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Center Channel Design Questions

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  • Mayhem13
    replied
    A horizontal WWTWW works IF the tweeter XO is low enough.....the RS28 and the SB26ATC can do 1khz in a typical home setting.

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  • jhollander
    replied
    With a split chamber for the M's you can design as two woofers. Use the point between the two M's as your x measurement in PCD. if you are doing a 2.5 you are back to breaking out the zma's and reducing the frd's spl.

    Having done a similar center I found no advantage to a .5 x-o with a 3 inch woofer. My opinion would be to take the extra SPL from the M's and make a 2-way.

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  • weinstro
    replied
    Thanks guys.

    If this were a vertical format I would consider pursuing the design on-axis only, but I would still be curious about the impact on vertical directivity.

    However, this design is a horizontal oriented center channel to be installed in a wide room, so off-axis behavior is something I need to at least examine.

    The sealed chamber for the W4-1757SB splits the vented enclosure for the Ms in 2. Each side has the same volume and an adjustable length slot port to allow adjustment of the box frequency for each pair independently. You could think of this as two (FR)MM with 2.5 way cross, mirrored horizontally about the full range driver.

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  • Chris Roemer
    replied
    If you DON'T need to see response off-axis (instead, take the easy way out), 4 woofers (wired series/parallel) should be the same FR as a single unit (but +6dB), and should have the same Z as a single unit (modified on the bottom (box rolloff) end - although that "box modified" impedance shouldn't really have an effect on your XO design - except for whatever you're doing for BSC, maybe).

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  • jhollander
    replied
    The FRDs can be taken as an average then adjusted down by about 6 dB (I don't have the exact formula in front of me) but the total combined response needs to match the measured combined response. Once you have the individual driver responses (ZMAs are not required) you can derive the Z.

    As the Ms are likely in the same cabinet you need to break out he ZMA's using a modeling program. The benefit of all the extra work is to be able to use PCD to move off axis and look at the response.

    The short answer is with all the drivers in the same cabinet you should tie together and test, long answer above is why you might want to break out the responses.

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  • weinstro
    replied
    Relative Acoustic offsets....

    For my design work to be sufficient, I need to determine relative acoustic offset between the ND91s and the W4-1757SB.

    My question is this: for an MMTMM design, should I tie all woofers together and test as if this were a single average woofer?

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  • jhollander
    replied
    Ouch

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  • weinstro
    replied
    So, I managed to tear out about 25% of the plywood finished face when routing the ends square.

    Where do I register for my free T-shirt?

    Box is otherwise ~90% complete.

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  • weinstro
    replied
    Both, although the cabinet is getting to the point where measuring will be upon me soon. Another learning curve...

    I keep finding myself in scenarios where the common baffle diffraction and room boundary simulators run into some limitations. So, measurement capability is pretty much a requirement.

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  • jhollander
    replied
    Sounds like a good plan to me. Are you going to measure or do simulations?

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  • weinstro
    replied
    Out of curiosity, I modeled the as-built box in HornResp, Unibox, and Transmission Line (Leonard Audio). The schematic for the HornResp model didn't really reflect what I built and I was unable to configure the model for a better fit, so I set it aside.

    Unibox predicts a small resonance at around 730 Hz (shown below) due to the port, but this package doesn't account for acoustic wave effects.

    Transmission Line predicts a more aggressive scenario. What's neat about this package is it will model stuffing densities. Most of the resonances can be damped out, but not the resonance associated with the port at 750 Hz or so. This shouldn't be a huge surprise as I don't intend to stuff the slot port.

    I'm hoping to cross from the ND91's to the fullrange driver at 800 Hz or so, but that remains to be seen (and heard). So, what do you think?

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  • jhollander
    replied
    I usually stuff all my enclosures as I worry about reflected sound inside the cabinet coming through the cone. For vented I keep the path to the port free then stuff to catch the rear wave. May be only an inch or so of batting stapled on top of Roxul or fiberglass at that short depth. Sealed I like layers of different densities finishing with poly batting near the magnet if the pole is vented. If you did make a MLTL might be difficult to eliminate those resonances.

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  • weinstro
    replied
    Lining and Stuffing-

    I'm in the process of completing the cabinet, and the height has come in at 4-3/4", a bit over 5" deep, and 47" wide. As a reminder, this is an MM(FR)MM center channel using ND91 woofers and a Tang Band W4-1757SB for tweeter/mid duties. The woofers are slot ported while the TB driver is in a separate sealed chamber.

    I'm thinking I should line the inside of the cabinet with something, but I'm wondering what folks use when the cabinets are very small spaces? There isn't a whole lot of space left over for 1" thick material, even with ND91 woofers. The cabinet is amply braced so I'm not all that worried about panel resonance, but the length of the cabinet and port is reminiscent of a transmission line where the driver offset is around 40-50%. So, there could be acoustic resonances to worry about.

    Similarly, the W4-1757SB will reside in a 1-2 L sealed box. Should I bother to throw some stuffing in there?

    Thanks and regards,

    Rob

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  • weinstro
    replied
    Re: Center Channel Design Questions

    Originally posted by Chris Roemer View Post
    ND105-4s will outplay DA115s by +5dB, +10dB at and below 50Hz.
    Thanks for the suggestion, but it's also too big - I'm constrained to 4.5" baffle height (114.3mm). The DA115 is 115.6mm wide.

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  • Chris Roemer
    replied
    Re: Center Channel Design Questions

    ND105-4s will outplay DA115s by +5dB, +10dB at and below 50Hz.

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