Acapella High Violoncello II loudspeaker
A horn acts as an acoustic transformer, converting the high acoustic impedance of a direct-radiating diaphragm to a much lower impedance better suited to excite the air loadwhich is why, more than a half-century ago, in the heyday of low-powered tube amps, horns were ubiquitous. But there are practical limitations to the increase in efficiency offered by a horn: at low frequencies, its output will depend on the diameter of the horn at its widest and at high frequencies, reflections of the sound from the opening will lead to coloration. For the best sound quality, therefore, a horn needs to be used over a relatively narrow passband, which in most circumstances is impractical. In the past, horns have been used over too wide a bandwidth, hence their reputation for "honky," colored sound.
In recent years, developments in the physical design of horns, especially in the cases of the biradial horns from JBL and Klipsch, along with careful implementation and use, have all but banished the coloration bugbear. Two German companies, Avantgarde and Acapella, have also worked hard in the past 20 years to make use of the potential for sound quality offered by horns. Avantgarde was first to market in the US, and Stereophile has reviewed and recommended several of that company's models. But I have been impressed by the sound produced by the Acapella designs at recent audio shows, and approached the brand's US distributor about a review. We settled on the High Violoncello II ($80,000/pair) as being most suitable for my room.
The Acapella High Violoncello II
This beautifully finished speaker is a large, three-way floorstander. Its rectangular enclosure, 50" tall and substantially constructed, houses three 11" treated paper-cone woofers, one of which internally loads the other two in isobaric fashion. Each of these has a substantial half-roll rubber surround and a stationary phase plug on the front of the pole-piece. Though the fact that the woofer enclosure is raised from the ground by the plinth with its four corner pillars, the enclosure is sealed rather than reflex-loaded by a downward-firing port, as its appearance might suggest. A V-shaped notch in the top of the enclosure embraces the midrange enclosure, which is secured with large Allen-head bolts fore and aft. (The front bolt also secures the woofer grille.) A 2" soft-dome midrange drive-unit is loaded with a large, circular horn that has what appears to be an exponential flare. Its bell is 18.5" (470mm) in diameter. The drive-unit's backwave is absorbed by the filling of the midrange enclosure, which is both massive and acoustically inert.
Below the midrange horn, the top 12" of the woofer enclosure is open to the rear, above the sealed compartment for the drive-units. The separate Ion TW1S tweeter module, housed in a perforated metal box and powered with its own AC cable, sits within this section. Viewed head-on, the metal box is hidden by the continuation of the front baffle; all that is visible is the distinctive 6"-diameter brass horn that loads and amplifies the tweeter's output.
The tweeter module is a completely self-contained unit, accepting a line-level input from an RCA jack and amplifying the signal with a class-A amplifier. What is unusual about this tweeter is that it has no mechanical diaphragm. Instead, an intense radio-frequency electrical field ionizes the air between two electrodes to produce a distinctive, violet-tinged yellow flame in the quartz combustion chamber at the base of the horn. The RF field is modulated by the audio signal, causing the almost massless flame to expand and contract in what should be a perfectly spherical pistonic manner, producing sound that is then coupled to the outside world with the 9"-long brass horn. The upper limit of the ionic tweeter is indeterminate but is arranged by Acapella to be at least 40kHz. As well as the AC inlet and the RCA jack, the rear of the module carries three fuses and a rotary tweeter-level control. Internal adjustments are provided for the crossover frequency, control of the flame conditions, and the automatic turn-on voltage (set at the factory for 30mV at 1kHz). Full details of the TW1S, which is also available separately, can be found here.