Apogee Caliper loudspeaker
The moving-ribbon principle for generating sound is as old as the loudspeaker, the prewar German Blatthaller PA system that announced the departure of Robert Mitchum by Lufthansa for all parts of Occupied Europe in The Winds of War (footnote 1) being a very large ribbon speaker. It is also one of the most simple. An alternating current is passed down a flat conductor suspended between the poles of a magnet; Fleming's Left-Hand Rule would indicate that the conductor should move back and forth at right angles to the directions of both magnetic field and current; and move it does, generating sound. It doesn't generate much sound unless the magnets are very powerful (read expensive) and/or the ribbon very large. The very low intrinsic impedance means that an average amplifier will be hard put to deliver sufficient current.
The ribbon does have advantages, though, over a conventional drive-unit. As with an electrostatic design, the diaphragm is driven over its entire area, giving an even dipole response above low frequencies, where the back wave will cancel that from the front, and the lack of cabinet implies the absence of a major source of coloration. Equally as important is the lack of coloration from diaphragm resonances. Apart from the one low-frequency resonance due to its suspension—and this will be very low if the ribbon is suspended floppily enough—there is hardly anything to color the sound.
In general, ribbons have been used only as tweeters (footnote 2): a small size is appropriate due to the need to reproduce only high frequencies, which leads to physically manageable (and affordable) magnets; the drive is via an impedance-matching transformer; and the low sensitivity is often compensated for with some kind of horn. The only large-scale manufacturer of a full-range loudspeaker resembling a ribbon is Magnepan, whose proprietary drive-unit—a grid of wires bonded to a Mylar diaphragm—acts in an analogous manner. Magnepan also uses a true ribbon tweeter in their MGIIIA and Tympani IV models. Nobody, however, was making a full-range ribbon loudspeaker, the theoretical advantages being outweighed by real-world disadvantages.
Such was the situation when Apogee Acoustics' designer, Leo Spiegel, retired from the aerospace industry at the beginning of the 1980s. Leo felt that a true full-range ribbon speaker would offer significant advances in clarity and neutrality over dynamic and electrostatic speakers, and with his son-in-law Jason Bloom duly set up a loudspeaker company to attempt the task of making such a speaker. Their first product, launched at the 1983 SCES, was a 7'-tall, full-range three-way ribbon loudspeaker, a system with astonishing dynamic range and bass extension, but, at over $6600/pair, hardly a volkspeaker.
Treble and midrange were handled by 80"-long, floppily suspended, corrugated aluminum ribbons hanging between the poles of a ceramic-magnet array in a steel frame. The bass unit, however, was a new departure in speaker design: a trapezoidal sheet of corrugated aluminum foil, reinforced with a transparent insulating Kapton tape on the rear, had a pattern of slits hand-cut in it, making, in effect, a large, flat coil. This coil is then mounted in front of an array of bar magnets, their poles so arranged that, as current flows through the aluminum foil, each section moves in the same direction. Unlike the tweeter/midrange ribbons, which are so floppy that they can hardly be said to be under tension, the woofer diaphragm is stretched across a wooden frame, much like a very thin metal drumskin. The art of the Apogee woofer lies in the fact that, coupled with the irregular shape, the tension is arranged so that, above the fundamental low-frequency "drumskin" resonance, the diaphragm will be effectively non-resonant.
The large number of magnets required and the labor-intensive production of the large diaphragms imply a size, weight, and price that rule out the possibility that the original Apogee would feature in any but a very small number of systems. Leo and Jason's game-plan, therefore, was to produce smaller and more affordable loudspeakers, which would sacrifice dynamic range, power handling, and bass extension but keep the key sonic attributes of the ribbon principle. Each year since 1983 has seen the introduction of a smaller Apogee ribbon speaker: first the three-way Scintilla at $3300, then the two-way Duetta at $2780, and now the Caliper, effectively a two-thirds size Duetta, costing a "mere" $1650.
The Caliper uses the smallest Apogee woofer yet, with an area of just 450 square inches (equivalent to the radiating area of three conventional 15" drivers!). The tweeter appears to be a cut-down version of the Duetta ribbon: three vertical strips of corrugated aluminum foil (the center one is slightly narrower), joined with a yellow Kapton tape backing, hang between the poles of the magnets. Closer examination, however, reveals that whereas the earlier Apogee tweeters hung free, the Caliper tweeter is clamped from the rear about two-thirds of its length from the bottom, and the top third "hinged" back slightly. This dogleg shape is said to improve the dispersion. Nearfield measurements indicated that the tweeter handles the range above 800Hz or so, with what appeared to be 12dB/octave slopes. Actually, the crossover is more sophisticated, gentle initial 6dB slopes giving a seamless transition region, with steeper slopes being used out-of-band.
Footnote 1: I think I've been watching too much television this summer; something to do with the NAD MR20 TV monitor I bought on Bill Sommerwerck's recommendation! But don't the Dr. Pepper ads have great sound? And you should hear how the Calipers handle the soundtrack of Miami Vice.—John Atkinson
Footnote 2: And as microphones. The superbly uncolored Coles/BBC 4038, used by Sheffield for their recent Firebird recording, is a ribbon.—John Atkinson