2x18" IB's v's 4x15" IB's

[moonfly]

I think Johns piece goes some way to show that it probably differs between manufacturers. Looking at the WinISD plots of the IB15's vs the Fi18's, the 2 Fi'18's seemed preferable over the AE's, even 4 of them.

Given the whipping Thomas dished out, I'm wandering how accurate I should assume the software model is. Its always been pretty accurate for Johns other drivers, so Ive got a fair bit of confidence in WinISD in relation to AE drivers. That said, the point you make about IB's is interesting in that respect.

I actually modelled both sets of drivers as sealed cabs, but used my actual underfloor volume rather than the guideline maximum stated on here. The hope was to accurately estimate where xmax would be. Part of the decision to choose Fi over AE IB drivers was due to the xmax results that simulation gave.

To increase the tolerance of the results, I then assumed that if I included floor bounce into the equation, and assumed the effect of that could be slightly more excursion than modelled, I would need a driver that came in below xmax. The Fi drivers do with 5mm to spare in WinISD, with the planned 500 watts, so hopefully Ive given myself a bit of a cushion there. I cant recall what xmech on the Fi's where though, can you?

[ThomasW]

It's not that software sims are bad, it's that they're of limited use. And unfortunately there's a tendency to use sims as the sole basis for driver or alignment comparisons. This is myopic.

If one models a sim of a smallish portable box with a Qtc of 0.5, it certainly doesn't sound the same as an IB with the same Qtc.

So a sim is helpful when it comes to helping decide how much Vd might be needed for a particular install. But there are other factors to be considered, and they may trump what the sim recommends.

[ThomasW]

Addendum,

IMO Xmech should never be a consideration. Things happen way too fast when we reach Xmax let alone Xmech.

And some people are factoring Xmech in like it was Xmax. Once passed Xmax distortion goes up exponentially, so the idea of factoring in Xmech in with regard to anything is IMO a bad idea.

As far as I'm concerned the 500 watt amp I've seen posted isn't adequate for a pair of Fi-18"s, I don't care what the sims say. Yes IB subs become more efficient as frequencies go lower, but watts are cheap, and it's way better to have too much power rather than drive an amp into hard clipping then toast a driver with DC.

[moonfly]

thing is, you would need to change the driver to do that comparison, so its not a comparison you can make is it. Different driver all sound different.

Personally, I dont want a Qtc thats too low, certainly not near .5, just preference from past experience. The Fi drivers are .65, which is ideal for me based on past experience (again) anyway, so I wasnt to worried about that. Adding 4 drivers does seem to change that in software models, with the AE's being quite high, and I just think that would end up all muddy and boomy given my room. Also the spl's and xmax figure were more favourable from the Fi's.

I know I'm basing a lot of this on the software model and past experience with box subs. If your asking me to throw that out the window, then fair enough, but that will put me entirely back into the unknown really. Problem I have with that is that unknown will be the same for all of us for my room, but surely I must have something I can work off to give me an idea of how the results will be, if not then everything is a guess, from driver choice, to input power, to number of drivers, and final Qtc sound. That just seems to rub me up the wrong way, for no reason other than so far so good with everything I think I know, and I just dont feel like I should scrap all that off.

[moonfly]

I dont think about xmech, I try keep within xmax. I like to think as xmech as the event horizon, or the crest of the waterfall, and just dont go there. I want to stay well within xmax ideally.

I take your point on that amp though, and given I want 500 watts, I think I am best having more on tap. I'll just need to keep an eye on the drivers to begin with.

[Wull]

I am building my manifold this Sunday.

As I am now using 4 x 15" Fi IB's, the manifold will be 57cm square.

3 x SD 810 = 3240. 57x57= 3249 cubic cm's

The loft space where the manifold will be fitted has 480 cubic ft, and my loft space which joins it which has air gaps all along has at least 1000 cubic ft.

Main loft 480 x 28.3 = 13584 ltrs.
Adjoining loft 1000 x 28.3 = 28300 ltrs.

vas required = 4 x 15" = 183.3 x 5 = 916.5 ltrs x 4 = 3666 ltrs

So my sums add up, yes?

Now, from what I have read I understand it is best to keep the manifold as short as possible. i already have to add 6" to the height to allow for the ceiling joists. My question is this, will it hurt to add a couple more inches in height to allow room for my loft insulation, or would you keep it to the minimum which would be 6" + 8" to the center, so the overall height would be 22"'s?

