isobaric loading: midrange problems?

[mondogenerator]

does anyone have experience with isobaric loading? basically i was considering that midrange response MAY be worsened over a single driver due to sound passing thru the 'front' speaker cone, from the rear of the pair. Has anyone used isobaric loading for a 2 way, where the xover low pass f3 is around 2kHz or so, and if so, was this the case?

Also, is there any difference, subjective or otherwise, between parallel and series wiring of the isobaric pair? id assume parallel wiring, but wondered what effect series wiring would have upon the isobaric method, as i understand it would affect BL and Qes, as in a dual VC woofer.

[Glowbug]

I don't really see the reason why you'd want to do it...reducing enclosure size isn't as important as it would be for a subwoofer enclosure (and even then, isobaric isn't as popular as it once was with the current crop of small-box drivers).

[Cal Weldon]

My thoughts too. Whatya thinkin' mondogenerator?

[MJK]

Originally Posted by mondogenerator

does anyone have experience with isobaric loading? basically i was considering that midrange response MAY be worsened over a single driver due to sound passing thru the 'front' speaker cone, from the rear of the pair. Has anyone used isobaric loading for a 2 way, where the xover low pass f3 is around 2kHz or so, and if so, was this the case?

Also, is there any difference, subjective or otherwise, between parallel and series wiring of the isobaric pair? id assume parallel wiring, but wondered what effect series wiring would have upon the isobaric method, as i understand it would affect BL and Qes, as in a dual VC woofer.

Isobaric loading is great for low frequencies since it will cut the enclosure volume in half. If you are using big drivers (> 12") this is a real advantage. At one time I had MathCad worksheets for drivers set up in an isobaric configuration and designed a ML TL for somebody using 13" Focal woofers, it worked very well.

You get into trouble at higher frequencies. Basically the small volume of air between the drivers acts as very stiff spring at low frequencies coupling the driver cone masses. But as you go higher in frequency you start to get additional resonances when the two drivers interact with this air spring. You really want to roll the isobaric configuration off before you reach into the midrange.

There was a difference between parallel and series connections, my recollection was that parallel had an advantage but I do not remember the details. It was almost 10 years ago that I looked at isobaric loading and did the Focal design.

[mondogenerator]

thanks everyone, i thought i had omitted the resonances caused by the difference in driver distnaces and their effact on midrange reproduction..

basically, i figured that as the volume is halved for and isobaric design, i COULD effectively use 2 pairs of isobarically coupled drivers(in series parallel to keep impedance the same as one), and in the same volume box as one driver also.

this could then form the '0.5' part of a 2.5 way design in a MLTL/BR which i was planning to use just as a 2 way. this is hoped would compensate (if i design the crossover right) to compensate for baffle step losses. Also, since midrange is an issue with isobaric designs, the same driver type will be used in the 'mid' section, and likely in sealed box form, non parallel walls etc.

this config would yeild a box of 12 litres for the iso quad '0.5' section , and a 5 litre(maybe more to lower Qb to less than 0.7).

however i have doubts as cone area isnt really improved as much as if i were to use a larger driver. plus i would have to consider where to cross the mid woofer HP filter to match it with the 'sub' section. i guess since the sealed box should roll of at about 12dB/Oct then i could leave it unfiltered, but i suspect this driver will be the first to complain about high volumes!!!

the major consideration i had was the doubling of BL, which to me seems to be a big benefit.

[chris661]

There's no need to put them into "clamshell" configuration - have the speakers mounted in a sealed box, which is part of the bigger box. Put the first speaker in normally, then the second one firing into the magnet of the first driver. Wire them in phase. It's still iso-brarik, but without the midrange problems.

[SY]

I had some speakers that used Dynaudio 30W54 in isobaric loading, but ran them right up into the midrange. No problems- but they used a trick to get around the issue. Front driver mounted in a smallish subchamber on the front wall. Back driver mounted on the bottom wall, i.e., 90 degrees with respect to the front driver. And the back driver fired into the subenclosure through a Variovent. This way, the speaker was isobaric at low frequencies, but not in the midrange. Tradeoff was some extra volume, but the bass quality was some of the best I've ever heard.

[MJK]

Originally Posted by chris661

Put the first speaker in normally, then the second one firing into the magnet of the first driver. Wire them in phase. It's still iso-brarik, but without the midrange problems.

The midrange problem is not solved by doing this, the bigger the trapped volume of air between the drivers the lower in frequency the system departs from the theoretical isobaric behavior.

[mondogenerator]

the sealed midbass/highs enclosure will be Qb = 0.7(ish) with an F3 of 90Hz(again ish)
unfortunately for me, a 2nd order LP for the quad iso sub box is a little annoying, with all the high values of C and L required, and as im looking to include or at least apply a little BSC maybe the LP will cross at about 150 instead, or maybe the components just adjusted to yeild a lower than 12dB/oct but higher than 6db/Oct slope.

the quad isobaric chamber will be formed with front to back speaker mounting, less than ideal but better aesthetically and eaiser to realise in construction(at least without silicone-ing the magnets of each woofer together so that the pole piece vent is also coupled to its partnering driver, which i WAS planning to do!)

this sub box will be either build in BR(though i have some doubts about port diameter and whether it would be too noisy), or MLTL: 20 to 1 ratio taper.
thats if its practical at least. have cal'd it for ONE driver but with 4 in iso pairs i think ill have to remodel it!!!!! due to BL more than anything else i guess.