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Isn't a TMM supposed to be 2.5 way?

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  • Isn't a TMM supposed to be 2.5 way?

    I keep reading everywhere that a TMM should use a 2.5 way XO but recently, Jeff B. has designed a few TMM 2 ways and Dayton has the T652 TMM with a 2 way XO also.

    A TMM with 6.5" woofers and a small tweeter would need to be crossed at about 1200hz to keep CTC between the lower woofer and tweeter at or below a 1 wavelength distance.

    Anyone can shed some light on this for me please?

    Thanks!

  • #2
    Re: Isn't a TMM supposed to be 2.5 way?

    Like any ROT, this is another that can be bent depending on a few things. One as you noted is ty he ctc distance. From what I have observed, you can use the center between the two mids as the point to measure as well instead of the individual drivers. Plus the use of the lower crossover point as Jeff is using as well.
    https://www.facebook.com/Mosaic-Audi...7373763888294/

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    • #3
      Re: Isn't a TMM supposed to be 2.5 way?

      Was waiting for someone to ask this.

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      • #4
        Re: Isn't a TMM supposed to be 2.5 way?

        You do not have to go by the ctc of the furthest driver as the two woofers act more like one driver (similar to a line array). I would assume you would consider in between the two woofers as center. Also you can cross higher than 1 wavelength distance you just sacrifice some power response in the vertical plane. This may not make much difference depending on listening position.

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        • #5
          Re: Isn't a TMM supposed to be 2.5 way?

          There I go again, breaking rules. Honestly, despite the fact that I think I introduced the Cascaded 2.5 Topology and I am well aware of many 2.5 way designs, I was surprised to read that you "keep reading everywhere that a TMM should use a 2.5 way XO". I'm not sure who takes these things and turns them into rules. Obviously, the spacing of the driver group and the crossover point are factors in the choice, but the decision is much more flexible than you make it sound. If you do a little modeling you will see what I mean.

          I actually began with a 2.5 Way design for the Solstice, but after having trouble getting a crossover that made any sense in the way the two woofers needed to work together to mitigate baffle step I dropped the idea pretty quickly and optimized a 2-way crossover instead. The 2.5 Way just required larger, more expensive inductors, and greater crossover complexity with no real advantages, or a very small one if there was one.

          If you think about it, for a baffle this size, there is baffle step that rises to a peak around 1 kHz. In order to cross to the tweeter at the low 1.7 kHz that I did (this crossover point was chosen because I decided to work the two woofers as one) the low pass crossover's transfer function has to begin to roll-off below 1000 Hz. If I tried to do this with a 2.5 way I could get very little separation between the two woofers below this point anyway.

          As others have pointed out, working in this range the woofers function primarily as one acoustic source since their CtC separation corresponds to about 2000 Hz and I am working well under that frequency. In reality there will be some lobing effects at vertical angles, but they are small due to the woofer roll-off rate, and all 2-ways deal with lobing in some form anyway. In this case though, I can even control where the null and lobe fall along the axis, which is beneficial too since I targeted a speaker whose listening window is pretty stable and free of nulls for sitting to standing for most people.

          In the long run, with small woofers and lower crossover points, a simple two-way TMM may make more sense.

          Jeff
          Click here for Jeff Bagby's Loudspeaker Design Software

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          • #6
            Re: Isn't a TMM supposed to be 2.5 way?

            I've been curious about this for a bit now. Thanks for the detailed explanation, Jeff.
            Failure is not an option. It comes bundled with Windows.

            We are passionate about great sound at whatever level we can afford, so don't let the audio atheists reduce the experience to a set of numbers and squiggly lines. - jbruner

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            • #7
              Re: Isn't a TMM supposed to be 2.5 way?

              Originally posted by Jeff B. View Post
              There I go again, breaking rules. Honestly, despite the fact that I think I introduced the Cascaded 2.5 Topology and I am well aware of many 2.5 way designs, I was surprised to read that you "keep reading everywhere that a TMM should use a 2.5 way XO". I'm not sure who takes these things and turns them into rules. Obviously, the spacing of the driver group and the crossover point are factors in the choice, but the decision is much more flexible than you make it sound. If you do a little modeling you will see what I mean.

              I actually began with a 2.5 Way design for the Solstice, but after having trouble getting a crossover that made any sense in the way the two woofers needed to work together to mitigate baffle step I dropped the idea pretty quickly and optimized a 2-way crossover instead. The 2.5 Way just required larger, more expensive inductors, and greater crossover complexity with no real advantages, or a very small one if there was one.

              If you think about it, for a baffle this size, there is baffle step that rises to a peak around 1 kHz. In order to cross to the tweeter at the low 1.7 kHz that I did (this crossover point was chosen because I decided to work the two woofers as one) the low pass crossover's transfer function has to begin to roll-off below 1000 Hz. If I tried to do this with a 2.5 way I could get very little separation between the two woofers below this point anyway.

              As others have pointed out, working in this range the woofers function primarily as one acoustic source since their CtC separation corresponds to about 2000 Hz and I am working well under that frequency. In reality there will be some lobing effects at vertical angles, but they are small due to the woofer roll-off rate, and all 2-ways deal with lobing in some form anyway. In this case though, I can even control where the null and lobe fall along the axis, which is beneficial too since I targeted a speaker whose listening window is pretty stable and free of nulls for sitting to standing for most people.

              In the long run, with small woofers and lower crossover points, a simple two-way TMM may make more sense.

              Jeff
              Jeff, I didn't mean to question your design choices or abilities, just trying to understand, and it seems many people had the same interrogation.
              Thanks for the TMM 101 lesson, very useful as usual. I can now go ahead with my TMM 2-way project with much more confidence!

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              • #8
                Re: Isn't a TMM supposed to be 2.5 way?

                There are a lot of these so called "rules" that people think are hard rules when in fact they are just rules of thumb. The important thing is knowing why they are rules of thumb and the tradeoffs you make when you stray away from them. As Jeff said there will be off axis lobing in the vertical axis but he knows that and can control how much there is and where it is aimed. You can break the the rules as long as you can get away with it.

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