Bag End Infrasub-12 powered subwoofer
By Robert Deutsch   •   November, 2004 Of all the subwoofers I've reviewed over the years, the one I remember as being the most satisfying overall is the Bag End Infrasub-18. It went lower than any sub I've had in my system, and its integration with the main speakers was the most natural. At any level that I could tolerate, the low bass had an authority that left other subwoofers sounding just a bit strained.

Although the Infrasub-18 is an excellent performer, it's an extremely plain-looking sub that makes no attempt to be décor-friendly, and it's big (23.5 x 21.5 x 18.5 inches), which makes placement problematic in many home theater systems. Bag End Loudspeakers does most of its business in the pro sector, where the practical concerns are quite different from those in the home.

The Infrasub-12 represents an attempt to provide the performance of the Infrasub-18 in a more home-friendly package. The styling hasn't improved: this is still a utilitarian-looking box painted black, but now it's a smaller box (18 x 15.5 x 15.75 inches), and the smaller size means that it draws less attention to itself. The frequency response still claims to be flat to 8Hz, and the 12-inch driver (vs. the 18-incher in the Infrasub-18) is driven by the same 400W amplifier. The Infrasub-18's driver has a paper cone with a doped corrugated surround, the latter not normally seen in subwoofers from other companies. The surround of the Infrasub-12's driver is the more common foam, which permits the greater excursion that's needed to produce comparable output from a smaller driver.

Patented Technology
Bag End subwoofers' main claim to technical fame is the electronic compensation used in the crossover to boost the frequency response below the resonance of a sealed-box speaker. This "dual integrator" is said to be free of the delay introduced by conventional lowpass filters, and it is claimed to produce flat frequency and phase response. Formerly called ELF (Extended Low Frequency), the dual-integrator bass-extension technology has been renamed Infra (not an acronym), with some evolutionary improvements.

One characteristic of the Infra bass-extension technology that has a potential drawback for surround-sound systems is that it uses a fixed crossover frequency (95Hz), and the crossover cannot be bypassed. Since surround preamp-processors and receivers already have a lowpass filter at their subwoofer output, the combination of that filter with the Infra system could lead to some untoward interactions. To avoid these, Bag End suggests running full-range left, center, and right line-level signals to the Infrasub-12's crossover, and then running cables from the three corresponding highpass outputs on the sub to the power amplifiers.

I tried this arrangement when I had the Infrasub-18, and it did produce an even more natural blend with the main speakers. But it requires three pairs of long interconnects, which was simply not practical in my present home theater system—nor would it be in most systems I know of. Bag End allows that the standard arrangement, with the sub's signal coming from the surround pre-pro's subwoofer output on a single cable, is an acceptable alternative. That's what I used, and I heard no frequency-response anomalies to make me think that there was something amiss in this arrangement.

I should point out that there's also a Pro version of the Infrasub-12 ($1980), which is aimed at the professional studio market but could work well in a high-end home theater. The Pro version has five balanced line-level inputs with five highpass filters and five balanced outputs, a configuration clearly designed to accept all five main channels from a pre-pro and send them to one or more separate power amps after extracting the bass information. It also has one more balanced input without a filter for the LFE signal from the pre-pro. This requires a lot of balanced interconnects, which are much more resistant to interference than unbalanced connections.

You Put the Sub Where?
As discussed in a sidebar to my review of the Primare SP31.7 and A30.5 Mk.II (January 2004), my new home theater reference setup has the electronics in a rack that's against the side wall behind the listening area, and the subwoofer is in the right rear corner. While most people instinctively want to put the subwoofer front and center or in one of the front corners, a rear corner can work very well. I'd used this location for a couple of subs before I received the Infrasub-12 for review, and I concluded that, in my room, the bass from this location was actually better than with the sub in the front.

The controls provided by the Infrasub-12 are quite limited: power switch, volume potentiometer (with detents), and polarity switch (0/180°). As noted above, there is no crossover-frequency adjustment. While these controls are pretty minimal, I had no trouble dialing in a smooth blend from the main speakers to the sub. To do this, and to get a handle on some simple measurable aspects of the Infrasub-12's performance, I used the Room Adaptive Bass Optimization System (R.A.B.O.S.) measurement system, which had been provided with the Infinity IL120s subwoofer (and which Infinity kindly let me keep). R.A.B.O.S. consists of a CD containing 23 test signals covering the 20–100Hz range, with the signals as little as 1Hz apart, and a special sound-pressure-level meter designed and calibrated for accuracy in this range.

