Celestion 3000 loudspeaker
While we were setting up the speakers, I stuck one of the stand spikes in my foot! Later on in the review period while I was repeating a measurement, one of the pair slipped off its stand, breaking the grille (which is essential to the speaker's optimal performance) loose from its retaining studs, and warping the ribbon's magnet structure. Luckily, this occurred after a replacement pair of 3000s was already on their way to me, but I was understandably starting to feel that this review was jinxed, particularly as I found it all too easy to get the 3000s to sound quite unimpressive.
I'm running ahead of myself, however, and I should devote a few column-inches to what the 3000 is all about.
Throughout the 1980s, Celestion's product line consolidated the pioneering work on metal-dome tweeters that had first made an appearance in 1981's SL6 loudspeaker. Metal-dome drive-units now dominate high-end box loudspeaker design, with OEM units from SEAS, VIFA, MB, and Elac widely used by manufacturers (footnote 1). It was perhaps not so surprising, therefore, that when Celestion announced their contenders for speaker supremacy in the '90s at the 1989 Berlin Funkaustellung, the models 3000, 5000, and 7000, these featured a very different drive-unit to cover the frequencies above 900Hz—a monopole ribbon! (By "monopole," it is meant that the radiation from the diaphragm is all in a forward direction, as with a conventional enclosure-mounted moving-coil unit, and not like the dipole ribbons featured in speakers from Magnepan, Carver, and Apogee.)
A ribbon driver is perhaps the most basic electromagnetic drive-unit. A rectangular strip of corrugated aluminum foil (12µm thick in Celestion's ribbon) carrying the signal current is suspended between the poles of strips of magnets. These are strontium/ferrite ceramic magnets (mounted on a steel backplate) in the case of Celestion driver and are supported by an extruded-aluminum former. When the current flows up the diaphragm, the latter moves toward the listener (or away, depending on the arrangement of North and South magnetic poles); when it flows down, the diaphragm moves in the opposite direction. Feed an alternating current to the ribbon and the result will be sound.
The advantages of ribbons, when properly designed, involve their relative lack of passband resonances, low inherent distortion, and intrinsic heatsinking; the disadvantages mainly concern their low sensitivity, extremely low impedance (typically around 1/10 of an ohm), the fact that at very high levels, the diaphragm motion can lead to metal fatigue, causing the ribbon to split longitudinally, and their labor-intensive construction, which obviates much of the automatic machinery used to manufacture conventional drive-units.
There is no doubt, however, that a good ribbon can offer a sound free from nearly all the fizzes, honks, and squawks that can be all too obvious with even quite good cone or dome drivers. The Celestion unit (see the illustration in Robert Harley's interview with Celestion designer Graham Bank elsewhere in this issue) has obviously been very carefully thought out: the end suspension incorporates silicone rubber sections which keep the ribbon tension constant even when it warms up under hard drive conditions; foam dampers minimize the production of standing waves in the ribbon due to the end clamp; and four lateral suspension threads ensure that the diaphragm is kept centered in the magnet gap.
The Celestion ribbon has a nominal impedance of 0.14 ohm, so it is fed signal via a stepdown transformer. The design of this transformer is not trivial; it has to source high currents while maintaining its frequency response to well above the audio band. Celestion's design has split primary windings with a total series impedance of 5 ohms—dividing the primary into two is said to halve the peak leakage flux, thus reducing eddy currents in the core material—while its 0.14 ohm secondary winding is wound in bifilar (double-wound) fashion.
As constructed, the Celestion ribbon is a natural dipole: it emits as much sound to its rear as to its front. To convert it to monopole operation, the rear radiation needs to be absorbed, and to this end Celestion loads the ribbon with its own transmission-line sub-enclosure (footnote 2), this some 4" wide and running the full depth of the cabinet. The ribbon is mounted at the front of this enclosure at an angle of 45°, and as the cavity is filled with sound-absorptive material, the ribbon's backwave undergoes a number of internal reflections, each time losing energy in the filling. By the time it reaches the ribbon again, it should be attenuated to the point of inaudibility. Assuming that all goes to plan, all the listener will hear, therefore, is the ribbon's frontal radiation.
Due to the low crossover frequency, matching the radiation pattern of the ribbon with that of the woofer is a little more problematic than usual. By mounting the ribbon unit at 45° on the speaker's internal edge and carefully contouring the edge of the ribbon plate, Celestion has therefore allowed the ribbon to "see" a larger baffle at low frequencies. To ensure that the presence of the woofer allows the ribbon to have an unobstructed acoustic view of the entire baffle, the woofer is mounted on a small ¾"-thick square sub-baffle and the grille actually has similar built-up sections above and below the woofer, which gives a flush, obstruction-free surface. Diffraction effects from the first "sharp" edges the ribbon's output sees, from the rear and outermost right-angle cabinet corners, should therefore be below the driver's operating range.
Footnote 1: Dynaudio and Morel seem to be the only companies sticking with high-quality soft-dome tweeters, while Focal is making much of their inverted-dome fiberglass and Kevlar units, which have a lot in common with metal-diaphragm units.
Footnote 2: Unlike the so-called "transmission line" speakers, in which the bass line terminates in a port and often doesn't even have any absorptive filling, the Celestion ribbon enclosure is sealed and is intended to act as a true transmission line. No energy from the backwave is ever intended to reach the outside world.