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Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

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  • Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

    Long posts can be boring so I'm splitting this up for easier consumption.

    Any interest in passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB?

    Pros
    - no enclosure or baffle of any type
    - any driver will work regardless of t/s parameters (within reasonable limitations and pending further investigation)
    - very easy to time align drivers
    - small baffle (or baffle-less) OB provides better than average chance of making a technically accurate speaker
    - could be an extremely attractive conversation piece

    Cons
    - very low sensitivity
    - very pricey crossover
    - little or no physical protection of the drivers
    Don't even try
    to sort out the lies
    it's worse to try to understand.

  • #2
    Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

    No enclosure or baffle of any type
    This should appeal to people with little or no tools, although ideally some type of skeletal frame should be made to hold the drivers together and upright.

    Any driver will work regardless of t/s parameters (within reasonable limits and pending further investigation)
    I'm only in the very beginning stage of investigating this type of alignment so I could be wrong but...
    It looks like any driver that has enough sensitivity and max spl at the lowest desired frequency will work. (Very low sensitivity drivers and small cone/small power handling/small xmax drivers are probably not a good candidate for this type of alignment.)

    The sweet spot seems to be about 6.5 inches for a midwoofer size, with at least 4mm xmax, with 90 db or more (more is better!) sensitivity. This looks like it will do about 102 db from 200 hz and up (if the driver can handle it thermally), and can possibly be crossed over as high as 3000 hz LR4, which might be possible from a robust but small neo tweeter (also baffle-less). Obviously, larger midwoofers will play louder (or lower) but can't be crossed over as high, so they might need a 2 or 3 inch fullrange instead of a tweeter. 5 inch drivers will be able to cross over higher but won't go as loud (or low).

    Very easy to time align drivers
    Tilt the woofer back a bit (the appropriate amount based on the actual drivers used) and the drivers are time aligned.

    Small baffle (or baffle-less) OB provides better than average chance of making a technically accurate speaker
    About 4 years ago I noticed some rumblings about large baffles leading to less than perfectly technically accurate speakers at Audiocircle (in the OB forum). A couple of years ago, a "naked" baffle revolution was underway at diyaudio, usually using string or rope or chain to hang drivers up without baffles in 4 way (or more) very complicated active systems. There's also info by John K. (and ABC Dipole) that explain why large baffles can be detrimental.
    Baffle-less OB presents it's own challenges but eliminates some obstacles as well.

    Could be an extremely attractive conversation piece.
    Drivers look nice, I think this speaks for itself.

    Very low sensitivity
    This is a problem, especially if you want to make a full passive 3 way (with real woofers for the bass below 200 hz). Using the example above (6.5 inch woofer, 90 db sensitivity) you lose almost 12 db of sensitivity when you equalize flat down to 200 hz with no baffle at all. If you are using an active woofer for the bass below 200 hz then the situation is definitely workable.

    Very pricey crossover

    The slopes are very steep when you try to equalize and cross over an OB speaker, especially one with no baffle and make it work in the loss zone below baffle step. Steep slopes cost $$$. Also, this speaker absolutely needs protection from low frequencies (below 200 hz, ~LR4) built in. Normal bookshelf speakers don't usually have that 200 hz highpass built in, and it's not cheap. The price of the crossover is (more than?) offset by the lack of expense for an enclosure though.

    Little or no physical protection of the drivers.
    These are meant to be sitting on the bass woofer box. If they fall to the floor for any reason something is going to break.
    Don't even try
    to sort out the lies
    it's worse to try to understand.

    Comment


    • #3
      Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB



      Here's a picture of a 6.5 inch driver without a baffle (blue line). The range we are interested in using the midwoofer for is from ~200 hz up to the first big peak (which will be lower for larger drivers and higher for smaller drivers). Usually active crossovers and eq are used to flatten out the response into a usable flat line but it is possible to do this passively. As you can see, the lower you want the midwoofer to play, the more sensitivity you lose - and it goes fast, at 6db/oct. So you could equalize it flat down to 80 hz if you wanted to but the sensitivity will be too low to be practical.

      We can see that when we use the driver in this portion of the diffraction zone (the 6db/oct loss zone below the baffle step) that the diffraction signature is a straight line (not lumpy), so the diffraction nets us a huge sensitivity loss as frequency decreases but a completely straight response curve which can be equalized to a completely flat response. Midbass drivers that we would consider to use in 2 ways are also very linear in this frequency range. That means that since the driver response and the diffraction profile of any given size of round baffle-less driver will be almost identical through the usable response range (with the possible exception of differing sensitivities), all capable drivers of a given size will model very closely in frequency response (with the possible exception of sensitivity) in the loss zone below baffle step.
      Don't even try
      to sort out the lies
      it's worse to try to understand.

      Comment


      • #4
        Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

        The Tweeter

        As I mentioned, I am hoping to be able to get by with a small but robust neo tweeter crossed ~2 or 3000 hz LR4. My initial simulations seem to support that this is possible, as long as the tweeter "sees" the midwoofer as part of it's baffle. We'll see how this works out but I may have to resort to a 2 or 3 inch midrange for a tweeter.
        Don't even try
        to sort out the lies
        it's worse to try to understand.

        Comment


        • #5
          Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

          It was only yesterday that I thought about simulating this alignment so I'm very early in the design process. This concept seems very possible but I could be wrong. If there is any interest we can discuss this further, in more detail, otherwise I will probably just present the project when I'm done if I am able to get workable and attractive results. I'll probably begin collecting measurements in the next few days but I don't own the type of tweeter I'd most like to test at this time though.
          Don't even try
          to sort out the lies
          it's worse to try to understand.

          Comment


          • #6
            Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

            Have you considered using a mini-dsp as an active crossover?

            Comment


            • #7
              Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

              I already have several different software active crossover options so if I were to go active I would use the ones I already have, not a mini dsp. And if I went active I would do a fullrange 4 way, but that's a different project for a different day.

              Anyway, the whole point of this was to avoid a 4 way with active crossovers and all that goes with them in favor of a very simple one amp/one wire plug and play passive speaker. (It needs an active woofer but that's all.)

              I'm personally not worried about the price of the passive crossover, I have lots of parts.

              This could end up looking uncluttered (unlike an active setup) and look VERY attractive. Kind of like a fully functional little sculpture of the hobby. I think they would be super cool little desktop or monitor speakers as well, as long as a little thought is put into their environment.
              Don't even try
              to sort out the lies
              it's worse to try to understand.

              Comment


              • #8
                Re: Passively crossed, ~200 hz and up bookshelf size baffle-less 2 way OB

                Tagman, you are right, the raaaaaaaaaaaaaaaaaaaaaaal would be a significant upgrade but I'm not sure how low it could cross over, and I'm thinking more in the price range of the ND28.
                Don't even try
                to sort out the lies
                it's worse to try to understand.

                Comment

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