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  1. #1
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    Default Isobaric enclosures..what advantages if any?

    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.

  2. #2
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    Default Re: Isobaric enclosures..what advantages if any?

    Smaller box.

  3. #3

    Default 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.

  4. #4
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    Default 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?

  5. #5
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by bill poster View Post
    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.

  6. #6
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by bill poster View Post
    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.
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  7. #7
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by bill poster View Post
    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.

  8. #8
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    Default 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?

  9. #9
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    Default Re: Isobaric enclosures..what advantages if any?

    Yes
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  10. #10

    Default 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):

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  11. #11
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by Dark Regent View Post
    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.

  12. #12

    Default 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.

  13. #13
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by Dark Regent View Post
    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.

  14. #14

    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by Jeff B. View Post
    The M&K of today isn't really the same company as the Miller & Kreisel that invented the modern subwoofer.
    That was back when "subwoofer" was not a type of loudspeaker driver, but rather was a separate enclosure into which a woofer had been assembled.

    Blast from the past:
    http://www.kreiselsound.com/
    "Our Nationís interests are best served by fostering a peaceful global system comprised
    of interdependent networks of trade, finance, information, law, people and governance."
    - from the 2007 U.S. Naval capstone doctrine
    A Cooperative Strategy for 21st Century Seapower
    .

  15. #15
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    Default 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.

  16. #16
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    Default 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?

  17. #17
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    Default Re: Isobaric enclosures..what advantages if any?

    Parallel ( assuming they are 8 ohm drivers )

    BTW: My recollection of Isobaric is the same as Jeff: There is constant pressure within a common body of enclosed air adjoining one side of each diaphragm
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

  18. #18
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    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by craigk View Post
    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.
    And if it were not isobaric, it would have been almost twice as big. Right?

  19. #19

    Default Re: Isobaric enclosures..what advantages if any?

    Quote Originally Posted by Sydney View Post
    Parallel ( assuming they are 8 ohm drivers )

    BTW: My recollection of Isobaric is the same as Jeff: There is constant pressure within a common body of enclosed air adjoining one side of each diaphragm
    A couple of things: First, an "isobaric" arrangement is an vented box is not close to the same thing as a sealed box isobaric system. Second, and you can look at my web page about this, the pressure in the "chamber" is not constant at all. It acts like a spring connecting the drivers. Think about it. For the sealed box arrangement, if the front driver is to move as if it is in a box twice as big as the rear box, then the pressure on the back side of the front driver must vary in the same manner as the pressure in the bigger box would. Then, since the rear driver moves just about in the same way, but with a little phase lead, and since the rear box is 1/2 the size of the normal box size, the pressure in the rear box varies twice as much as in the larger box. The result is that the pressure difference between teh front and rear of the rear driver is the same as that as it would be in the bigger box. So the pressure in the isobaric chamber basically varies in the same manner as that of the rear box, but with 1/2 the magnitude. It is the 1:2 difference in pressure that results in the apparent reduction of Vas by 1/2.

    In the sealed box system, the front and rear drivers are connected by a spring and the rear driver is loaded by another spring (the rear box). In a vented system the front and rear drivers are again connected by a spring but the rear driver is loaded by a mass (port) connected to the rear driver by a spring.
    John k.... Music and Design NaO Dipole Loudspeakers.

  20. #20
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    Default Re: Isobaric enclosures..what advantages if any?

    the pressure in the "chamber" is not constant at all. It acts like a spring connecting the drivers.
    Of course air is compressible.
    Harry Olson was working on similar compound speakers with the same design goals. The patent issued and the reference ( the source of my statement ) was to Ivor Tiefenbrum.
    In the abstract of the patent:
    "A bass unit for a loudspeaker system which has a pair of loudspeakers mounted one behind the other in a casing to define a chamber of air therebetween. The loudspeakers are operated in phase with one another so that the pressure of air in the chamber remains substantially constant ( emp. mine );
    In the body of the patent is where the used of the phrase "ISOBARIC" is established.
    The germane point is the chambering between drivers and the goal ( which is different from multiple drivers sharing a cab ).
    While the isobaric moniker given has long been a source of ambiguity it is what it is and continues to be used just like the popular use of subwoofer - rather than a infrasonic transducer.
    We can all relax now
    Last edited by Sydney; 08-14-2012 at 12:27 PM. Reason: add. for clarity
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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