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| Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63951 is a reply to message #63945] |
Mon, 30 August 2010 18:28   |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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| BigmouthinDC wrote on Sun, 29 August 2010 21:55 | Thanks for the positive comments and thanks for making your plans available. Without the plans this effort wouldn't have happened.
I haven't forgot our discussion about the use of foam. When the horns were loose I made a cardboard template of the inside profiles of the horns so that I could attempt to fashion a foam plug. My plans are to make a plug, but to also send you a larger sheet of foam along with a piece of the screen fabric so that you can do some measurements of what might be the best strategy for behind screen placement. This assumes you still have an interest.
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Absolutely. Send me some of the screen material and foam and I'll make some measurements. It will be an interesting study.
My gut feeling is we want to span the gap between source and screen with a material that is as close to the same acoustic resistance as the screen as possible. The idea is to prevent an acoustic transitions from source to freespace to screen. To achieve this goal, we will want foam filling the entire horn, all the way back to the phase plug at the entrance of the throat. This should extend out past the horn to reach the screen.
The midwoofer won't need quite as much attention as the wavelengths are longer, but it does reach up high in the midrange into the overtone area where wavelengths are just a few inches. So I would place a block of foam that spans the distance from the baffle to the screen, covering the woofer. You don't want the woofer to rub the foam, naturally, but you do want the foam to be as close to the woofer as possible, without actually touching it.
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| Re: Three 4Pi's in Basic Black for Home Theater Build Thread [message #63956 is a reply to message #63951] |
Tue, 31 August 2010 09:16 |
BigmouthinDC
Messages: 41
Registered: February 2010
Location: No VA
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Baron |
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Wayne another technical question. I wonder if you have any experience with the 4PIs in a baffle wall mounting.
Currently my front wall behind the screen is entirely treated with linacoustic but I could also build a baffle wall flush with the fronts of the speaker the same size as the screen.
http://www.thx.com/professional/cinema-certification/speaker-layout-and-baffle-wall/
In my research I found this quote in a JBL manual:
Baffle loading
For optimal results, JBL Professional recommends the installation of screen channel speakers in a full
baffle wall, such as recommended by Lucasfilm/THXÒ and others. Good results can also be achieved using
reinforced baffle "wings" on either side of the speaker system stacks, as long as they are constructed using
high quality materials and are free of resonances and vibrations.
The preferred baffle wall design extends from interior theatre wall-to-wall, and floor to ceiling, essentially
creating a room behind the screen, into which the speakers are flush mounted, firing into the auditorium.
For practical and budgetary reasons, this concept can be modified, still yielding good results, but optimal
performance is typically achieved in a full baffle wall condition.
Full Baffle Wall
The speakers should be located in the baffle wall so that the front of the LF cabinet is flush with the front of
the baffle wall acoustical treatment material (typically black faced duct-liner).
Another post on a discussion of baffle walls had this qoute from Dennis Erskine (well respected theater designer, board member CEDIA)
"A baffle wall should:
run stage to ceiling, wall to wall
be very rigid
allow no resonances in the cavity behind the wall
be covered with 1" (sometimes more depending on speaker) of a black absorptive material (reduces reflections between the screen and the wall.
have the speakers resiliently mounted to the baffle
have no air gaps between the speaker body and the baffle wall
have all front speakers including the front subs in the same continuous baffle.
The speakers themselves should be covered with the black absorptive material with cut outs for the drivers.
Problems. You best have your speaker placement exact for proper listening and sound stage creation ... you're not moving them later. Understand, speaker frequency response will change (partially why the baffle is treated with absorption).
Do a search on 2 pi speaker response or spatial loading to get started. I believe QSC Audio has a paper on their website about this. Really good speaker companies will have FR plots and data available for their speakers for 2 pi installations."
Any thoughts?
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| In-wall flush mounting [message #63961 is a reply to message #63956] |
Tue, 31 August 2010 17:08 |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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All of my speakers respond well to this kind of placement. They are all designed to be used in quarter-space or eighth-space, and I don't use baffle step compensation filters. My thinking has always been that the baffles are rather large, so the transition happens in the lower midrange, close to where room modes take over. That's not where you really want to bump up the power, but then again, you don't want it to be lean down low either. So instead of including baffle step filters, I've suggested placements that reduce the problem acoustically. And as for room modes, a different but related matter, they can be effectively mitigated using a multisub approach.
Placing the speaker close to the boundary reduces baffle step, among other things. The closer the speaker baffle is to the back wall, the less the baffle step. If you can recess the speaker in the wall flush, the baffle step goes to zero. It also makes the reflection off the back wall go to zero too. Those are both important goals.
Some people try to reduce the negative influence of the back wall reflection by moving their speakers out away from the back wall. That's reasonable, a good idea, I suppose. It reduces the amplitude of the rear reflection because of distance. It increases the delay of the reflection too. There's a window of time where reflections are most troublesome, and by moving the speakers away from the back wall, they hope to decrease the early reflections, and to delay them out of that window.
But I go the other way, for several reasons. It is hard to space the speakers far enough away from the back wall to do much good in most living spaces. Most rooms in peoples' homes aren't big enough. So I use another approach. I prefer to use directional speakers that limit the rear wave in the first place, which decreases the amplitude of the rear wave by virtue of directivity. Instead of trying to make the rear reflection late, I try to make it so early it is indistinguishable from the direct wave. As you decrease the distance from baffle to boundary, when the distance becomes zero, so does the reflection delay. So that's the direction I go.
That's why I like using directional speakers designed to mount right up against the rear wall or in corners. The rear wave reflection becomes less of a problem and baffle step is nearly eliminated, because there is so little transition region. If the speakers are mounted with their baffles completely flush with the walls, or their drivers mounted at the apex of the corners, baffle step and rear reflection are completely eliminated.
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