Driver Choice

[ThomasW]

No there have been no IBs built using the 21" drivers. Main reasons are.....

1) they're new to the market and $pendy
2) makes no sense to pay extra for a motor structure that won't be used.
3) in many instances it's difficult to shoe-horn in a 18" let alone a 21"

IMO there's no reason to get into the "paralysis from over-analysis" situation when building an IB. If you don't think the Behringer amps are good enough, pay the piper and get one of the bigger QSC or Crown units.

[FOH]

Thanks Thomas,
1.) They sure are expensive, however they've come down recently.

2.) I read a lot, but I'm learning. Do you mean that the driver is optimized for a small sealed box, not IB? The excessive motor structure? Is this what's added to overcome the limitations imposed by the small box?

3.) Regarding size, many people post and discuss drivers, and never really see anything bigger than a 15" in person. Yes the 18"s are huge, and one in front of you never ceases to amaze. So in turn, a 21", would be impressive. But that's the point.

Total swept area being the goal, the 21"s just seem like the next natural progression toward achieving that.

I modeled the (2)Mal 21 vs. the (2)Fi 18, and at 10Hz., the 21 had a 7db advantage at rated power, 115db vs. 108db. These are my first attempts at using WinISD, however I found the spl advantage impressive.

Thanks,
Analysis Paralysis....too funny, I'll have to mention that to my therapist

[ThomasW]

You're comparing apples to oranges.

As discussed in the FAQ, when drivers are used in an IB sub they're excursion limited not thermal limited like drivers used a 'portable' boxes. Since there's no point in wasting money paying for unneeded motor structure, IB drivers have smaller magnets and sometimes smaller voice coils. This allows the driver to cost less than a driver designed for a smallish sealed or ported box.

When looking at output levels it's important to have a level playing field. This means one needs to compare systems having the same Vd. As you're aware this certainly isn't happening when the comparison is between a pair of Mal 21" and a pair of Fi IB-18"s

Other than bragging rights I don't see anything particularly appealing about 21" drivers. 18" are large enough to be a logistics issue. I wouldn't want to hassle with a 21"-22" units unless circumstances where such that I had no other option.....

Finally don't get too fixated on output levels at 10Hz. This is very dependent on the amount of 'room gain' created by the listening space. I know 10Hz has somehow become some magical design goal. Thing is all 10Hz output does is shake the room, higher frequencies can also create room shake.

[wookus]

Thomas,

As I begin to look more closely at the layout of my basement walls vs my theater walls, I see that I really only have two choices:

1) A single big manifold that will hold eight drivers and have only a few inches of wiggle room.

2) Two cubes, with cones inside the manifold, and having a few more inches of wiggle room.

The only difference to me would be convenience; each individual cube would be lighter and thus easier to lift.

What would you recommend?

This should get me started. Once I start the building, in a couple of weeks, I'll start a new thread under the "My Project" heading. Is that the proper procedure?

Thanks,
Dave

[wookus]

Thomas,

Sorry to keep going on and on...

If I do go with the two cubes, in order to place them so they're space evenly with respect to the theater walls, there would only be about 12" between them. Since the cones would be inside the manifolds this means 12" between the drivers facing each other. Is this okay?

I'm guessing the distance between the facing drivers isn't crucial as long as it satisfies the spacing mentioned in FAQ #15. Is that about right?

Thanks for your help and your patience,
Dave

[ThomasW]

What is it that forcing this close spacing of the manifolds? Perhaps you could post a picture of the area of the basement where the sub will go?

[wookus]

Hi Thomas,

The space looked so big until I started to measure and take into account that my wife will want to be able to open the doors on the cabinets - she's funny like that.

The manifold(s) would go right around where the "on" fluorescent light is (both lights will be moved). The distance from the right hand wall to the left hand cabinets (with doors open) is 94". If it were required, I could cut into the right hand wall to create an opening (to be covered by grill cloth) for a driver to fire towards.
There is lots of room in the basement looking into the picture (perpendicular to the lights), but directly above, in the theater room there isn't as much leeway. The space available from the cabinets facing the picture is about 48".

It's no problem fitting the manifold(s) in the space. I just am not sure how much clearance I need to leave between a driver firing towards a wall and the wall, or between drivers in two manifolds that fire towards each other.

One big manifold able to hold eight drivers (four on each side - a gigantic square hole in the theater floor wouldn't pass WAF) could be, say, around 82" long. This wouldn't allow much clearance between the manifold and the right hand wall (and the cabinets),roughly 7" each side of the manifold, but the drivers would be firing along the walls rather than into them.

Two cubical manifolds with driver's cones inside the manifolds, could probably be 24" square (extra space to allow for the driver's being inside the manifolds). Going across the basement that would be space - 24" manifold - space - 24" manifold - space. That would leave about 46" total space or just under 16" for each space, if the spaces are divided evenly.

Unfortunately, since the walls in the basement don't line up with the walls in the theater, if I want the drivers to be placed evenly with respect to the theater, the spaces are reduced by a few inches. I know it's confusing and I apologize for that.


Yet again, thanks so much for your help.

Dave

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[ThomasW]

Given the rather narrow nature of the room, combined with the seating position close to the screen I don't see how you could possibly need more than 4-18"s unless you have 105dB efficient huge prosound speakers for mains