Ceiling manifold next to screen = vibrations?

[bigjon]

After two years of sonic bliss with my first IB I am craving more, and find myself in the planning stage of IB v2.0. I am swapping my HT room with my son. He will get a bigger bedroom and I will get to apply everything I've learned since v1.0!

Thanks to a dormer window upstairs the only feasible manifold position is in the ceiling directly in front of the screen. I am tentatively using [url=http://www.monoprice.com/products/product.asp?c_id=108&cp_id=10829&cs_id=1082904&p_id=6582&seq=1&format=2
]this[/url] motorized 106" screen, mounting it to the same ceiling joist that supports the manifold (or on the next joist if possible).

Do I have cause to worry about the pants-flapping effect of an IB on a screen? I suspect structural vibrations would not be a problem as much as air pressure within 1ft of the manifold on such a large surface? Do all cultists use fixed screens?

[ThomasW]

Dec 18, 2010 22:01:54 GMT -7 bigjon said:

After two years of sonic bliss with my first IB I am craving more, and find myself in the planning stage of IB v2.0. I am swapping my HT room with my son. He will get a bigger bedroom and I will get to apply everything I've learned since v1.0!

Thanks to a dormer window upstairs the only feasible manifold position is in the ceiling directly in front of the screen. I am tentatively using [url=http://www.monoprice.com/products/product.asp?c_id=108&cp_id=10829&cs_id=1082904&p_id=6582&seq=1&format=2
]this

[/url] motorized 106" screen, mounting it to the same ceiling joist that supports the manifold (or on the next joist if possible).

Do I have cause to worry about the pants-flapping effect of an IB on a screen? I suspect structural vibrations would not be a problem as much as air pressure within 1ft of the manifold on such a large surface? Do all cultists use fixed screens?[/quote]

Sort of hard to answer questions without info.....

Below is all info posted about your existing install (2 Tempest-X 15" drivers? I guess) .... and we have no info other than location and a possible screen type about the proposed upgrade.

Dec 18, 2007 19:31:29 GMT -7 bigjon said:

Well I just ordered a two-pack so let the circulation begin!

On a side note, DIYCable's customer service rocks - I had a detailed reply to my support request in just a few minutes.

I'll post pics and comments once everything is installed. Thanks guys!

[bigjon]

Sorry, I did not give too much info because I am still unsure of all the parameters and didn't want to bog down in too much detail.

I currently have two Tempest-X that I could reuse for this room but I would like more headroom than the smaller room alone can provide. The Tempest-X is no longer in production, so I will probably sell them and buy two Fi IB318 with manifold space for two more later. These will be driven by an EP2500 I already have.

This will be a small room at only 1170 cubic feet. Notice the shaded area that shows possible positions for the manifold near the screen. The manifold would be at most 6"-9" in front of the screen. The proposed manifold width of 13" is based on the assumption that the joists have standard spacing of 16in on center.

Edit: Sorry, not sure why the image isn't showing up - here it is as a link.

[Wull]

My IB sits 8" in front of my PJ screen, and have had no problem what so ever with screen movement

[Wull]

Pic

[ThomasW]

You should be ok unless you go off the deep end and end up putting in 6-8 18"s

Thing is non tab-tensioned screens tend to swing on their own, they can moved with the furnace or AC operating in the room. I used one years ago and ended up putting velcro on the bottom of the wall and bottom of the screen. When the screen was lowered the velcro held it tight.

Now I use a tab tensioned screen (warning these are a little on the $pendy side). It takes a lot more pressure to move it.

[bigjon]

Thanks for the input! I can't see going with 6-8 of those drivers unless I had a much bigger room (never say never I suppose!). Tab tensioning is nice but out of my budget for now. I might be able to fashion some angled U-brackets to guide a regular motorized screen into a locked position against the wall as it is lowered. Will have to give this some more thought. Too bad my room doesn't allow for another fixed screen.

magicj1, I like the way you situated the drivers in your manifold (2 innies, 2 outies). Did you have to cut a joist to get enough clearance? If so, have you had any problems?

