Conversion of IB to LLT

**Bob_99** · **02-14-08, 03:00 PM**

I started to look at building an IB subwoofer and some of the discussion about building an LLT instead seems very interesting but I have several questions that I'm hoping Steve will answer.

When converting an IB to an LLT, can the same drivers be used? That is the ones that are built for IB usage or is it better to get a different type of driver?

Can a box be built around a manifold type of setup or do the drivers have to be directly facing the room?

I don't know anything about DIY subwoofers but I'm assuming that the vent has to output into the listening room. Is that correct? Also, how far away from the drivers does it have to be?

Thank you.

Bob

**Geoff St. Germain** · **02-14-08, 03:28 PM**

It depends on the driver. The same driver can probably be used, but it depends on the specs of the driver as to how much volume it's going to need to get a good frequency response for the tuning you intend to use. Rodny converted his IB to an LLT and was able to use the same Sound Splinter RL-p15s.

The box could be built around a manifold. There's no requirement to have the driver facing the seating position or into the room. For the same reason it works in the IB it would work in the LLT.

The vent does have to output into the listening room. The vent can be as close to the drivers as it can reasonably fit. I would not want to put it a great distance away as there could be some effects once the vent and driver distance starts getting really large, such as a distance larger than half the wavelength of the tuning frequency or so.

**Bob_99** · **02-14-08, 03:34 PM**

Thank you, Geoff. I appreciate your response. If I were to have a rectangular manifold with two 15" drivers on each long side (four drivers total), is it feasible to put the vent output on one of the short sides of the manifold or is that too close?

Bob

**Geoff St. Germain** · **02-14-08, 03:38 PM**

It depends on how large the short side is compared with the port size. If they become comparable you can run into problems as the effective length of the port can be altered. Ideally the port should be at least the diameter of the port away from any obstructions.

**SteveCallas** · **02-15-08, 12:26 AM**

What are the drivers you are wanting to use? Typically an "IB specific" driver works great, as they have a large Vas and can really put volume to good use.

As for the port, like Geoff mentioned, you would need to make sure this manifold opening isn't too small in volume, otherwise the air in this space will couple with the air entering and exiting the port, thus artificially extending the length of the port and messing with your tuning. You also want to minimize any turbulent air flow from the port, so small spaces with corners aren't ideal.

**Bob_99** · **02-15-08, 06:59 AM**

Thank you both. I'll plan on venting through the ceiling instead. As for drivers, while I was considering four 15", I haven't decided on any specific one although the Ficaraudio IB looks interesting (SD:810,Xmax:22.0,Vas:269.5,Displacement 3.56 liters).

The room is 17'x37'x9' and the TV is against one of the short walls with the speakers firing down the long lenght. The seating position is about 12 ft. from the wall and I was planning on putting the manifold centered over the TV between the two main speakers (roughly 13.5 ft. on a straight line from the seating position).

Do you think that a total of about 14 liter displacement is enough or should I consider going to a larger driver? If I go to four 18", I would have to probably span a joist for the manifold.

Any comments are greatly appreciated.

Thank you.

Bob

**Geoff St. Germain** · **02-15-08, 10:10 AM**

The 18 will get you more output. It has both a large cone area and a larger Xmax. It looks to have about double the displacement of the 15 (7.26 L). Of course, you're going to need very large enclosures around 4 or either of these drivers.

**Bob_99** · **02-15-08, 12:14 PM**

I'm trying to use WinISD to model both the 15" and 18" speakers and while this is all new to me, I think that I entered the parameters correctly. The initial results confirm what you said about the enclosure. Can you suggest what might work with the room with a reasonably large box (i.e. the number and size of drivers)?

Thanks again.

Bob

**Geoff St. Germain** · **02-15-08, 01:14 PM**

I was getting somewhere in the 3000 L range for 4 of the IB 18s and around 2200 L for 4 of the IB 15s. Tuning was 12 Hz for the 18s and 14 Hz for the 15s. I think the excursion limited SPL for the 18s was in the 129 dB @ 1m range and 127 dB @ 1m for the 15s. 20 Hz was down about 5 dB and 15 Hz around 7 dB.

**Bob_99** · **02-15-08, 01:33 PM**

Interesting. I was quite a bit off with WinISD which means that I still have a lot to learning to do to use it correctly. My guestimate was sort of close as I figured about 70 cu ft versus the 78 cu ft (2200 l) that you obtained for the four 15s.

I think that I'll play around with Win ISD and a few other drivers for a while so I can get a better idea on how to continue.

Again, thanks for the help.

Bob

**SteveCallas** · **02-16-08, 03:33 PM**

Uhh, so which drivers are we talking about?

