Before I turn to the subject of this review— Avantgarde Acoustic’s fabulous, $16,000, powered, digitally optimized, virtually plug ’n’ play Zero 1 horn loudspeaker system, which, to spill the beans in the very first sentence, is far and away the most ingenious and sonically successful compact horn loudspeaker I’ve ever heard (and in many ways one of the freshest, most brilliantly engineered, and strikingly styled loudspeakers of any kind I’ve come across in years)—I’m going to talk a bit about horns and my past experience with them. Those of you who feel like I’m taking the long way around the barn, can skip to the chase on page 130 (column two). Those of you who haven’t have had much experience with horn loudspeakers may want to read on.
As was the case with subwoofers (until the JLAudio e110 showed up just a few months ago), I’ve taken a “been there, done that” attitude toward horn loudspeakers for the past decade or two. I gave them a lengthy shot around the close of the past millennium—using Avantgarde Acoustic’s four-way Trio Compact horn loudspeaker system as my reference for two-and-a-half years. By the end of that time I’d lost sight of all the things that horns do better than other speakers (and they do a whole bunch of things better) in the light of all the things they do worse.
Chief among the original Trio Compact’s shortcomings was its inability to consistently turn the one trick I consider most important in any stereo component: disappear as a sound source.
Despite its incomparable transient speed, still unexcelled dynamic range, near-’stat-like resolution and tone color, and ability to make certain hard-to-realistically-reproduce instruments and ensembles (such as grand piano and symphony orchestra) seem astonishingly “there,” sooner or later the Trio ended up betraying its presence by sounding like three separate tubes yoked to an inferior cone subwoofer. In other words, the Trio lacked the seamless driver-to-driver coherence that is one of the chief prerequisites of a “disappearing act.”
Of course, the Trio Compacts I owned were the very first iterations of Avantgarde’s strikingly beautiful spherical-horn system, and a good deal has changed in the German company’s thinking over the last twenty years (as the Zero 1 and my recent experience with the latest Trio/Basshorn system attest). Even at that, I’ve never forgotten those occasions on which the original Trio shone. No other speaker I’ve used as a reference has combined speed of attack, dynamic range, resolution, and beautiful tone color (especially when it was driven by SET amps) in quite the same measures as the Avantgarde Trio Compact. But then no other speaker has had the advantages of horn-loading.
The most mature loudspeaker technology and quite literally the first, horns benefit and to some extent suffer from the enormous amount of research devoted to their design. According to horn guru Dr. Bruce Edgar, there is still a lack of clear consensus about how to build a proper horn (and any number of wrong ideas and dead ends in the literature—not surprising given that horns have been studied and written about for better than a century). What isn’t in doubt, and never has been, are a horn’s manifold intrinsic strengths, the first and foremost of which is much higher efficiency.
A horn is able to provide higher SPLs (and greater transient speed and dynamic range) at a given listening position and given wattage for two reasons. First, the horn’s tapered shape increases the directivity of the driver’s wavelaunch, concentrating and intensifying the sound rather in the same way that a flashlight’s beam becomes brighter and more intense when that beam is focused rather than diffused. (The horn’s tapered shape and consequent highly-directional wavelaunch has the substantial additional benefit of reducing the deleterious effects of room reflections, since, unlike conventional cone or dome drivers or planar dipoles, horns don’t radiate substantial amounts of their energy hemispherically or in a figure-eight pattern.)
