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four π plans [message #64238 is a reply to message #64236] Sun, 10 October 2010 15:55 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Illuminati (33rd Degree)

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Re: four π plans [message #64241 is a reply to message #64175] Sun, 10 October 2010 19:13 Go to previous messageGo to next message
CDY2179 is currently offline  CDY2179
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Registered: October 2010
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Wayne, I was planning on lining the cabinet with some extra 2" compressed wool, it has similar coeffecient factors as Owens Corning 703.

http://www.atsacoustics.com/item--Roxul-Acoustical-Fire-Batts-Mineral-Wool-2-inch-Case-of-6--1006.html

I used this for my DIY acoustic panels and have 3 or 4 sheets left over. Would this be better than R13?
Re: four π plans [message #64242 is a reply to message #64241] Sun, 10 October 2010 20:44 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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I don't know, but the insulation and its position are pretty important. For example, if you don't span the cross-section with a sheet placed on the brace between the woofer and tweeter, you'll notice a blip in the lower midrange. So don't experiment with the insulation unless you know it is equivalent or you have some measurement gear to check it out.

Re: Just bought some JBL 3677...? about the CD and horn [message #64295 is a reply to message #64175] Tue, 12 October 2010 10:52 Go to previous messageGo to next message
skywave-rider is currently offline  skywave-rider
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Viscount
Thumbsup on building the 4Pi as designed!
Re: Just bought some JBL 3677...? about the CD and horn [message #64316 is a reply to message #64175] Wed, 13 October 2010 11:42 Go to previous messageGo to next message
CDY2179 is currently offline  CDY2179
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OK about to build my boxes. My center will be going in the wall..of course it'll stick out of the wall some. I'll be keeping the proper volume and tuning. My question is how shallow can I make it and not effect performance. The woofer is 6" deep, my plans are to put it into a box that is 7.5" deep(inside clearance).
Dealing with internal standing waves [message #64321 is a reply to message #64316] Wed, 13 October 2010 13:09 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
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It's hard to say, really. The port is 10" long, so you'll have to bend it if you make the box any shallower than it already is. But that's OK, just make the port like the seven π cabinet. Beyond that, built it, test it and see.

My suggestion is to make a sacrificial cabinet out of MDF. It's cheap enough and the box is easy to build. Make it the depth you want and with a port like what's used in the seven π loudspeaker. That will tune the Helmholtz frequency right, so you'll know what you have there. What you won't know is what the internal standing waves do to response. So build the cabinet and make some acoustic measurements. You can even do an impedance sweep if acoustic measurements are hard to do.

If you see any noticable blips, then the box isn't good enough and you'll need to move something, port position, woofer position and/or cabinet geometry. And don't forget to use insulation - three sides and often a sheet spannning the cross-section, somewhere in the middle of the box. That will damp internal standing waves in the lower midrange better than just lining the walls inside.

Re: Just bought some JBL 3677...? about the CD and horn [message #64325 is a reply to message #64175] Wed, 13 October 2010 16:02 Go to previous messageGo to next message
CDY2179 is currently offline  CDY2179
Messages: 16
Registered: October 2010
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Sounds good. I plan on using the compressed wool I spoke of earlier. It's absorption factors are much better than r-13..because it's compressed it's equivelent to over a foot of fluffy (standard insulation)..that's why it's used as acoustic panels at first reflections. Crossbraces will be wrapped as well.

I'll probably use two 2.75" ports as the 3677 uses which means the ports only have to be 1.75" long each...but I'll probably use REW to test tuning to see if I need to tweak a port to get a better response.

Because I'll only have 1.5" of clearance behind the woofers magnet..can I leave the portion of box directly behind the magnet untreated with stuffing? I want to make sure the vent can breath...I was thinking I could just cut a circle slightly larger than the magnet.
Re: Just bought some JBL 3677...? about the CD and horn [message #64326 is a reply to message #64325] Wed, 13 October 2010 16:42 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
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I think so, yes. With the back so close to the woofer, you'll have no choice, really, because you don't want damping material sucked into the cooling vents. That would make them turn into one-way valves, which would not only affect cooling but would change tuning and even introduce a slight amount of asymmetry in cone motion. So keep the damping material away from the woofer's cooling vents.

Re: Just bought some JBL 3677...? about the CD and horn [message #64373 is a reply to message #64175] Mon, 18 October 2010 15:14 Go to previous messageGo to next message
CDY2179 is currently offline  CDY2179
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Registered: October 2010
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Hey Wayne, wondering if I could pick you brain one more time. I've built and painted my corner cabinets they're 43" tall with a 30 deg angle for the baffle. I've braced it really well and wrapped it all with r-13 with my acoustic material behind the driver only and r-13 everywhere else.

once I lined everything it got pretty tight in the two places I used crossbracing. Anyways tonight I,m gonna install the ports and finish them up. Would you think it would be better to put the ports off to the sides but near the woofer on the baffle like the 3677 orignally came or midway down the baffle or lower in the cabinet? Or does it not matter?
Internal standing waves and port position [message #64374 is a reply to message #64373] Mon, 18 October 2010 16:02 Go to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18691
Registered: January 2001
Illuminati (33rd Degree)

Sorry to sound like a broken record, but ultimately you're still dealing with the same issue. The thing you're concerned about is internal standing waves. You can't prevent them from lining up inside the box, but you can prevent them from adversely affecting the response curve. The way to do this is to put the port in a benign position and/or damp the inside of the cabinet with strategically placed absorbent material. The problem is it isn't always easy to know what positions work best. But you can model the cabinet mathematically, and verify with measurements. Without that, you're sort of working in the dark.

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