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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63658 is a reply to message #63641]

Wed, 04 August 2010 14:53

BigmouthinDC
Messages: 41
Registered: February 2010
Location: No VA

Baron

Some more progress. Three boxes less bracing and backs completed.

Found some scrap 1x2 oak in my scrap pile for the braces as specified in the plans and threads.

http://www.PiSpeakers.com/DIY/BigmouthinDC/BigmouthinDC_4Pi_HomeTheater_07_UnfinishedCabinets.jpg

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63659 is a reply to message #63658]

Wed, 04 August 2010 16:44

Wayne Parham
Messages: 18784
Registered: January 2001

Illuminati (33rd Degree)

Looking good. Don't forget to put insulation on the top brace, spanning the cross-section of the cabinet. It kind of forms a pseudo-transparent separator between woofer and port. I say pseudo-transparent because it is completely transparent at low frequencies where the bass tuning works, but attenuates midrange frequencies to damp internal standing waves. This is done in addition to lining three internal panels, the bottom, one side and the back.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63679 is a reply to message #63366]

Thu, 05 August 2010 08:36

krikor
Messages: 31
Registered: May 2009
Location: Detroit

Baron

Seeing this makes me want to find the time to build a pair of these cabinets as well... and soon! I've got all the parts including the JBL drivers just sitting in a closet. So many projects, so little time this summer.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63685 is a reply to message #63679]

Thu, 05 August 2010 09:26

BigmouthinDC
Messages: 41
Registered: February 2010
Location: No VA

Baron

Wayne is this what the Doctor ordered for bracing?
I found some 3/4 x 1 1/2 and 3/4 x 2 Oak in my scrap pile. I positioned it about as low as I can go to create the "shelf" for the recommended insulation installation separating the woofer from the port.

http://www.PiSpeakers.com/DIY/BigmouthinDC/BigmouthinDC_4Pi_HomeTheater_08_Bracing.jpg

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63686 is a reply to message #63685]

Thu, 05 August 2010 10:33

krikor
Messages: 31
Registered: May 2009
Location: Detroit

Baron

Wayne,

Something I've been wondering about with regards to the 4Pi port that is evident in this photo... since the port is so close to the cabinet side wall, could you just build a three-sided port and use the cab sidewall as the fourth side?

It would mean moving the port over slightly, or narrowing the cab a bit and making adjusting in height/depth to keep the same volume.

Any reason you wouldn't want to do this?

Just thinking about how I can streamline my (eventual) build.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63687 is a reply to message #63686]

Thu, 05 August 2010 10:54

BigmouthinDC
Messages: 41
Registered: February 2010
Location: No VA

Baron

When you look at the front of the speaker and the size of the woofer I can't imagine it being much narrower. I applied a copious amount of Green Glue between that side wall and the port to act as another constrained layer dampening surface to tame vibration and resonances.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63689 is a reply to message #63366]

Thu, 05 August 2010 11:01

krikor
Messages: 31
Registered: May 2009
Location: Detroit

Baron

Ah yes, I forgot about the diameter of that big JBL woofer. Actually, your approach makes more sense using the port wall as additional stiffening agent/vibration sink for the cabinet.

I like what you did with the baffles, especially the rounded over port, and I am now planning to do similar. Will probably use baltic birch or a veneer for the front layer since my speakers won't be hidden.

Thanks for the photos and details.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63691 is a reply to message #63366]

Thu, 05 August 2010 11:23

Wayne Parham
Messages: 18784
Registered: January 2001

Illuminati (33rd Degree)

BigmouthinDC wrote on Thu, 05 August 2010 09:26

Wayne is this what the Doctor ordered for bracing?

Yes, that's good. That's exactly how I do the braces.

BigmouthinDC wrote on Thu, 05 August 2010 09:26

I found some 3/4 x 1 1/2 and 3/4 x 2 Oak in my scrap pile. I positioned it about as low as I can go to create the "shelf" for the recommended insulation installation separating the woofer from the port.

That's perfect.

krikor wrote on Thu, 05 August 2010 10:33

Something I've been wondering about with regards to the 4Pi port that is evident in this photo... since the port is so close to the cabinet side wall, could you just build a three-sided port and use the cab sidewall as the fourth side?

You could, and that's actually how I modeled the cabinet when I first started assessing the impact of internal standing waves. But I didn't like the looks of the cabinet with the port that close to the edge, so I shifted it inwards about an inch by adding the extra material.

As you guys have discussed, the cabinet dimensions can't be reduced, and really, you don't have a lot of choices with port position either. But there is some wiggle room, I suppose. You could put it right out at the edge. Still, with the extra cost of a couple pieces of wood, I think you'll be happier with the port where it is drawn in the plans. It looks really funny to me when it's placed higher than the tweeter and shifted outward. Where it is now, it's even with the tweeter.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63715 is a reply to message #63691]

Fri, 06 August 2010 07:45

BigmouthinDC
Messages: 41
Registered: February 2010
Location: No VA

Baron

Wayne, Design question of the day.

As I mentioned earlier I need to avoid any direct reflections back to the screen. The JM white R13 fiberglass I picked up for insulation will definitely reflect light at the back of the port. I've thought of 4 solutions.

1) Staple some black acoustically transparent fabric to the back of the port. I have some black GOM FR701 fabric on hand but could easily run and grab some Joannes speaker cover fabric

2) Use the same fabric and instead of stapling to the back of the port drape over the offending insulation leaving a gap between the port and the fabric. Since the side of the cabinet with the port is supposed to also get insulation this isn't going to be real easy as there will be two surfaces to cover.

3) Get out a spray can of black flat and put a light coat on the visible fiberglass

4) I have some Linacoustic duct liner which is one inch thick compressed fiberglass insulation with a black coating. I could cut a little pocket in the R13 and position behind the port.

My preference is #1 but I'm not sure whether or not even a little air movement resistance may affect the tuning.

Thanks for the assistance so far!

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63716 is a reply to message #63715]

Fri, 06 August 2010 08:00

Wayne Parham
Messages: 18784
Registered: January 2001

Illuminati (33rd Degree)

Don't use a screen on the port because it will be too restrictive.

Use a can of black spray paint and lightly mist the insulation in the area behind the port. Don't saturate it, or the fibers will become stiff and lose effectiveness. Test it with your hand after it's dry and make sure it's still the same consistency as the untreated fibers. I've done this before and it works very well.

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