Infinite Baffle. Best bet if possible?

[johninCR]

johninCR:

An IB sub with manifold mounted drivers doesn't change Fs because the manifold opening isn't a restriction to cone movement. If it is, you would be talking a bandpass alignment, and that's a different ballgame entirely. So, if you find this to be the case, your opening isn't big enough.

As to your other ideas, I'm not sure I follow what you're talking about clearly at all. On one hand it sounds like you want a sealed volume between your facing drivers that are out of phase... with only one side of this facing the room. Given the availability and cost of good high-excursion drivers well suited to IB use I really don't see the complexity of this to be worthwhile.....

I read somewhere that the alignment lowers the Fs and I'm trying to determine if that is correct. To say that 2 drivers in relatively close proximity firing directly at each other has no effect on their response doesn't make sense, at least to me.

The other part of my discussion is to fully understand the pros and cons of a push/pull alignment in a 2 driver manifold. No sealed boxes, just the same 2 driver IB manifold but with the drivers facing the same direction and wired out of phase, nothing complex about the construction.

Regarding price and availability, I'm down in Costa Rica where good drivers are nonexistent and shipping and import duties more than doubles the cost of woofers purchased online, so I make the most with what is available. I'm trying learn as much as possible before I finalize the design of my 10 driver/side pair of dipole subs. Unfortunately I can't go IB, so dipole will have to do, but I still should be able to get some substantial performance from only $300 worth of drivers.

 

[joe carrow]

Heres a question though, if I made a ported enclosure inside the attic, and the driver fired into the listening room, but the port fired into the attic would I loose the extension of a port, or does it matter? Basically does the port have to fire into the same air space as the driver?

I've been following this thread from the start, and it's very interesting.

The way I understand it, the port's contribution is very important. The driver's excursion ought to reach a minimum at the tuning frequency; the port's contribution should be at its maximum there.

So having the port fire into the attic would suck out all the low end. If, hypothetically, you did that- I think that you would notice the lack of bass, and that malady would be improved by blocking the ports.

I think that people will agree that different enclosures have different strengths and weaknesses. I'll let you read up on the relative benefits and drawbacks of each one, or someone with more experience can step up and give the rundown- but in my opinion it should be a showdown between IB and horn.

 

[ThomasW]

fourdoor,

There is a HUGE difference in sound quality between even the best ported box and IB. I've built what many consider to be a pretty impressive ported sub. And there's no comparison between it and an IB

With an IB you have the somewhat rare opportunity to own one of the finest sound quality subs available at any price. Why would you want to put the drivers in a box and have all the box induced colorations and limitations?

Or will the large volume simply dissapate the air being moved?

Yes and the fact that the insulation will absorb a ton of sound

Since you have available attic space directly centered between the mains, I suggest placing the IB there.

 

[Hybrid fourdoor]

One of the reasons I was thinking about mounting them off to the side was so that I could mount them at a 45* angle or something. That way there just not pointing straight down, since there is some concern that that could affect the drivers.

I know a manifold would fix this problem (mount them vertically), but I kind of want them each firing directly into the room.


This would work though;

An externally hosted image should be here but it was not working when we last tested it.

 

[johninCR]

I'd still put them up front. Instead of protruding down, have them recessed between the rafters. Each could be mounted on the short side of what would look like half of a rectangular box that was cut in half on the diagonal. You'll want the ceiling a dark color anyway to reduce the reflected light from your projection screen.

You really should consider the manifold setup for vibration issues.

 

[Hybrid fourdoor]

Actually we plan on painting the room a darkish grey color. I'm kinda hoping its enough (color wise) to use a projector without a screen.


Although it would be cool to have the overlapping fabric like a lot of theaters have

 

[joe carrow]

One of the reasons I was thinking about mounting them off to the side was so that I could mount them at a 45* angle or something. That way there just not pointing straight down, since there is some concern that that could affect the drivers.

I know a manifold would fix this problem (mount them vertically), but I kind of want them each firing directly into the room.


This would work though;

An externally hosted image should be here but it was not working when we last tested it.

