small even with isobaric configuration.

I will now exit this thread.

I wasn't specifically referring to lower THD. I was referring to a reduction of directional non-linearities in the driver's motor since the two motors are pushing in opposite directions simultaneously and are effectively averaged since they're coupled by the air trapped between them.

That makes it a 6th order bandpass, as the chamber housing the driver has a low-pass function. There won't be any THD reduction, as the front and rear waves combine way too far apart, with a huge amount of phase shift between the two as a result .

More accurately, the fore-aft asymmetries of the driver linearity are canceled, which reduces 2nd order distortion, which is admittedly of lower audibility and concern. Lower concern or not, it does work if the trapped space between the cones is small. The other case Bill is referring to is exemplified in the M&K designs where the bottom woofer is mounted magnet out, while both drivers move out from the box at the same time.

Show off!!!

That's exactly why I said "common definition". Every driver configuration like this, including dual opposed was referred to as isobaric by the general public for years. I got countless rolled eyes and laughs out of people when I would correct them back in the 90's, even "pro" cabinet makers.