Going by Collo's Gap Calculations (in the FAQ) I need 6 inches of clearance between the manifold and my attic wall. To do this I'll need to put the magnet of the wall-facing drivers inside the manifold. To keep the manifold thin enough to fit between two joists I can make all speakers face the same direction, with two innies and two outies nested a few inches apart. This will allow for a manifold opening of 13in X 37in, which is very close to the total Sd of all four drivers.

[ThomasW]

Dec 20, 2010 14:02:37 GMT -7 bigjon said:

Going by Collo's Gap Calculations (in the FAQ) I need 6 inches of clearance between the manifold and my attic wall. To do this I'll need to put the magnet of the wall-facing drivers inside the manifold. To keep the manifold thin enough to fit between two joists I can make all speakers face the same direction, with two innies and two outies nested a few inches apart. This will allow for a manifold opening of 13in X 37in, which is very close to the total Sd of all four drivers.

Collo's recommendations do not relate to the amount of clearance behind the drivers, They relate to the minimal opening for the frontwave.

One of the fundamental ideas behind IB subs is having unlimited and unrestricted space for the rearwave. This is part of what gives the IB it's superior sound. So it would be best for you to rethink how and where you place the drivers.

[bigjon]

Dec 20, 2010 19:18:47 GMT -7 ThomasW said:

One of the fundamental ideas behind IB subs is having unlimited and unrestricted space for the rearwave. This is part of what gives the IB it's superior sound. So it would be best for you to rethink how and where you place the drivers.

To be clear, I have ~unlimited attic space for the rear wave (1000ft^3 plus) so you are saying the placement of the drivers near an obstruction (edge of the attic space) would color the sound? Just want to be clear that the volume is there, but the rear wave will basically be corner loaded by its placement in the corner of a large attic space. I had not considered this to be a factor for the rear wave.

[ThomasW]

Dec 20, 2010 21:16:17 GMT -7 bigjon said:

Dec 20, 2010 19:18:47 GMT -7 ThomasW said:

so you are saying the placement of the drivers near an obstruction (edge of the attic space) would color the sound?

Yes the rearwave can reflect of nearby objects, those reflections can interfere with the optimal performance of the driver.

[bigjon]

Dec 20, 2010 22:00:29 GMT -7 ThomasW said:

Yes the rearwave can reflect of nearby objects, those reflections can interfere with the optimal performance of the driver.

Thanks, this makes sense but somehow I had not considered it. Here is a view of my attic with the red area the only viable location for a manifold (above the screen). My original plan put the drivers on the east and west side of the manifold, which would have obstructed the rear wave of the east driver. Based on your recommendation I have turned the drivers to the north and south sides of the manifold, which gives them about 4 feet of space to a wall. This is still not perfect but would you say this is sufficient distance to the wall? The volume of the attic is well over ten times Vas of the drivers but as you can see there are walls in odd places (my house is cape cod style).

In this configuration I think my only option for 4 drivers is to stack them into a deeper manifold. I know this affects SQ, but so does reduced headroom with 2 drivers instead of 4. In your opinion is a deeper manifold worth the tradeoff to have more drivers?

Thanks for all your help!

Attachments:

[ThomasW]

Dec 21, 2010 19:54:51 GMT -7 bigjon said:

This is still not perfect but would you say this is sufficient distance to the wall?

That's fine

In this configuration I think my only option for 4 drivers is to stack them into a deeper manifold. I know this affects SQ, but so does reduced headroom with 2 drivers instead of 4. In your opinion is a deeper manifold worth the tradeoff to have more drivers?

Stacking 15"s or 18"s 2 layers deep isn't a problem. Problems start occurring when manifolds get ~6'+ deep, then cancellations can occur between the drivers closest vs those furthest to the outlet

[bigjon]

Thanks for the good news - you just made my day! Now back to the drawing board. I emailed Nick @ Fi about the IB 318s but they are starting another company and moving the IB line over to it. They are out of magnets for them until next month which is when they will release the HT brand. This is probably old news to you, but hopefully good news for me as it will be great timing if my other plans stay on course (as if!).