Hey, I was going to mention this before- it really shouldn't matter what direction the speakers are pointing. You should cross them over low enough that you would NOT be able to listen to the subs with your eyes closed and point your finger at where the subwoofer is. I hear that 80 hz is as high as a sub should play; maybe a bit more, like 120. Done properly, it should sound like it's not really coming from anywhere. Like godzilla is really stomping in your listening room.

I agree with other posters; I think that your best bet would be a four woofers mounted vertically in two manifolds; one manifold on either side of the double beam in the ceiling. I say that the two woofers in each manifold should fire towards each other.

I don't think of it as two sound fields that overlap and smack into the opposing woofer- I think of it more like they're working together to push air out of and suck it into the manifold.

 

[ThomasW]

I'd put all 4 in a single manifold centered between the mains.

There is no audible difference between direct firing drivers; and those mounted in a manifold, as long as the manifold outlet opening is properly sized.

 

[johninCR]

Hybrid,

With the power and high contrast ratios of the newer projectors, screens have become somewhat optional. I've tried using grey paint to make a high contrast screen but I didn't notice any contrast benefit over white and the grey impacted colors negatively. You'll want to have a flat black painted frame of 3-4" around the image. Framing the image makes a big difference in the perceived brightness and contrast and will clean up the edges of the image, which often have some excess light due to keystone correction and other factors.

Regarding room colors, you definitely want a dark ceiling and back wall with a matte finish (not glossy), due to reflected light from the screen. Colors for the rest of the room depends alot on the power of your projector. If it's more than about 1500 lumens, then using very dark colors throughout the room will make the image too bright and hurt your eyes during bright scenes. Also, don't feel locked in to a drab dark grey. Any dark colors work fine. Just be sure not to use glossy paint.

Regarding the sub setup, with multiple drivers, manifold alignments are much better due to the cancellation of the mechanical energy of the drivers without loss of sound output or quality. The vibration in your ceiling caused by the mechanical operation of 4 15" drivers will be substantial otherwise.

Also something to keep in mind is the acoustics of your theater. With that kind of clean bass output, I'd highly recommend some bass traps. For a new construction the way to go would be a slot resonator (helmholtz resonator) with tuning mainly at the primary room modes. They're easy to build, take up very little space, and look good too. Here are a couple of links (be sure to note the link with the formula correction as the incorrect formula for tuning is widespread on the internet, but the first link is good for construction info) http://www.saecollege.de/reference_material/pages/Low Mid Frequencies.htm

http://forum.studiotips.com/viewtopic.php?t=94

 

[kelticwizard]

Pardon the extremely crude drawing, but here is another way to build a bass manifold which allows straight up and down mounting.

I have never built one of these.

However, the center of the woofer will be 7.5 inches from the ceiling level. If you plan to cross over at 80 Hz, that will be 1/22 of the wavelength of 80 Hz. There is a general rule of thumb that 1/10 of a wavelength difference is negligible-you will be well past that. So I would think building the manifold that way would be a possibility.

The advantage is that it allows straight up mounting for the drivers.

 

[dhenryp]

Regarding push-pull IB, I think I'm the one Thomas W was refering to when he said someone on his IB forum had tried it. See the attached picture. I've got a fairly big manifold with four AV15s and when I did some push pull configuration testing before I installed the manifold I did not notice any huffing from the rear mounted drivers. I ended up switching to the "standard" configuration with all drivers facing in because it made mounting in my trussed attic easier. I attached all the drivers with screws from the inside of the manifold. I did not have easy access to attach one of the drivers with screws from the outside of the manifold.

The bottom line: I'm very pleased with the configuration I have and doubt very much I could detect an improvement with a push pull arrangement.

You can see more photos here:

http://f2.pg.photos.yahoo.com/ph/dhenryp/slideshow?&.dir=/cf64&.src=ph

One last comment: I think you really should mount the drivers opposing to cancel out vibrations. You would be amazed at the vibration caused by a single 15" driver moving over long excursions. I did some OB testing with the AV15s before I assembled the IB. I made a simple open backed box with one driver facing forward. When I played 20hz at large excursions the box would shake violently. Even sitting on top of the box with all my weight (180 lbs) did not stop the shaking, although I must admit it was an interesting experience...

