Isobaric IB – thoughts?


  • Participant
    moonfly on #1373

    OK, so I just posted this on another forum, but Im interested to see what thoughts John might have on this –

    One thought I have had is of a sort of Isobaric IB design. I think I can only create enough cabinet space to give me 4 x the Vas of 4 drivers. Power is a non issue as I have a 4 channel power amp that can give around 1KW per channel and I dont need anything like that kind of power. Isobaric should have the approximate effect of doubling the internal volume of the cabinet, or rather halving the required cabinet volume required. The net result should mean I end up with roughly 8 times the Vas in terms of effective cabinet volume. If I can only use 4 drivers anyway, then I go to pondering on using the other 4 by implementing the isobaric design. This should lower system Q for a given internal volume, and the extra power needed is a non issue as these drivers only need 50-100 watts each anyway. I realise I would essentially be limited to what is a 4 driver system, and potentially only a dual sub setup if I end up with a pair of opposed manifolds. In theory, the design could work though, but I know of no one thats ever tried anything like this and think that potentially the results could be a mystery. I kind of like the idea of doing something totally unique though, and I could even mount the drivers facing each other so they operate in a push pull alignment. Theory suggests this sort of setup would give me the most accurate sound quality and bass response imaginable.

    Any thoughts?


    Member
    Stereodude on #11432

    @moonfly wrote:

    Any thoughts?

    Why bother? Seems like a waste to double the number of drivers for very little real benefit. There’s no box… I’d certainly try it without the isobaric design first.


    Participant
    stryke on #11433

    In reality, the 10x Vas number is a theoretically perfect point. In reality it is not a necessity. Here is a comparison of the IB15HT-8 modeled in red with 1x Vas, orange with 4x Vas, and green with 10x Vas. You can see that there is a big jump in low end efficiency with the increase to 4x Vas, but from there to 10x there is very minimal gain. Even down at 10hz there is only .66dB of difference.


    Participant
    moonfly on #11434

    Stereodude – I have 8 drivers, but don’t have the capacity for a true IB install so the install would essentially be a sealed cabinet, albeit a very large one. As a result I only have enough cabinet volume to accommodate 4 drivers and attaining 4 times the Vas of those 4 drivers. This leaves me with 4 spare drivers anyway, so on the flip side, the argument might be, why not use them anyway. Power is a non issue as I have loads of that.

    John, I realise the spl output figures aren’t really the deciding factor in this, what I’m more concerned about is that a 4 driver system in only 4 time Vas could result in a boomy system. The system Q should drop with the isobaric design if the cabinet volume remains unchanged, so I was thinking that as I have the extra drivers anyway, why not try the isobaric. I’ve also read the isobaric design lends itself to more accurate high quality bass, which is always desirable for me personally.


    Participant
    stryke on #11435

    Take a look a the modeled curve included. If you have 4 drivers with 4x Vas, there is no concern over things being boomy. Realize these are drivers with a large Vas already so this is less critical than a driver with much smaller Vas. Also a room is not air tight, will have leaks, and walls are not fully rigid so it won’t model like it is exactly the size it is.

    You say you already have 8 drivers. If you had around 7000L you could easily fit all 8 in with 2x the Vas. Qts is around .73 assuming the room is perfectly sealed and air tight. This is very close to the Qts of .7 of the driver. You can never go lower than that. With 2x the Vas you are already getting very close to “infinite” numbers. This is a well damped system that will not be boomy at all.

    In reality the room will appear even bigger because there are doors, windows, duct work, outlets, etc that would create losses. I know over on the cult forum they recommend the 10x Vas but it simply isn’t needed. Your main benefit will be in using all 8 drivers to get more efficiency, more headroom and lower distortion at any given SPL level.


    Participant
    moonfly on #11436

    Thanks for the input. FYI, I’m in the UK, the room wont be ducted and is essentially concrete construction. The room wont be totally airtight but the subwoofer cabinet likely would. I realise I cant get below the driver qts, and I’m fine with that, I just don’t want it raising up excessively. I run and Anthem avr with arc2 and own and Onkyo with xt32 as well having REW and a mini dsp. I well covered in terms of eq, I just don’t want a high q system. If I can get to say 10,000 litres, are you recommending I go for an 8 driver system. Do you think the isobaric design would improve quality? Could, or should I test a single driver I’m say 1000 litres and evaluate that?


    Participant
    stryke on #11437

    There will be only a small difference in the Qtc of the system. The isobarik design can help with motor non-linearity and inductance non-linearity. Both of these are already addressed in the motor design of these woofers though. Any benefit of the isobarik mounting will be small compared to the lowering of distortion you will get from using more drivers. Each woofer has to move only half as far for the same SPL level then which is a massive benefit for lower distortion.


    Participant
    moonfly on #11438

    While pondering on this, Ive had another thought cross my mind. The LLT design has always been good for efficiency and low end output, do you think the IB drivers would work in that scenario, but isobarically loaded. This would give low end output from 4 drivers akin to an 8 sealed driver IB, more in fact under 15hz. Tuned low enough, to like 10 hz, Im thinking it could work, and again with the cabinet space saving attribute isobaric loading can bring.

    Any thoughts on that John?


    Participant
    moonfly on #11439

    Any thoughts John? Im considering selling my drivers off and just purchasing some AV series woofers, but want to discount all options first.


    Participant
    stryke on #11440

    The motor strength of the IB woofers is just too low for use in a vented enclosure. The IB18’s in the infinite baffle even with only 1-2x Vas would typically be preferred to a group of AV15’s in sealed enclosures.


    Participant
    moonfly on #21841

    Thanks for the input John, I think I may have come up with a solution that will allow the proper IB install. If the build goes ahead Ill post on the boards here.


    Participant
    moonfly on #21842

    I do actually have one decision to make before I go ahead I could use some input with. Im thinking I will have roughly 7-8000 litres capacity give or take. I have 8 drivers, so the question is, how many should I use in this volume of space? Obviously If I wire up 8 I can always remove some and blank off the baffle spaces, but Im interested in your thoughts.

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