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Cone surface also enters into this relationship. As cone surface increases, so does radiation resistance. Hence we see woofers whose size exceeds that of midranges and tweeters. As pressurization losses increase, scaling up cone surfaces becomes the counter measure. So far so modest. The efficiency of conventional speakers means that only about 1-3% of input energy is converted into acoustic sound pressure. Now enter horn assistance. For one, the air can no longer escape laterally because of the horn's wall. And because the diameter of the horn increases as determined by flare rate, the radiation resistance of the small driver at the horn throat is transformed into the larger virtual surface of the horn mouth that meets the room air. Radiation resistance increases, acoustic coupling of driver to air improves and voltage sensitivity rises. This means that the same input signal converts into higher SPLs. So far so groovy particularly for widebanders whose high sensitivity in the mid to upper bands always leaves much to be desired down low. Coupling such drivers to a back-loaded horn compensates at least in theory.

Practically and for the Allegro, we're dealing with a 2-meter thrice-folded hyperbolic horn. The latter two items pay tribute to a reality that bites. Without the folds, men would get shit from their ladies unless those were in love with room-filling sculptures. The hyperbolic bit refers to the flare rate between throat and mouth, i.e. the degree of curvature. Over the first 3/5th, that angle here is quite low but increases over the last 2/5th to shorten the necessary path length over an exponential horn. After all, length and horns are joined at the hip. The cross-sectional image shown on the previous page is incidentally of Allegro's predecessor. Hüpfel is less inclined to publish the actual innards of the current model but isn't equally reluctant on less copy-prone matters.

As stated earlier, the horn acts like a high-pass for direct frontal output. But the horn itself requires a low-pass or it would remain active at frequencies higher than desirable. This gets restricted by the throat chamber. It controls what frequencies are passed and strategically applied felt liner on the horn walls applies further selective filtering. The enclosure is a constrained-layer affair of MDF, Ply and Bitumen plates to minimize cabinet talk. The latter is further addressed by construction details and manufacturing process. Parallel walls are reduced to prevent gross standing waves. The cabinet maker also bonds the chassis planks under high pressure to glue together the final enclosure under considerable tension. This is claimed to create a certain springiness which reduces susceptibility to external disturbances.

Those choosing the Allegro will acquire a piece of furniture. Dimensions of 125 x 40 x 47cm alone (HxWxD) are somewhat tame by horn standards but certainly not demure enough to disappear. Those looking for visual disappearance acts would never pursue horns in the first place of course. But there's more. The cosmetic appeal of the Allegro includes being the product of one-up handcrafting. This clearly is no assembly line carbon copy. It should thus serve admirers of widebanders who as a species already practice highly confident individuation.

High voltage sensitivity -- the Allegro claims 96.5dB -- becomes apparent at whisper sessions. Most basic is the fact that your volume control will sit far more left than usual. Most important is what remains alive at such low levels. To get at any speaker's best usually requires room levels. Not with the Allegro. This gets us straight at one of the core virtues of this Austrian widebander. It's quite a practical virtue too. Most of us simply aren't fortunate enough to listen to music at 11:00 or 16:00 during workdays when noise wouldn't upset neighbors or kids. In the evenings post red wine cork popping, levels must come down. Then it's a true bother if stuff sounds like elevator music without real bass, highs or any dynamic nuance. Not here. As should perhaps be expected from the species of back-loaded higher-efficiency widebanders in general, differentiation between loud and quiet functions splendidly already when loud is still quiet. Music retains life rather than trickles along. What's more, tonal balance remains even. Nothing goes amiss in the extremes. If the Allegro is a bit more present in the upper midrange, it becomes advantageous at background levels where voices gain an extra shot of vitality.