 

[kelticwizard]

Couple more things. As long as you are making this a dream system, are you sure that four long throw 15's are enough? They will give you a total displacement of 70 cubic inches each, for 280 cubic inches total, but with todays DVDs, etc, have you considered more?

How about 8? I'm serious.

The following thread contains a chart about how much air must be moved before bass frequencies can be reproduced in a room at a certain volume. Big bass at high volume uses up displacement fast. Perhaps you want to consider more.

For instance, with 280 cu in displacement, you would be able to reproduce a 14 Hz note at 110 SPL. But if the note is that low, you might want to reproduce it at 116 SPL. You won't be able to do it, unless you double the displacement. This doesn't count room gain, but still. 14 Hz is plenty low, yet it is on some DVDs.

http://www.diyaudio.com/forums/showthread.php?threadid=5668&highlight=

Another thing. The manifold will be set up in a vertical line. Yet 2X2 mounting increases efficiency by 6 dB. I wonder if that is not the way to go. By 2X2 I mean like the following:
00
00

Maybe 8 can be set up like this
000
000
00

Just something to think about.

 

[kelticwizard]

Finally, I don't know how much time you have to plan this, but have you considered finding out what the room modes will be? There are some freeware programs available at Steve Ekblad's site:
http://www.audiogrid.com/audio/

Visual Ears seems to be a good program, available for $59. Apparently you put the speakers in a position, and it tells you the room reaction:
http://www.kbacoustics.com/visualears/


I have never used these programs, by the way, but room placement does matter.

As long as this is in the planning stage, you might want to start a different thread, like

"Ceiling Mounted IB SUB, Need Help Calculating Room Modes". There are people on here who have used these programs. I would give it a shot before committing my IB to a specific place in the room I was building.

 

[dhenryp]

I would not worry too much bout four 15" long throw drivers not being enough. My room is 12x18' so it's about 25% smaller than yours. I've got four AV15s powered by a 1200 watt Crown pro amp. I can easily hit 115db on the explosion in the opening scene of Start WARS Attack of the Clones. I'm sure I could go much louder (125? more?) but 115db at low frequencies feels quite alarming. It will literally rattle the dishes in the kitchen cabinets one floor below. It will physically move the windows in the window sills an amount you can easliy see. Much higher volume I think might break the glass.

I think four AV15s is just about right for your size room. I think my 1200 watts is overkill and I could easily get by with half that, maybe less. I got a good price on it and I will never have to worry about clipping.

 

[cjd]

johninCR:

If you wire out of phase, you have the same result that wiring any two bass drivers in a sealed cabinet out of phase - where's the bass? To be 100% sure, I went down and tested with my IB. I did this without adjusting the volume at all, but... Best output was two drivers in parallel. A single driver was next best. Worst response was out of phase. It was just barely there.

kelticwizard: The complications of your manifold don't outweigh the benefits, whatever they may be (I don't come up with any). And, having just tested with a single driver I can only reiterate Thomas's suggestion that you mount the drivers facing each-other! The amount of noise I picked up in my wall suddenly vibrating just between the two drivers and a single driver was quite surprising. Almost none with the pair (a heater grate I haven't successfully secured yet) went to drywall vibrating against the studs (between screws).

A traditional box manifold makes the most sense to me. If you want to showcase your drivers or something, make it big and put some rope-lights up in it or something.

FWIW, my IB crosses at 100hz right now. It is centered between the mains and is wall-mounted (manifold), which makes this possible. I designed the speakers (VERY cheap components, but my first ground-up design) and setup around my existing amp which has a fixed crossover point of 100hz. The mains actually just don't have output to speak of below 100 so it works well enough. For HT, it's fine. For music it isn't too bad since it's centered, but I can localize the bass a bit more than I would like.

