| Re: Contemplating a change [message #69416 is a reply to message #69414] |
Fri, 09 September 2011 09:50   |
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Wayne Parham
Messages: 18792
Registered: January 2001
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Illuminati (33rd Degree) |
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It's hard to say. On the one hand, I always tell folks contemplating mods to think twice, because these designs are mature and have been thoroughly tested. On the other hand, you've had those boxes for years, so I would assume you like the midbass/midrange from them. That leads me to believe the internal standing waves aren't mucking up the lower midrange. They really can in a box this size.
There are two things to consider:
First is the position of the forward lobe. This is established by the crossover and the positions of the drivers on the baffle. So be sure the tweeter is the same distance from the woofer as shown in the plans. It can be centered, but do not make it any further away. Keep the tweeter close to the woofer, like an inch edge-to-edge, just enough room to get a brace in between them. And do not adjust the fore/aft distance. Mount both the woofer and the horn on the baffle. You can mount both on the surface, or route a groove to mount both flush. But do them both the same way so the relative fore/aft position relationship is maintained.
Second thing is the internal standing waves, as I mentioned above. They line up in the lower midrange, so it is vitally important to mitigate anything that might cause ripples in response. The insulation inside helps, of course, but it is also beneficial to put the driver and port in positions where the influence of standing wave modes are reduced. I also suggest using a sheet of insulation spanning the cross-section of the cabinet, as it helps reduce midrange energy better than the sheets lining the walls. It should be placed between woofer and port. Bass goes right through but midrange is attenuated.
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| Re: Contemplating a change [message #69419 is a reply to message #69417] |
Fri, 09 September 2011 11:07 |
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Wayne Parham
Messages: 18792
Registered: January 2001
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Illuminati (33rd Degree) |
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I probably wouldn't change your box at all, except to cut a hole for the tweeter. Don't change port size or reposition it. If you have to do that, I'd start from scratch and build as shown in the plans.
As I said, the whole deal with cabinet dimensions and midwoofer/port placement is these determine where standing wave pressure nodes line up. You don't want them to be lining up in a way that causes ripples in response, which is a difficult problem because cabinets this size tend to have standing wave modes in the 100Hz to 200Hz region. This is low enough the insulation doesn't do a very good job of absorbing those waves. That's why I always put a sheet inside that spans the cross-section in addition to the sheets that line the walls.
My suggestion would be to either build new boxes as shown in the plans or leave your cabinets as they are, and just cut a hole where the tweeter should go. Put it close to the woofer, just an inch above or so. At least that way, you'll have the same bass and lower midrange response you're used to. The difference will be from about 1kHz up. The new setup will be matched in the horizontal plane, and will limit the HF vertical dispersion at large angles, so you won't have any ceiling slap. It will put vertical nulls out about 20° above and below the speaker, outside the HF angle.
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