In all my years of car audio, I've never done an IB setup (no box, for those who aren't familiar with the term) before. So, I decided to try one in my lexus. So far, I really like it. Its not for 'big boom' but for smooth, extended low frequency response, quick transients, and lack of 'coloration', its outstanding.

I went with an atypical IB sub, and used a Crossfire XT212. This thing is a monster, designed to go in a ported box...its the little brother to the 15's I have in my caprice. It has big Xmax (1.1 inches), midrange Qts (.47), decent Fs (31hz) and great power handling (1300 watts RMS thermal).

Anyway, I was modeling this thing for a sealed or ported box, and accidentally put 15 cu ft into BassBox instead of 1.5. What I got was a really low Qtc of .37, and a super gradual roll off. Since anything over about 3 times the Vas is basically infinite (the Vas of this woofer is less than 1), so I essentially simulated no box.

Doing classic calculations, I might have written this alignment off due to the F3 being 75hz. Its really only that high due to the extremely gradual roll off. In a classic Butterworth or Bessel, this would have meant fairly poor low frequency response, but in a highly critically damped situation like this, it works out. I've posted a response curve, if anyone is interested. The black line is the speaker response, the red line is the theoretical in car response, which takes into consideration the acoustic coupling and cabin gain below 50hz that takes place in the car.

In the car, the response is really almost perfect. Its like a good home theater, where you really don't even notice the sub, unless the bass is very low, or until you turn it off. There is no 'boominess' at all.

The hardest part, and I'm still fighting some of this, is the flex of the baffle. I put a 3/4" piece of MDF across the rear deck, with about 10 lbs. of modeling clay between the MDF and the factory metal to help dampen and seal it. I still get some deflection, though, so its probably time to weld.
I also added a loading board in front of the driver to add modal reinforcement to some of the lower fundementals, and to keep the back window from resonating as much.

Long story short, this may be my favorite SQ setup of all times. I know most of you on here are into the 'big boom' stuff, but if you ever get a chance to try an IB for sound quality, you should. You might be surprised.

Toby

 

wouldnt mind seeing the db numbers on that.

 

Good write up! Nice to see more tech. I've tossed aroudnt he idea of IB, but with smaller subs.

 

i use to ib back in the day - in fact orion mader a sub specific for ib - they sounded great - just like you say not real boomy, not present until you turn it off

 

I remember the IB orions. JL had some IB's, as well. I never really liked the sound of them, and now I think I know why.

Most subs designed for IB are of a fairly high Qts...0.6 or higher, and a high Vas. This basically means they have weaker motor structures, and looser suspensions. Suspension (Qms) does play a role in Qts, but not to the extent that the motor force (Qes) does. By having a higher Qts, the resulting 'enclosure' alignment would have a higher Qtc...more on the order of .55 or higher. This would make them more pleasing to the average sub buyer, who is looking for the strong higher sub bass impact that enclosed subs provided, and just didn't have the space or money for an enclosure.

Most 'loud' enclosures have a Qtc of .707 (butterworth) up to about 1.2. The higher the Q, the more output around the knee of the curve, and the sharper the rolloff below the knee. So, for a pop, hip hop, rap listener, a Qtc of 1.1, and a F3 of around 60 hz, is going to 'pound'. Someone looking for that sound will be very disappointed by an IB sub with a Qtc of <.5, and an F3 of 75ish, even though this alignment is much smoother, and will be much more efficient down lower, due to the shallower roll off. This type of sound is often considered 'dry'. likewise, the 1.1 Qtc won't appeal to an audiophile...transients will be muddy, and the box will be very one notish...varying bass frequencies will sound the same, due to the exaggerated output around the knee point.

What really killed the IB subs in autosound was the advent of the low Qts, low Vas, small box woofers. One thing to understand is that the lower the Qtc number, the more control the cone is under. Put a woofer in a small box, and control goes down, not up. Think of it this way, the Qtc is reflective of the tendancy for the cone to return to 0, and come to a stop when the signal is removed. The air in a small box acts like a big spring. This would seem like a good thing for controlling cone movement, but it isn't. When the amp says stop, the voice coil says stop, but the 'spring' in the box has energy to dissapate, so it bounces a couple more times. It is this 'bounce' and the harmonics it creates, that makes the higher Q alignments seem fuller, and louder. It is actually louder, in the area around the knee. So, if your target Qtc is a .7, a woofer with a Qts of .45 will need to have some of its control removed by putting it in a smaller box vs. starting with a .6 or greater Qts woofer. Combine that with a low Vas figure, and you have a woofer that will get you to the .7 Qtc in a very small box. The trade offs are speed, accuracy, and low frequency extension. Efficiency will also suffer, but that's why you get big excursions and high power handling. Its also worth noting that while, for a given woofer, putting it in a smaller box will increase its mechanical power handling (to a point) it will not be as efficient as it would in a large box. Put another way, a woofer will reach Xmax in a larger box with a lower voltage than it would in a smaller box. So, you can use less power to get the same output.

So, IB's get even more attractive. The big drawbacks are still the alignment (the average person still tends to favor the higher Q alignments), and the execution. The most critical part of an IB setup (or any setup, for that matter)is that the front wave be completely seperated from the rear wave, or you get cancellation. In a box, this is farily simple. In a baffle, in a car, this can require lots of work. Every possible source of wave mixing has to be addressed. Break out the dynamat, fiberglass, mdf, foam, whatever. The other concept is rigidity (what I'm fighting with now). Its fairly easy to make a 12x20 wall of a box rigid...its much different when its a 16x40 rear deck in a car. Another way to get close to this is by using a 'small box woofer' in a big box, provided you have the space.

I think I've really dragged this thread into the 'boring' category now...maybe I can cap it off with a discussion on Aperiodic enclosures.

Toby

 

I'll get some once I get everything tied down and tuned, but don't expect big numbers. You can't have everything...I've traded SPL for smooth roll off, and low frequency extension.

I could probably woop up on your ported box at 15 or 20 hz, though...

Toby

 

I used to have 4 pioneer 6x9 on a vertical wall behind the rear seatsfacing forward in a 1980 toyota corolla. You couldn't even see them unless you popped the trunk. No box like you stated I was running a high power clarion 25X4 tape deck. the 6x9s all had radio shack cross overs on them and it hit mind you this was in 1987

 

hey Toby when is you next event?

 

this is something i've been wanting to try, but i just don't want to put the effort in to make a proper IB setup. i'll be compromising and sticking a small enclosure sub in a big box (probably going to be sticking a single 13kv.2 which is designed for 0.8-1.1 cubes into a 2.x cube box... should be interesting squeezing that much space out of my neon) i'll need the extra efficiency since i won't be blowing the sub out power-wise.

one thing i've been debating in this though is speaker placement. on one side, aesthetically, i like seeing the sub pointing out into the trunk space. on the other side, i've heard over and over that in a trunk, i'll probably get better sound pointing the sub towards the back seat. ideally i'd like to just take out my back seats and make a wall. the realist in me though knows i need my backseat. so i'm stuck compromising again. any input on that side of things?

 

May 13th.

Toby