I set up the SPL meter on a tripod in the central listening area, and, having first established that the in-phase/0° position of the polarity switch produced a louder sound in the crossover region, I repeatedly played the R.A.B.O.S. CD test signals, adjusting the Infrasub-12's level control to produce the smoothest frequency response. Except for a 5dB bump between 35Hz and 43Hz and a smaller one between 77Hz and 85Hz, the frequency response was impressively flat, varying less than +/-1dB from 22Hz to 100Hz. The level at 20Hz was down 2dB from a plateau between 22Hz and 30Hz.

Basso Cantante
Playing music from CD, DVD-Audio, and SACD sources, the Infrasub-12's most notable characteristic was a sense of naturalness, of blending with the main speakers in a way that did not call attention to itself, providing bass extension without the low bass becoming detached from the rest of the frequency range. This is one musical sub. Even when the music did not include instruments in the low bass range—like Chris Norman's The Beauty of the North (CD, Dorian DOR-90190), which features flute, violin, accordion, and various gentle acoustic instruments—there was an enhancement of realism through a greater sense of hall ambience. This is one of the lesser-known benefits of having a subwoofer, but it requires a subwoofer that is a real subwoofer, not just a supplemental woofer. Like the larger Infrasub-18, the Infrasub-12 delivered on the promise of providing the kind of musical foundation that is felt as well as heard. The 16Hz note in the Saint-Saëns Organ Symphony was palpably there, although perhaps not quite as prominently as with the Infrasub-18.

With films on DVD, the Infrasub-12 continued to impress—as long as the levels weren't too high. I don't have an SPL meter that is sufficiently well-calibrated to be able to specify exactly what "too high" means in this context; let's just say they were higher than the level I normally listen at, but lower than what I've experienced in commercial movie theaters (where I've been known to put bits of rolled-up tissue in my ears to preserve my hearing). I did not have the Infrasub-18 on hand for comparison, but I remember it as taking in stride everything I threw at it, at times shaking the walls and furniture—and from a location that was not as optimal as what I have now.

As mentioned earlier, the R.A.B.O.S. measurements showed that the Infrasub-12's frequency response in my room was down 2dB at 20Hz—excellent performance by normal standards, but Bag End claims that the Infrasub-12 is down only 3dB at 8Hz. Based on listening to material that I know to have some very low bass, my feeling was that this claim is a bit optimistic. R.A.B.O.S. doesn't have test signals below 20Hz, but I do have a Hewlett-Packard signal generator with no such limitation. Feeding the output of the signal generator into the Infrasub-12's input, I listened and recorded the levels indicated by the R.A.B.O.S. SPL meter as I turned the signal frequency down from 20Hz.

With 20Hz as the reference, the Infrasub-12's frequency response was down 6dB at 18Hz, 10dB at 16Hz, and 12dB at 11Hz. At this point, the sub's amplifier shut off, and I did not pursue testing at lower frequencies. It's important to keep in mind that the R.A.B.O.S. SPL meter is not calibrated for frequencies below 20Hz, but it is designed specifically for subwoofer measurement, and my ears told me that the Infrasub-12 was struggling more and more as the frequency was lowered. I remember the Infrasub-18 doing considerably better in this test.

Conclusions
Based on my experience of the Bag End Infrasub-18, I had high expectations for the Infrasub-12, and they were only partially fulfilled. The Infrasub-12 does very well with music, providing an almost ideal blend with the main speakers, but it neither goes as low nor as loud as its senior sibling, and there are some less expensive subwoofers around that perform as well or better in a home theater context. At $849, the slightly bigger Hsu Research VTF-3 plays louder than the Infrasub-12, and its low-end extension is comparable. If you want it all and can handle the size, the Infrasub-18 is almost the same price as the Infrasub-12 ($1770 vs. $1690); as a result, it would be my first choice in this price range.