September 2001

To mimic the basic principle of horn loading, use your hands to create a trumpet-like bell in front of your mouth. Now talk into this shape. Your voice gets louder, but its character is altered too. Horn speakers can do much the same thing. Based on our example, it seems patently obvious that they can distort tonality and change timbre. Perhaps that's acceptable for PA roadies, sound-reinforcement gigs and other head-banging, lo-fi enterprises. But for superior reproduction of music at home, horns are clearly bad, right?

But maybe there is more to horn loudspeakers. Certain manufacturers think so, and Avantgarde is one of them. Jim Smith, the real-life proprietor of Avantgarde-USA, has been an audio fixture since the early days of ARC and Magnepan. He's run audio stores, recorded music and worked as sales manager for various audio firms. He was ready to call the audio business quits. Then a chance encounter with one of the first pairs of Avantgarde speakers in the US caused him to hop aboard a plan -- and plane. He headed for the Avantgarde factory in Germany and became the company's exclusive representative in the New World. His main argument in favor of horns cites the inability of most traditional speaker systems to replicate the tremendous dynamic range of live music.

Horns add considerable acoustic gain to the driver resulting in a loudspeaker of much higher efficiency than conventional designs. As a result, horn speakers are quite common in the world of sound reinforcement where high decibel levels are more important than high fidelity. Unfortunately, the colorations that can accompany these designs are also apparent too. British designer Phil Jones, creator of the award-winning Platinum Air Pulse 3.1 full-range horn speaker, confirmed Avantgarde's position: high-end horn applications should eschew high compression ratios to avoid tonal anomalies and timbral deviations. In sound reinforcement, high compression ratios allow output levels that would be acutely dangerous in a home environment, where they're neither required nor desired.

According to theory, the low compression ratios used by Avantgarde and other high-end firms avoid the cupped-hands coloration but still benefit from the basic advantages of horn loading. (Of course, transducers superior to coliseum hardware are essential as well.) All dynamic transducers are pistonic devices. As such, they operate with diminishing linearity when pushed to the outer limits of their range of motion. For equivalent output levels, a horn-loaded driver undergoes significantly reduced excursion from a directly air-loaded competitor. Ten to 15dB of acoustical gain means that, everything else being equal, a horn-loaded transducer never abandons its area of linearity -- even at high levels, it's barely moving. This also makes it measurably faster. For any given amplitude level, the excursion and rarefaction cycle of out/in motion spans a far shorter distance than that of a non-horn driver. The increased efficiency of horn loading naturally requires lower power levels. High speaker sensitivities, in upper 90s or low 100s, turn single-ended micro-power triode circuits into optimal mates.

Truth or fiction?

Disregard for a moment our audiophile dream list of desirable attributes such as soundstage holography, midrange voluptuousness and bass slam. If we honestly compared our system to a live event, one area would likely pale: dynamics.

Most speakers placed in conventionally sized rooms end up in what a real venue would call the extreme nearfield. An 11-foot distance at home might look like the farfield, but in a jazz club it's front row. At that distance, a real-life drum kit is frighteningly loud, transient rim-shot explosions and cymbal crashes startlingly violent. When a diva goes full throttle, her voice singes hair in the first row of a symphony hall. When a pianist falls into the keys from two feet up, it isn't sweet but stridently metallic.

It's quite obvious that live music is filled with serious short-term bursts of powerful decibel levels, sharp leading edges and brilliant transients. The confounding part is how differently our hearing reacts to live music versus at-home playback. In an acoustic venue, dynamic peaks are exhilarating and hair-raising. In a living room, attempts at realistic replication are usually plagued by convulsions. Clearly, most stereo rigs go into nasty-sounding and distorted compression when we set average volume levels such that peaks approach real-life scale.

Only gluttons for aural punishment won't instinctively lower playback levels to re-enter a safety zone of pleasantness. And that's where, according to Avantgarde, we're missing out. Playing it safe means playing it boring. The oceanic roar of the original event is sadly diminished to a teapot tempest. The magical journey on the open aural seas is imprisoned on a tiny windjammer inside a bottle. No emotional devastation -- just bonsai audio!

Fancy words or fact?

Here are certain design aspects of the Avantgardes I'm especially curious about.

Controlled dispersion: Consider the subject of wide dispersion. Unless a speaker was specifically designed to have smooth and broad off-axis response, its direct and reflected response curves will be dissimilar. When combined at the listener's ears, room boundaries will induce peaks and suck-outs. To minimize room colorations with such designs, many audiophiles instinctively revert to room treatments that minimize side-wall reflections for clearer sound. Others place their speakers so far away from room boundaries that most interior designers and spouses throw fits.

Consistent performance: According to Avantgarde, the spherical shape of their precision-molded horns causes 85% of the sound to be aimed directly at the listener. Only about 15% is said to escape as reflection, albeit highly attenuated in output. Above the pass band of the active woofer section, this speaker should, in theory, remain less encumbered by room-boundary logistics than conventional speakers. The claimed controlled directivity or dispersion pattern should make them easier to place and (from room to room) less dissimilar-sounding than regular designs.

Crossover simplicity: Fewer electrical parts in the signal chain usually translate into enhanced transparency and ease if the mechanical/acoustical functions operate in a linear fashion. As a physical function of their so-called throats (the innermost area of the flare's narrowest portion), the respective lower cut-off frequency of the Avantgarde horn drops acoustically by 18dB/octave. The low-pass thus remains free from electrical crossover parts. The upper cut-off frequency is a combined function of mechanical driver roll-off, a first-order electrical network and augmentation from a small inner chamber that acts as secondary acoustical 6dB/octave bandpass filter. These acoustical and electrical first-order slopes combine to an effective 12dB/octave roll-off, but they preserve the phase-linear benefits of an electrical 6dB/octave network.

Back to reality

The ready availability of many patently colored horn speakers speaks loudly and very efficiently about their inherent challenges. Could Avantgarde's engineers marry their very real theoretical advantages -- controlled dispersion, high efficiency, economy of transducer excursion and concomitant low distortion figures, superior mechanical rise times, and potentially huge dynamic range -- with the kind of flat frequency response and tonal honesty we audiophiles demand?

The Avantgarde Duo is a dual-horn actively bass-amplified four-driver, three-way speaker system. The load is a nominal 8 ohms, frequency response a claimed 22Hz to 20kHz, and sensitivity 103dB. The Duo's high-frequency horn covers an ultra-wide bandwidth down to 900Hz, with the passive crossover point at 2kHz. The midrange horn extends to 170Hz, where it hands off to dual 10" powered and sealed woofers. Why not a bass horn? Horn diameter and depth are a function of wavelength. Even a folded horn -- or spiral, as in the Platinum Air Pulse 3.1 -- would require a positively gargantuan cabinet. Consider this: the Duos' larger horn diameter is 26.25". It covers second- and third-order low and midbass harmonics and all midrange fundamentals. The company's flagship adds 11", for three feet of width, just to lower the crossover point by a mere 70 cycles to 100Hz! Avantgarde's engineers obviously intended to keep their speakers as living-room friendly as possible.

Where do we go from here?

To the music, of course. A forthcoming "Earmarked!" column has a pair of Avantgarde Duos playing in my space. I look forward to sharing my sonic findings next month.

...Srajan Ebaen
srajan@soundstage.com 

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