Help to choose IB woofer placement

[harryharry]

Hi,

I have bought 2 18" FI IB3 woofers and plan to use them in my 16 m2 (173 sq ft) home cinema. I hope 2 woofers is enough or I will add 2 more.

I am planning to use 1 B&O icepower 1000W on each woofer in a stereo sub woofer setup. The eq is done feeding the sound through a computer with a good sound card (Lynx Two B). I use correction filters made in Audiolense (audiolense.no) in a convolution VST plug-in (convolver plugin).


I have three placement options:

Option 1:
To create a line array with stereo subs with 60cm x 60cm x 300 cm (2 ft x 2 ft x 10 ft) port/box. I need to make hole in a 15 cm concrete wall for the air intake/port. I am a bit afraid that the length of this port is a bit too long.



Option 2:
To make holes under the screen. This will look better than option 1 but is very difficult since I have to cut through 40 cm (16 inch) concrete plus 10 cm (4 inch) wood on both sides. I hope to be able to avoid this option.



Option 3:
To place the subs on the back wall. This is very easy since i already have a hole in the concrete. Several postings in this forum suggest that this might smear the stereo image.




I prefer option 1 but I am a bit afraid that the port is going to be to long.

PS. I can not reverse the room so that the back becomes the front.

[ThomasW]

I'm not sure how to reply to this. Are we dealing with a concrete floor, or is there basement/crawlspace or an attic available?

IB subs don't have a 'port' so I don't know what you're referring to when you say .....

I prefer option 1 but I am a bit afraid that the port is going to be to long.

[chrisbee]

The difficulty with a false baffle wall means the dimensions of the room will become even smaller.

Option B makes most sense but cutting through 16" of concrete in around 18" diameter is a major undertaking for an amateur with hammer and chisel. ;D

I wonder what a factory ventilation engineer would charge to cut some nice round holes with a big diamond tipped hole saw? I'm not even sure the hole saws are available in that length. Though they could cut from both sides if access is reasonable.

As Thomas says: Can't you go up through the ceiling or down through the floor instead of through the walls?

[harryharry]

I can only expand through the walls in the front, the side and the back unfortunately. The cinema is in a basement and above it is the living room.

I agree that option B makes most sense however I am hoping to avoid cutting through this concrete. I have earlier made hole for a door in the same wall and I can assure you that I really do not want to make any holes in this wall again. It was hard work. In addition I have to cut through wood and move electricity on both side of the wall. That is why I prefer option 1.

Here is a better drawing of option 1:



This is not a port but a box with a air intake. I am a bit concerned however that it will act as a port since it is so long. However I do not believe this is the case.

Is option 1 called a false baffle?

[ThomasW]

This is all so backward since we usually problem solve around the installation then the person buys the drivers.

We don't use air intakes for IB subs.

Pretty much all you can do is build the 2 X2 X10 box, line it with fiberglass and hope for the best.

[chrisbee]

Thomas, Slow down.

hh is probably thinking he could increase the volume behind the bafle wall by having an "air intake". If this "air intake" really is large enough in area it will probably help the IB cause.

An opposed driver manifold set behind the baffle wall will keep the construction simple without requiring heavy reinforcement or strong braces to the back wall. It may not be the perfect IB but it will still be better than putting his IB drivers in small boxes.

So, hh, how large an area would this "air intake" be and how long would it be? Is it connecting to another room?

[ThomasW]

Oct 8, 2009 4:49:32 GMT -7 @chrisbee said:

So, hh, how large an area would this "air intake" be and how long would it be?

No smaller than the 2 X 2 cross section.

[harryharry]

The air intake is going to be 2 x 2 feet. I can make the box and air intake even bigger, say 2 x 3 feet. But I really do not think this will help. The room the air intake is connected to is about the same size as the cinema.

[chrisbee]

You still haven't told us the length of the "port" to this other room. Is it a huge secret?

You could build a manifold or array in the other room using the entire room as the enclosure and "pipe" the output to the cinema room. If the port were made large enough it would help to detune its pipe characteristics. Using the other room to house your drivers would free up space in the cinema.

Or just build the drivers into a vertical array in the corner of your cinema room and exhaust the back wave through the largest possible port area into the other room. We promise not to tell Thomas what you are doing. ;D

Provided the port isn't too long you could have one hell of a hybrid subwoofer! Stretched IB/ ported or bandpass? Who cares if it works?

The corner opening won't do your stereo image any harm at all provided you keep the crossover at 80Hz or lower.

I have often considered doing this with four of my 15" drivers in a box out in my lean-to workshop. They would fire through a fairly large plywood port and exit right betwen my main speakers. But I was always terrifed Thomas would send some local Cult members over to dismantle it in the middle of the night. ;D

[harryharry]

The length of the box (port) is 10 feet.

I was hoping to use a stereo sub close to the main speakers so I can experiment with even higher crossover than 80 Hz. I also believe that it is easier to get a homogeneous bass response if the drivers a placed a bit apart.

[ThomasW]

We don't know the Le of the Fi drivers. The assumption is it's realtively high. This means they aren't the best choice when a higher crossover point is being used.

Since you can cut a 2' X 2' opening into an adjacent space, I think you should do what Chris mentioned, put a manifold in that opening.

If you want to augment the lower mid bass output from the mains, get a couple high quality 10" or 12" drivers, put them in sealed boxes used as stands for the mains.