Jeremy, you pose some good questions.
I'm in a unique position. I'm not ready to go into much detail, as I've not completed all my impressions and testing yet. But I own both a capable IB, and high quality small sealed subs. My room has both a quad-set of 18" Fi-IB3-18s, in a manifold IB, and multiple small-sealed subs. I built the IB, and then complemented it's output by purchasing two Seaton SubMersives for filler subs/modal control experimentation, etc.
Originally Posted by Bill Fitzmaurice
IBs don't inherently go any lower with any more ease than any other design.
Bill, I respectfully disagree.
I'd suggest that's not entirely true. One chief theoretical advantage of the IB approach, is the ease at which it can delve into the bottom octaves. Yes, the same frequency response can be attained with either a sealed or IB design. However, without the acoustic suspension air-spring, the drive signal is utilized with a greater level of effectiveness, with less being disapated in heat and potentially negative effects associated with compression. Additionally, the quality that many IB enthusiasts claim is inherent to a properly executed IB, may include both the lack of compression and be the lessening of latent energy re-encountering the driver via reflections and pressure.
Those two characteristics, in my opinion, very much differentiate the two approaches; small sealed, and IB. There's a third as well, the way we power each type .. and that advantage goes to the small sealed.
Now having multiple small-sealed certainly can facilitate a much easier path to one's acoustic goals for a multi-seat HT or music listening space. Trying to overcome the room's dominance and influence in the bass octaves, is clearly benefited from multiple sub sources, carefully positioned, EQ'd, and aligned in time.
Jeremy a little review;
Within the sealed alignment, there are two sub-sets; IB and small-sealed ... clearly the acoustic back pressure or air spring is the primary mechanism differentiating them. Small sealed, properly executed, utilizes that air spring in essence to facilitate a variable limiter keeping the huge excursions in check. Thus you can have massive amounts of power on tap both above and below the "knee" of the response curve, .. so big inherent advantage their IMO.
Heat is a by product of current flow. Typically, a driver can take enormous amounts of power, but they're limited with regard to dissipating the heat. This creates compression effects, which onsets at relatively modest power levels, but doesn't become a big problem until higher levels. A driver changes electricity into acoustic pressure, ....any deviation toward more heat is bad, an can be problematic in conveying realistic dynamics and accurately tracking the material.
For example, whether it's the subs or the mains, if any section enters into compression, noticeable spectral shifts occur, robbing the playback of adequate dynamic realism. This may not be as nasty as clipping, or over-loading of other types, but it really detracts from realism.
Also, inherent to a small sealed is the native response shape prior to EQ. Just like the Fi IB3-18 in a smaller box. Typically, a small sealed has a somewhat smooth, yet peaked, or mounded in the middle and un-extended frequency response before equalization. This is due to the low efficiency, up against the air spring ... as frequency lowers .. the box fights back. However, one can EQ back in the response shape desired to attain a smooth, extended response in room, as long as the driver has enough magnetic motor strength.
This works very well, and is the fundamental principle behind essentially all small-sealed designs. But, it costs amplifier power, thus more heat. If excessive, the heat can easily rob the driver of efficiency in the form of compression, so even more power is added. Although it sounds quite detrimental (and certainly can be), all small sealed subs work this way and many of them just fine with few noticable issues.
Now, with an IB sub, the drivers aren't thermally limited .. like the sealed. The IB approach is excursion limited. The lack of acoustic loading on the back-wave side of the drivers, allows for fantastic bottom octave efficiency and native extension without EQ. This allows for far fewer negative effects of compression, and allows for superb dynamic delineation of transient detail. Now the trade-off is typically, you need more drivers for equitable output in an IB, vs that of a small sealed approach. This is because of the power limitations of the IB excursion limited design. In the lower frequencies, you will not encounter thermal issues with an IB. The limiting factor will be running out of throw (excursion) down in the bottom octaves.
Those are the theoretical differences, which favor the IB. Greater accuracy in tracking signal transients, due to both a lower level of compression, and lessening the smearing effects of latent acoustic energy influencing the output. The IB's potential disadvantages are within the realm of placement, and adapting to room's acoustic needs. Realistically, the small sealed has advantages in placement flexibility, which is huge.
Also, in my opinion, the manner in which one can power a small sealed has disticnt advantages. This is what call the IB power conundrum. If the IB design runs without a high pass filter (which is the whole point in the first place), then the power available is limited above the knee. The 18" Fi's encounter their excursion limits about 300 watts or so @10hz. So operating with no hi-pass, you're limited to a modest amount of power. Now, they'll play much louder with tons of impact with an hi-pass and much more power. However, that negates the wonderful, natural extension of an IB in the first place. This is why there's more drivers needed with an IB system as compared to a typical small sealed .... The IB power conundrum.
The sealed approach on the other hand, has all it's power availed to the driver, .. both above and below the knee. This is because the vari-limiter imposed by the air-spring. This is best case use of power, which is significant in use, .. because spectrally much is required from 25hz-35hz. So, either high-pass and pour the power to them, .. or multiply the drivers 2x or 4x.
Wrapping up, both design's inherent problems, have clear workarounds ... with the exception of the latent energy issue of the small-sealed. How much of an issue is it? I don't know, but it is a clear theoretical advantage to the properly executed IB.
Both approaches are fine. My IB sounds different than my SubMersives, .. they're both great. Any differences pale in comparism to the real hurdle one faces; the acoustic distortions of the room.
Good luck, and I hope this helps