Isobaric enclosures..what advantages if any?

[bill poster]

I'm looking at the Wilson Benesch designs and struggling to see how Isobarics offer any kind of advantage over a conventional bass reflex enclosure... maybe someone can explain.
Do they use a different type of isobaric system? .. yes, 2 drive units in the same volume as a BR and offering the same performance but offer 3db less per watt doesn't really make much sense to me.

If the drivers are wired in series is it still 3db down? And regardless of the extra power needed for an Isobaric, does having 2 drivers ultimately offer a higher SPL? Thats the only advantage I can see, if so.

[rpb]

Re: Isobaric enclosures..what advantages if any?

Smaller box.

[Tin_Ears]

Re: Isobaric enclosures..what advantages if any?

Using two drivers in isobaric configuration cuts the Vas in half so you can use an enclosure with smaller volume but it really doesn't work well unless you mount them face-to-face and then the back of one driver protrudes outside the box. The latter configuration is said to help cancel motor and suspension nonlinearities but nonlinearities can also be cancelled by building two enclosures and reverse-mounting one of the drivers. That last configuration makes the most sense to me because distortion is minimized in two ways... nonlinearities cancelled and half the cone excursion for a given SPL. Your subwoofer setup will have twice the box size instead of half but it'll sound better and play louder.:D

[bill poster]

Re: Isobaric enclosures..what advantages if any?

Thanks Tin Ears, I replied bf i read your post so edited So mounting the drivers face to face with one driver facing 'backwards' outside the box.. that box is then half the size? what about the impedance being halved? wd wiring in series cancel this out?

[rpb]

Re: Isobaric enclosures..what advantages if any?

Ok, the enclosure is 50% smaller, but what about the impedance being halved? wd wiring in series cancel this out?

Lets say you model an 8 ohm 15" woofer that alone needed a ported 16 cu-ft box to have an f3 of 14 hz. By going to isobaric, the total box size could drop to about 9 cu-ft. The f3 would still be 14 hz. The impedance would now be 4 ohms. A second advantage would be that each driver gets half of the power, so the thermal power handling is improved. The maximum spl output would be about the same as using one woofer.

[Sydney]

Re: Isobaric enclosures..what advantages if any?

Ok, the enclosure is 50% smaller, but what about the impedance being halved? wd wiring in series cancel this out?

Remember there are 2 parts: the electro and the mechanical.
As stated the Vas is reduced in half which allows a box half as small.
How the drivers are wired affect electrical sensitivity so it's not so really a matter of "canceling out" anything.

If the compound pair is wired in parallel the sensitivity will be the same as a single driver ( sensitivity will go up 3db due to parallel wiring but decrease 3db due to doubling of Cone Mass ).
If wired in series the sensitivity would decrease an additional 3db.

[billfitzmaurice]

Re: Isobaric enclosures..what advantages if any?

struggling to see how Isobarics offer any kind of advantage over a conventional bass reflex enclosure..

IMO, none. They were semi-popular a few decades ago, when average driver Vas was far higher than today, and it could take an 8 cubic foot or larger box for extension into the 30Hz range or lower.

does having 2 drivers ultimately offer a higher SPL?

No. Since only one cone is exposed the Vd of the two drivers is the same as that of one, so displacement limited output is the same as one driver. Getting the net box size down from 8 cu ft to 4 cu ft might be worth paying for two drivers while getting the output of one might make some sense, but getting the net box volume to 1 cu ft versus 2 cu ft, not so much.

[bill poster]

Re: Isobaric enclosures..what advantages if any?

Ok, so it really is only useful if your box volume is limited for what ever reason.

Last question- spl/output is the same(if wired in parallel), so cone excursion wd be the same as a single driver BR?

[Sydney]

Re: Isobaric enclosures..what advantages if any?

Yes

[Dark Regent]

Re: Isobaric enclosures..what advantages if any?

http://www.danmarx.org/audioinnovation/theories.html

There's a bit of a breakdown on this page about the differences in driver mounting. There is also a bit at the bottom of the page highlighting the differences between Isobaric and Compound (Push/Pull) a la M&K. It might be of some interest to you.

Compound (non-isobaric):

[Jeff B.]

Re: Isobaric enclosures..what advantages if any?

http://www.danmarx.org/audioinnovation/theories.html

There's a bit of a breakdown on this page about the differences in driver mounting. There is also a bit at the bottom of the page highlighting the differences between Isobaric and Compound (Push/Pull) a la M&K. It might be of some interest to you.

Compound (non-isobaric):

I would have to take a bit of exception with this. For as long as I have been around (no cracks, please), and in every technical paper I have read, and in every bass alignment / box modeling program that I know of - "isobaric" and compound are exactly the same thing, "isobaric" simply being Linn's name for it. You are simply showing two woofers in the same box. That's all. It is not what has always been known as a compound arrangement.

[Dark Regent]

Re: Isobaric enclosures..what advantages if any?

Yeah the naming is confusing, requires the Push/Pull qualifier. The way I understood it, isobaric was used quite a bit in the past with the drivers wired in phase to make up for the weaker motors of the time. Compound (Push/Pull, Non-isobaric) as explained by Vance Dickason and M&K is achieved by wiring the drivers out of phase in order to cancel out harmonic distortion created by non-linearities in driver operation. Maybe I'm perpetuating a misnomer but it's not something I made up, promise.
It's what I was told when I was trying to clone a M&K sub.

Bassbox Pro allows for modeling of both orientations.

[Jeff B.]

Re: Isobaric enclosures..what advantages if any?

Yeah the naming is confusing, requires the Push/Pull qualifier. The way I understood it, isobaric was used quite a bit in the past with the drivers wired in phase to make up for the weaker motors of the time. Compound (Push/Pull, Non-isobaric) as explained by Vance Dickason and M&K is achieved by wiring the drivers out of phase in order to cancel out harmonic distortion created by non-linearities in driver operation. Maybe I'm perpetuating a misnomer but it's not something I made up, promise.
It's what I was told when I was trying to clone a M&K sub.

Bassbox Pro allows for modeling of both orientations.

No, I just looked to be sure, but the copy of the LDC that I have has the correct discription for a compound - the some one I used (Vol.5 Pg 33). Like I said, everything I have ever seen, including all software equates this with "Isobaric". Vance also distinguishes this from push-pull, which a compound arrangement doesn't necessarily have to be, but can be if desired.

I learned these discriptions 30 years ago from Bullock and D'Appolito and I have never seen them used in another way. If M&K is using it differently then they changed it for their own purposes or didn't understand what they were refering to, with is a distinct possibility. The M&K of today isn't really the same company as the Miller & Kreisel that invented the modern subwoofer.

[craigk]

Re: Isobaric enclosures..what advantages if any?

i am not sure about the benefits of isoberic loading. i know that one of the best sounding speakers i have ever listened to uses it in the design. the totem mani II. do a search on this site. some one posted pictures of the woofer configuration a while back. the amount of very good, clean base is amazing. totem did not try to go small with the cab design. it is a fairly large speaker.

[bill poster]

Re: Isobaric enclosures..what advantages if any?

More questions.. ;)

I'll probably go for a vented Isobaric but will a sealed Isobarik essentially give the same roll-off as a single driver sealed enclosure (volume size x2)?

Just one further question to clarify, do i link in parallel or series to keep the same sensitivity and impedance?