When making center channel, do you NEED dual midranges?

Discussion in 'Speakers & Subwoofers' started by JesseW, Jan 22, 2004.

  1. JesseW

    JesseW Extra

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    i noticed that a lot of center channel setups in 5.1 audio have 2 mids and a tweet. Could i just do a single mid and a single tweet?

    Thanx
    JW
     
  2. WayneO

    WayneO Supporting Actor

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    Sure, no problem. You'd want to keep the speakers vertical to keep the sound centered. Most center specific speakers are horizontal and therefore need two mids to keep the sound centered, but you could most certainly put it vertical if you'd like as well.
     
  3. Arthur S

    Arthur S Cinematographer

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    Jesse

    Almost all centers have the MTM arrangement. Actually this is not an ideal design. The problem is that the 2 mids cause some cancellation effects.

    The reason most centers have the MTM arrangement is because it Looks more attractive sitting on top of a TV (long and low).

    The biggest complaint about centers is that dialogue is hard to hear. For this reason I would suggest that you find a rather bright sounding one that will make dialogue easier to understand.

    On the other hand, many listeners just raise the level of the center by 3db so they can understand the dialogue.

    Artie
     
  4. Kevin C Brown

    Kevin C Brown Producer

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    What Arthur said. If you ever read the measurements portion of a midrange-tweeter-midrange designed center channel review in S&V or SGHT, they always comment on off-axis performance. They usually call it lobing, but the mids deconstructively interfere as you go off-axis and you get poor sound quality as a result.
     
  5. JesseW

    JesseW Extra

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    Makes perfect sense, since no two drivers are truely identicle..

    but i wonder: Why do some of even the highest respected (and priced) brands use dual midrange in their center channels?
     
  6. Kevin C Brown

    Kevin C Brown Producer

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    Because they think people don't know any better. [​IMG]

    Seriously, better designs include the tweeter being coaxial with the midrange (Vandersteen does this), or still using two midranges, but limiting the frequencies at which they overlap, or making one a passive radiator. Some makers put a tweeter above a single midrange, and then have two woofers that handle lower frequencies on either side. Improves the situation, but still doesn't eliminate it entirely.
     
  7. Chris A H

    Chris A H Stunt Coordinator

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    Interesting article here on center channel design and a DIY model example.
     
  8. JimmyK

    JimmyK Second Unit

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    Jim


    Interesting. So that's why my NHT Audiocenter2 sounds better than other centers I've tried. It also has the tweeter and midrange drivers in vertical alignment with the woofers on either side.

    JimmyK
     
  9. Mike Dzurko

    Mike Dzurko Stunt Coordinator

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    Chris:

    Good article you posted. I fully agree that either a coax or a vertical stack of mid-tweeter with flanking woofers offers far better performance than an MTM laying on it's side. As long as the crossover to the mid is low enough, the vertical mid-tweeter array with flanking woofers works extremely well.

    As to the original question, the answer is pure and simple, marketing. The form factor of low and wide above or below a TV works much better for most people.
     
  10. Joel()Les

    Joel()Les Agent

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    Sounds like a simple solution to the problem would be to buy an extra matching bookshelf speaker and put it above the TV. If you are already using bookshelf speakers it should match perfectly. One drawback of that is magnetic sheilding. How hard is it to magnetically shield a speaker? I noticed that in Chris's linked article there was no mention of doing anything special for shielding.
     
  11. Kevin C Brown

    Kevin C Brown Producer

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    The other downside of putting a 3rd bookshelf speaker on/above the TV, is that it looks funny to most people. But you'd get the best performance that way too. [​IMG]
     
  12. Mike Dzurko

    Mike Dzurko Stunt Coordinator

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    It is pretty difficult to magnetically shield a speaker after the fact. Best is to use drivers that are already magnetically shielded. They have a second, bucking magnet attached to the back of the regular magnet, then a cup is installed over that. You can get pretty good shielding with just the additional magnet, but not 100%
     
  13. Jon Lane

    Jon Lane Stunt Coordinator

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    Believe it or not, a horizontal MTM that strictly adheres to the principles of odd-order transfer between drivers and that has reasonable alignment of the drivers' acoustic centers can approach +/- 1.5dB across it's axis. That's why centers that are flat (horizontal) are MTM's. Or should be. [​IMG] Lobing and interference occur with other alignments, but not odd-order Butterworth's, IIRC.
     
  14. Kevin C Brown

    Kevin C Brown Producer

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    Jon- I don't see it. Here's an example:

    If I'm exactly in front of the speaker, then I am equi-distant away from both mids, so yes, then I'm OK. But as I walk to either side, the distance between me and the 2 drivers now becomes unequal, and bam, that's where the deconstructive interference comes in. I'm essentiually creating a phase difference between the two drivers because of the now unequal distance to each of them. I think it's a geometric thing, and actually doesn't have anything to do with crossovers, etc.
     
  15. Jon Lane

    Jon Lane Stunt Coordinator

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    Admitted, it's a little counterintuitive, but the theory is published and documented in Dr. Joe d'Appolito's work and others.

    Again, IIRC, additional requirements for this to work include midrange spacing that is close enough for them to sum as one driver at the frequencies they produce and with their acoustic center midway, or right on the tweeter. Once this is done, and when the correct crossovers are used, the system moves towards a point source with minor deviations across a usable axis. I'll also revise my implication that there's no lobing to say that there's usable, minimized lobing.
     
  16. Kevin C Brown

    Kevin C Brown Producer

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    I can live with that. [​IMG]

    Same-o: the "tweeter over the midrange but with two flanking woofers" design also minimzes it, but can't totally eliminate it.
     

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