**Avus_M3** · **02-20-08, 01:27 AM**

They are talking about the IB specific drivers from Fi.

**v1rtu0s1ty** · **02-27-08, 11:36 AM**

Just curious guys since I have not heard LLT. I also have not heard my IB as well since it isn't installed on the ht room yet, it's in free air right now. What does the LLT have in regards to sound that IB doesn't?

**Bob_99** · **02-27-08, 11:45 AM**

Rather than rehash everything, have you checked out this thread?

http://www.hometheatershack.com/foru...hy-ib-sub.html

As with all things in audio, there are many opinions but it still comes down to what you hear.

Bob

**v1rtu0s1ty** · **02-27-08, 11:53 AM**

Quote:

Bob_99 wrote:

Rather than rehash everything, have you checked out this thread?

http://www.hometheatershack.com/foru...hy-ib-sub.html

As with all things in audio, there are many opinions but it still comes down to what you hear.

Bob

Thanks for the link.

EDIT: Wow, that was a great thread. So EQ is the magic for our IB.

**Bob_99** · **03-03-08, 09:32 AM**

Another quick question about an LLT in the attic. Having just gone through a couple of days of single digit temperatures, I have some concern about cold air pouring through an opening in the ceiling. I think I remember a thread that mentioned insulating the outside of the box is not a good idea but I was wondering if building an insulated box around the box would work and if so, what distance would be needed between the two boxes. I was thinking of building a frame of chicken wire and attaching insulation to it as the outer box. Can anyone comment on this idea?

Thanks.

Bob

**Geoff St. Germain** · **03-03-08, 12:19 PM**

I can't see why you couldn't insulate around the outside of the box. Any idea what the reason was for people to be suggesting not to?

**Bob_99** · **03-03-08, 01:38 PM**

Geoff,

I'm not an expert on sub construction but I believe it had to do with changing the resonance of the box

. I wish I could find the original post but for the life of me, I can't remember where I saw it

.

Bob

**Rodny Alvarez** · **03-03-08, 03:35 PM**

Quote:

I think I remember a thread that mentioned insulating the outside of the box is not a good idea
Bob

I have insulation on mine, and it sounds just fine

I've seen some people at the cult with insulation on top of the subs (IB), to tone down the sound to the rest of the house.

**Bob_99** · **03-03-08, 03:50 PM**

Thanks for the information, Rodny. Very nice setup. It looks like you put a lot of work into it.

Bob

**Bob_99** · **03-26-08, 08:08 AM**

I have a couple more questions regarding LLT. Is the shape of the box critical? And how much power do you deliver to a driver? For example, if it's rated for 250W RMS, do you have to provide that much power for optimal performnce? I tried reading about it but after going to a dozen different sites, my head is still spinning from all the different specs of drivers and amps. I've tried modelling with WinISD but I don't quite understand what value to put into the 'Input Power' field.

Thanks again for any input.

Bob

**SteveCallas** · **03-26-08, 09:19 PM**

Box shape really doesn't matter as long as the internal port opening has plenty of clearance (a minimum of the diameter open and clear in all directions). In terms of real world usage, get the most power for the budget you have allotted. In terms of simulations, use enough power to cause the pre-tuning excursion hump to reach excursion limits - that should give you a good idea of output to expect when you tab over to spl.

**Bob_99** · **03-27-08, 08:13 AM**

Steve,

Thanks for your response. I think what I was trying to ask is if a driver rated for X watts delivers YDb which we say is moderately high, is the driver going to deliver the same quality sound if listened at 85Db (Y - Z where, let's say Z can be 30 to 40 Db) which I would assume required less power (assume one half X). At what point is a driver being under driven if that is possible? Or am I just not understanding any of this? Thanks for you patience.

One more question. Some folks make a point of the group delay in ported designs but if you give an EQ boost to the low end of an IB system, doesn't that also add a group delay? I know just enough to be aware that if you do anything to the signal, there's always a price to pay somewhere.

Bob

**SteveCallas** · **03-28-08, 07:08 PM**

Actually, if you are feeding your sub less power (less heat generated in the voice oil, less thermal compression), and it is utilizing less excursion (less motor nonlinearities), the sound quality would be even better, as there will be less distortion. This is why multiple drivers are better than one not just because they can play louder, but because they can play cleaner at the same output levels than a single driver of the same make.

EQing does indeed degrade transient response, and rarely if ever will you see a sealed subwoofer (including an IB) only given a single boost at a <15hz frequency. This is because that usually isn't enough boost too stay flat into the low teens with a smallish sealed subwoofer due to poor low frequency efficiency, and in the case of an IB, because there are few if any EQ solutions that allow you to boost that low. Additionally, using EQ for that boost doesn't result in significantly reduced distortion or greater headroom like a port does.

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