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Three 4Pi's in Basic Black for Home Theater Build Thread [message #63366]

Fri, 09 July 2010 23:06

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

Baron

OK, so I've jumped in with both feet and ordered/received my 2226's and Horns for three speakers.

They will be going in this theater space and this is both the before and after picture as the speakers will be behind an SMX acoustically transparent screen.

http://www.PiSpeakers.com/DIY/BigmouthinDC/BigmouthinDC_4Pi_HomeTheater_01_Screen.jpg

So the first question as I start to work on the baffles is I notice that although the horns have a textured surface they still have a considerable shine which I am going to need to abate otherwise the screen which has a 5% openness factor will create reflections off both the horns and the shiny metal flanges on the woofer.

Have there been any proven techniques for dulling the shine on either?

Here is a look under the hood of the area for the speakers. For size reference the center speaker is sitting on a 15 inch sub.

http://www.PiSpeakers.com/DIY/BigmouthinDC/BigmouthinDC_4Pi_HomeTheater_02_OldSpeakers.jpg

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63369 is a reply to message #63366]

Sat, 10 July 2010 08:15

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

That's awesome. It will turn out very nice, I'm sure. My home theater system used four π mains and three π subs for many years, and I loved it.

The horns are very easy to dull. Lightly sand them with some 220 grit sandpaper and you'll take the shine right off.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63371 is a reply to message #63369]

Sat, 10 July 2010 18:32

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

Baron

I tried the sanding and it was just knocking down the high spots but the shine remained. So I grabbed some solvent (Goof off) and wiped the horn which I knew would dull it (and it did) but it looked like crap when it dried. So I shot a coat of Krylon Ultra Matt black and I have the result I was looking for.

http://www.PiSpeakers.com/DIY/BigmouthinDC/BigmouthinDC_4Pi_HomeTheater_03_H290.jpg

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63373 is a reply to message #63371]

Sun, 11 July 2010 08:37

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

There you go, that's perfect! I've sanded those horns and dulled them, the trick is to remove the finish, not just the high spots. Tried sandblasting them too, which may work if the flow pressure is reduced but the place I go to sandblast stuff has a pretty strong feed that just cuts into 'em like butter.

Sometimes I think it would be worth refinishing these H290 horns, sanding them to remove casting marks and the like. They could be made much more attractive. I never was as concerned with dulling the finish as I was with removing the riser marks. But I can see how a dull finish is important to prevent them from reflecting a glare in an HT setup like yours, where the speakers will be behind the screen.

At any rate, I love the looks of your finish. Good job! It looks great! Will you paint the brushed edge of the 2226 woofers too?

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63375 is a reply to message #63373]

Sun, 11 July 2010 11:45

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

Baron

Wayne Parham wrote on Sun, 11 July 2010 08:37

Will you paint the brushed edge of the 2226 woofers too?

I think it would be a good plan, any ideas how to pull it off without risking damaging anything?

Just a note about the refection problems, In the picture I posted above I had to hang some black GOM fabric over the speakers to cut the reflections of the semi-gloss black painted cabinets that were very visible without the fabric covers. I am trying to do everything to avoid adding any additional material between the speakers and the listeners. I know the screen has some attenuation on the upper frequencies that may need some equalization.

This is from the SMX web site, blue line is speaker, the pink is with the screen in place.

http://www.smxscreen.com/images/smx-and-speaker.jpg

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63385 is a reply to message #63375]

Mon, 12 July 2010 09:51

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Oh my, it looks like the screen has introduced ripple of about 5dB between 3kHz and 10kHz, as much as 10dB between 4kHz and 5kHz. From that, I would guess the screen is fairly reflective, not fully transparent acoustically. Those peaks and dips look like self-interference from the reflection.

I think I would almost prefer a slightly more absorbent material because even though it would attenuate the sound, perhaps needing a smidge of EQ, it may not introduce that much ripple. Any other options? Have you measured other materials? I might be inclined to even try various fabrics to see what I could come up with.

As for the brushed edge on the 2226, I'd just use some flat black enamel and apply it with a small paint brush. You can get it at any hobby shop, the little bottles of paint used by model builders would work great.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63386 is a reply to message #63385]

Mon, 12 July 2010 12:17

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

Baron

Thank you for your brutal honesty. Actually this is a brand new screen that I purchased for my 2.35:1 remodel after looking at various alternatives. It has earned high reviews from HT enthusiasts and professionals. The published test results for perforated screens (Stewart Screen) are much worse.

So that is the screen I'm going with unless someone comes up with a magic fabric as a replacement.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63387 is a reply to message #63386]

Mon, 12 July 2010 14:18

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Sorry. I didn't mean to be brutally honest. I don't really like coming across that way. But I guess it probably did sound like that.

In truth, there are a lot of things in the environment that cause 10dB swings in what would be an otherwise perfectly flat response curve in an anechoic environment. Heck, just the reflections off the walls do that in the region below 200Hz or so.

Then again, I'm not sure I would be willing to accept response swings so large in that frequency range, right where our hearing is most sensitive. Here are a couple things I'd try:

First, angling the speakers might help because the much of the reflected energy would be directed away from the speaker instead of back towards the source. The self-interference ripple is caused by a reflection that combines with the source, and is therefore strongest when it is straight on.

When a boundary is a quarter-wave away from the source, the reflection is a half-wavelength late, which forms destructive interference. When the boundary is a half-wavelength away, the reflection is a full-wavelength late, which forms constructive interference. The relationship between source and reflection changes with wavelength i.e. frequency, so the resulting response curve develops ripples, as some frequencies combine constructively and others combine destructively.

If the reflection isn't directed back towards the source, the ripple would be reduced. So angling them inward should help. On-axis sound is at an incident angle that reflects away from the source. Far side off-axis is even further angled away from the source. Only near side off-axis sound is reflected back towards the source. Of course, that's the problem with this approach. These speakers generate sufficient output off-axis that there will be a fairly significant reflection back to the source even with a lot of toe-in. But it's worth trying. Try the recommended 45° toe-in to see if it helps reduce screen ripple.

Second, putting some open cell foam or other semi-absorbant material between the speaker and the screen may smooth the ripple. It would probably only be required in front of the tweeter if the screen is placed close enough. Increase tweeter output slightly to compensate. It would make the reflective "boundary" formed by the screen be somewhat "fuzzy". Instead of having a partially reflective boundary that is at a single distance from the source, you'll have a region of semi-transparent, semi-reflective material for the sound to pass through. The self-interference reflection could not line up with well-defined quarter-wave nodes.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63388 is a reply to message #63387]

Mon, 12 July 2010 20:14

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

Baron

I want the brutal honesty even if I can't or won't follow all of your ideas. I've got some problems with the toe in as I'm pinched for space on the left side. Also not sure how it would help the center channel.

I like the idea of experimenting with the open cell foam. How about a sheet of foam over the horn area (or horn and port) How about the stuff that the Geddes disciples are using to stuff their horns with?

http://www.swisstropicals.com/Web%20pictures%20600%20dpi/Poret/Poret%20Colors%201.JPG

I obviously like the black version.

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Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63389 is a reply to message #63388]

Mon, 12 July 2010 20:37

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

I think this is a good application for this open cell foam. I'd place it between the horn mouth and the screen, perhaps attaching it to the screen in front of the horn so sound has to travel through the span, not just the screen. Don't leave a gap because we're trying to eliminate transitions, having only one, where the sound leaves the screen towards the listeners. You can fill the horn with it as Geddes prescribes too, which will cause the sound to travel through the same sort of space all the way from the phase plug to the screen.

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