In the original Trio, bass below 120Hz (the cut-off frequency of the Trio’s horn-loaded woofer) was supplied by a conventional, free-standing, cone subwoofer (or a pair of these per Trio), housed in a rectangular wooden box, powered by a built-in 60W solid-state amplifier, and crossed over via a built-in high-pass filter at slopes that were either 6dB or 12dB per octave—which one I can’t at the moment recall. (There is a very loud amusement park just outside my window.)
To say that this sub/amp/XO was the Trio’s foremost weakness is to put the case mildly. The truth is that any “subwoofer” that is called upon to play up into the power range (100–400Hz), as the Avantgarde’s was, is really acting more like a “mid/woofer” than a “subwoofer.” In other words, the sub didn’t go all that deep in its bottom range and played too markedly in its top one—its sluggish, comparatively ill-defined sound coloring, veiling, and “slowing up” the horn-loaded woofer’s fabulous articulation in the lower mids.
Back in the day, I tried just about everything I could think of to fix this problem, and though I could ameliorate it by adjustments to placement and crossover hinge-point, I could never eliminate it. This “overlap” of disparate sounds in the bass and power range, coupled with the way the horns occasionally called attention to themselves as individual drivers, was the main reason why the original Trio—for all its power, majesty, and resolution on much music—eventually wore me out.
I remember thinking at the time: “Why can’t Avantgarde build some kind of horn-loaded sub to fully match the speed, color, and detail of its horn-loaded woofer?” Of course, I already knew the intractable answer to that question: Size.
I’m not going to go into the math behind the design and construction of very-low-frequency horns because, frankly, I don’t understand it. But the long and short (actually, there is no short) of it is that to reproduce, say, a 30Hz note (which has a wavelength of 37.7 feet), a horn has to be roughly one-quarter that wavelength long (a little more than nine feet) and about one-third that wavelength wide at its mouth (a little more than twelve feet in diameter).
Obviously, a pair of nine-foot-long, twelve-foot-wide subwoofers is going to have what I would reckon to be a lowish wife-acceptance factor, not to mention room acceptance factor—you’d have to live in a palace to house such monsters. (Just for the record, if you wanted 20Hz response, a horn would have to be better than fourteen-feet long and nineteen feet wide at its mouth.)
However, there are ways around these problems, such as corner-loading or “folding” the horn within an enclosure. While corner or folded horns are still large, they are far more manageable in size (as any Klipsch owner can tell you) than a “true” low-bass horn.
Which leads me to Avantgarde’s solution to horn-loaded bass: what it calls the Basshorn, three pairs of which you see pictured between the Trio-G2s on p. 138.
Avantgarde’s Basshorns differ from the other legs of the Trio by not employing spherical horns attached to cone drivers; instead their twin, 12-inch, paper-cone woofers (with neodymium magnets) fire into a mix of exponential and spherical curves, 55 inches (four-and-a-half feet) in length and approximately one square meter (about nine square feet) in diameter per module. Their stacking structure produces a cylindrical wavefront with calculated low-frequency cutoffs of 55Hz for two Basshorns (which have a collective mouth area of about thirteen square feet), 38Hz for four Basshorns (which have a collective mouth area of 36 square feet), and 24Hz for six Basshorns (which have a collective mouth area of 54 square feet). All of the Basshorns are actively equalized below their cutoff frequencies to play down to 20Hz via a system, invented by Matthias Ruff, called ADRIC (Active Dynamic Radiation Impedance Compensation). Typically, the problems with horn-loaded drivers forced to work below their cutoff frequencies are caused by back reflections hitting the driver diaphragms, causing impedance changes and resonant peaks. Matthias has created an inverse curve to compensate for such changes and predictable resonances, which when applied allows the two 250W Class AB amplifiers that come with every Basshorn to see a stable, resonance-free load, and allows the listener to hear low bass without the usual blaring/farting noises of a horn eq’d below cutoff.
So, you may well be asking, what do all these changes add up to? The short answer is: The most lifelike conventional horn system I’ve yet heard in an actual listening room. Though the Trio/Basshorn system may not be quite as seamless, coherent, and colorlessly neutral as its beautifully DSP’d cousins, the Avantgarde Zero-1 Pro and the Magico Ultimate 3, it comes close, and does certain other things that I value better than either.