Thanks Will.

[chrisbee]

Hi Will

Since all your drivers are going to be on the same level I'd just add what you need to the manifold height to accommodate the insulation.

Going back to the two loft spaces.. how are they separated and how are they connected?

Is there a large hole between the two spaces?

Could there be a large hole made between them without too much difficulty?

BTW: What is your ceiling joist spacing?

[Wull]

Hi Chrisbee.

The lofts are joined, but a wall runs along the side of them, holding the roof's purlins up as the pitch of the roof drops across my living room. There are 14"/6" gaps in between the purlins that run right a long the wall.

I could make a hole next to the manifold if this would be beneficial? I had to do this once before when installing a new gas fire flu. This sort of work is not beyond me as I built my house, it's a barn conversion.

My friend is a joiner, he is going to help me with the manifold, and altering the ceiling to suit.

[ThomasW]

Sept 2, 2010 8:59:59 GMT -7 Wull said:

As I am now using 4 x 15" Fi IB's, the manifold will be 57cm square.

3 x SD 810 = 3240. 57x57= 3249 cubic cm's

If it's easier to work with the box can be 18"-20" cube. That decrease in size will have no audible effect on performance

vas required = 4 x 15" = 183.3 x 5 = 916.5 ltrs x 4 = 3666 ltrs

So my sums add up, yes?

You have plenty of room

My question is this, will it hurt to add a couple more inches in height to allow room for my loft insulation, or would you keep it to the minimum which would be 6" + 8" to the center, so the overall height would be 22"'s?.

Not a problem. When we say keep it short we're not talking about a few inches, we're talking about avoiding building something 6' long

[Wull]

If it's easier to work with the box can be 18"-20" cube. That decrease in size will have no audible effect on performance
That's good to know, I will make it 20/20 then, it's a bit more house friendly.



Not a problem. When we say keep it short we're not talking about a few inches, we're talking about avoiding building something 6' long

Cheers Thomas. Roll on Sunday

[Wull]

How do you know, or what is the best way to ensure if using a 4 IB manifold that all IB's are working to their optimum?

What's happened is I have had to position my manifold so one of my IB's is close to an internal wall, I have been able to make a hole in this wall so nearly all of that IB is visible through the hole on the other side of the wall, but part of the IB is not visible

Am I best running each IB in turn and check with a SPL meter?

Hope this makes sense

[brady]

Can you post a pic?

[ThomasW]

Sept 18, 2010 4:51:41 GMT -7 Wull said:

Hope this makes sense

It doesn't.

Per the previous poster we need to see a picture f the installation

[Wull]

This picture is taken from the outside of the manifold looking through into the 2nd loft area. The white rock is a lime stone lintel.

Attachments:

[Wull]

This picture is taken in the 2nd loft through the hole I have made in the wall.

Attachments:

[Wull]

There is a 10" gap between the wall and the manifold. Unfortunately their is a huge lime stone lintel which blocks part of the IB's view. This can not be removed, I will lay insulation over this.

Attachments:

[ThomasW]

Questions like these are supposed to be asked prior to cutting holes, building manifolds etc., ...

The last image where the driver will be so close to the wall, is a situation where the manifold should have been built to reverse mount the driver (magnet inside box)

At this point any place where a driver is too close to a wall line the wall with thick insulation to absorb the rearwave.

[Wull]

Cheers Thomas.

Yeah I realise I should have done a lot more research, messed up their.

I think there would still be room with in the manifold to install an inner skin, so I could mount them inside the manifold if required.

Would I be right in thinking that I could run only two of the opposed IB's and record their results, then run the other two to see if the output is equal. Or is it not this simple??

[ThomasW]

Measuring them in pairs will show no difference in their overall output.

If you have enough room to re-skin the manifold so a pair of drivers can be reverse mounted that's a good option.

Actually mounting one pair magnets in, and the other pair magnets out will cancel even order harmonics. In theory that improves the SQ.

[Wull]

Thomas, If I am able to do this 'put one set of IB's inside the manifold' are the IB's connected/wired the same way as if all the IB's are installed on the outside of the manifold, or do I need to reverse one set so they are all working in the same direction?

Also, instead of using a lining inside the manifold, could I mount two of my IB's like Jon Foulks 'outie' IB install?
So the IB is facing into my loft on the outside of the manifold with the magnets facing in?

Thanks for your time.

Will