Second, a horn plays louder, with greater speed and dynamic range, because it more efficiently couples its driver to the air of the room via a phenomenon known as “acoustic impedance matching.” Like a megaphone, a horn constricts the area and volume of air that the driver (or human voice, in the case of a megaphone) works “into.” As a result of this constriction, the acoustic impedance of the air trapped in the horn’s throat (the narrowest part of the horn immediately in front of the driver) comes much closer to the high acoustic impedance of the driver’s diaphragm. (When the impedance—the electrical, mechanical, magnetic, or thermal opposition of a system to the flow of energy—of a source and a load are matched, power is transferred maximally.) This superior impedance matching of air and driver allows a horn to generate higher pressures from smaller movements of its diaphragm. Moreover, as the horn’s tapered shape gradually increases in area toward its mouth (the widest part of the horn that opens onto the listening room), those high-pressure soundwaves generated in the horn’s throat by miniscule vibrations of the driver’s diaphragm grow lower in pressure and progressively larger in displacement as they travel down the horn’s length, allowing them to couple more efficiently to the low-impedance air of the listening room. A horn-loaded driver is in many ways the ideal acoustical-energy delivery system, typically providing ten times more sound power than a cone speaker would from the same amplifier output.
But playing much louder with much less amplifier power is only one of a horn loudspeaker’s inherent virtues. Because the diaphragm of the driver attached to the horn works so much more efficiently (thanks to increased directionality and acoustic impedance matching), the driver itself has far less work to do than a non-horn-loaded driver, such as a typical direct-radiating cone or membrane that has to move air without the benefit of impedance matching. The horn-loaded driver’s much smaller excursions mean much lower inertia and distortion, which translate into a blur-less clarity, electrifying speed and pace, and sensational dynamic range that have to be heard to be fully appreciated. No other kind of loudspeaker can move air as efficiently as a horn speaker does— and on powerful instruments or large ensembles the effect can be startlingly realistic.
That’s the inherent positive side of horns. The negative side, unfortunately, is also built into them.
Because of horn-loading, the very-high-pressure sound waves generated in the horn’s throat are literally reflected off the throat walls. Any irregularities in those walls (any bumps or dips or material or structural resonances) and any high-Q resonances in the drivers themselves (when a compression driver is run out-of-passband, it decouples from the horn, particularly in its lower frequencies, generating distortion) will add a characteristic turbulence to the signal that ends up being amplified along with the music. The sonic result of this added distortion is the “cupped hands” or “horn coloration” that you typically hear on P.A. systems—like someone talking with his hands so tightly cupped around his mouth that they slightly pinch his nose. Such colorations also have the psychoacoustic side effect of localizing the drivers, making them sound even more like individual tubes than like a coherent loudspeaker system.
Additionally, though properly designed horns are inherently phase-correct transducers, the various resonances of the materials the horns are made of and the necessarily (because of the physical size of the tubes) much wider disposition of the drivers in space vis-à-vis each other can make overall time/phase/frequency coherence a dicey proposition. The small cone and dome drivers of a latter-day dynamic loudspeaker are typically located to the exact micrometer on a baffle—to ensure a time/phase/frequency-coherent wavelaunch. Though the positioning of drivers in a horn system is also mathematically precise, the horns’ physical size, their inherent resonances, and, paradoxically, their more highly directional wavelaunch tend to work against such coherence at normal seating distances, once again making you increasingly aware that you’re listening to separate drivers playing in separate frequency ranges.
Nowhere is this sense of incoherence more prominent than in the bass, which in many contemporary horn systems (such as my Avantgarde Trio Compact from the late nineties) is often handled by a conventional cone subwoofer. Why not use a separate horn for the bass, you ask? Because the long wavelengths of bass frequencies would necessitate a horn with a mouth the size of a three-or-four-car garage! (Back in the day, Nelson Pass actually turned a large garage in the hills above the Berkeley campus into a horn-loaded woofer, which played so loudly and went so deep that cops from all over the valley were regularly called to his residence to tell him to, uh, “turn it down.”)
The other solution to reproducing bass frequencies in a horn system is via a so-called folded horn—a long, zigzag-shaped, flaring duct built inside a cabinet into which a woofer fires. The path-length and flare-rate of the duct determine the low-frequency cut-off point of the horn, although the resonances of the cabinet and of the duct itself can result in the same horn-like colorations in the bass that you often hear in a horn speaker’s mids and treble. (Avantgarde currently uses a superior version of a quarter-wave folded horn in its Basshorn system, but that’s a story for another day.)