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