Oh... though off topic, since it came up here - do yourself a real screen. I painted a whole pile of test panels and found an odd concoction and multiple layers to give me the whites I wanted (grey screens kill the image for me and even some silver) and black level 98% of where a grey screen got me, with vibrant colors and crazy detail - my wife's first comment (watching an older traditional Japanese Animated) was "it's so 3d" yet the style is very much NOT 3d like more modern animateds (which are better yet). PJ is an X1.

And, black velvet surround beats the pants off anything else. It really does. I got mine 50% off with a coupon.

C

 

[joe carrow]

johninCR:

If you wire out of phase, you have the same result that wiring any two bass drivers in a sealed cabinet out of phase - where's the bass? To be 100% sure, I went down and tested with my IB. I did this without adjusting the volume at all, but... Best output was two drivers in parallel. A single driver was next best. Worst response was out of phase. It was just barely there.
C

I was going to reply earlier, but once i had drawn this up in MS Paint the conversation had taken off, and there wasn't much left for me to say. This is what I drew:

If your drivers are like this, than the ones with the magnets pointing into the manifold need to be wired in reverse polarity to the ones with the magnets pointing out. If they're all mounted the same direction(out), then you're absolutely right, and reversing polarity will kill the bass.

The idea behind this configuration has to do with the speakers canceling out even order harmonics, decreasing distortion. It has been said that the benefits of this are less noticable with good drivers, but very worthwhile with cheap ones.

I think that the confusion arose from JohninCR's statement that if the drivers are facing the same direction they need to have reversed polarity. If you say that "same direction" means "south" or "east", then he was right- but I think it makes more sense to talk about if they're firing "in" or "out"

Interesting discussion. I hope that some day I will be building one of these

 

[johninCR]

Regarding push-pull IB, I think I'm the one Thomas W was refering to when he said someone on his IB forum had tried it. See the attached picture. I've got a fairly big manifold with four AV15s and when I did some push pull configuration testing before I installed the manifold I did not notice any huffing from the rear mounted drivers. I ended up switching to the "standard" configuration with all drivers facing in because it made mounting in my trussed attic easier. I attached all the drivers with screws from the inside of the manifold. I did not have easy access to attach one of the drivers with screws from the outside of the manifold.

You've got good drivers, so the benefit of push/pull is probably small, but if Linkwitz uses it for his dipole woofers then why not? The harmonic distortion at lower frequency may not very audible at low frequencies, but they do tend to make things rattle more in the room.

When you get the other driver back, just mount the 2 difficult to get to drivers from the inside and the 2 that are easy to get to as seen in your attic pic from the outside. I'd think that push/pull with opposing drivers would be better than your trial setup anyway because it would be a true push/pull instead of a push/push, pull/pull.

 

[Jennice]

Hybrid 4D,

Just a suggestion re. screen (with out going too far off topic).

If you want to paint directly on the wall, check out "screen Goo" I know it's an odd name, but it's supposed to be a paint mixture that rresults in the same surface as real screens. (It's also costly, though!)

I borrowed a projector (heading for the Sanyo PLV-Z2 this fall), and tried it with glass fiber "wall paper", with a sample of real, white, screen material, and with the reverse of white wax-coated table-cloth. The wax-coated material comes quite close to the real thing. Real enough for me to have a direct side-by-side comparison to tell what is which. Price? about 1/70th of the real fabric for the same size!

Alternatively, have you tried to search the web for fixed frame DIY screen? There's a million ideas out there...


Good luck.
Jennice

(Trying not to reveal my jealousy )

 

[johninCR]

Sorry about the confusion and unnecessary tests CJD, but once you start talking about push/pull then I think in terms of the direction. Yes 2 drivers facing opposite directions with both firing into the chamber or firing to the outside need to be wired in phase. With push/pull (1 firing in and 1 firing out), facing the same direction, they should be wired out of phase.

 

[johninCR]

Don't waste your money on "Screen Goo". The only thing special about the paint on matte white screens is that it is flexible and adheres to the vinyl screen material without cracking or peeling. High gain screens are a different matter, but are unnecessary in a home theater with today's projectors unless you want a lot of lights on, but high gain screens have their compromises too including narrow viewing angles.