B&W Nautilus 801 loudspeaker The Evolution of a Speaker part 2
B&W developed a "surroundless" Kevlar cone by using an open-cell PVC foam bonded to the cone. The material matches the cone's impedance at eight places around the circumference, and energy is passed directly into the surround with no reflection. However, on either side of these points, the surround's impedance is either above or below the Kevlar's characteristic impedance, creating two different---and theoretically canceling---conditions for reflection. As a result, the midrange driver has a greater radiating area than would be the case if it used a conventional roll surround.
But the driver still had a bandwidth too broad to work with the Nautilus tapered tube. Other configurations were explored, including a sphere, which offered the advantage of a diffraction-free shape. 300mm (12") seemed to be the magic size---if it were made larger, imaging suffered; smaller, and the unit sounded closed-in, as when you cup your hands around your mouth when speaking. Additionally, the way the drive-unit fit into the sphere was found to have an effect on sound quality. This, it turned out, could be eliminated only if the interior and exterior walls of the sphere blended smoothly with the rim of the driver chassis. This topographic challenge was solved when B&W cast the sphere with a thin wall close to the drive-unit, and a thicker wall farther away; thus the internal and external spherical profiles are slightly offset.
Unfortunately, there remained a problem with the spherical approach: spheres support strong internal cross modes, which are quite evident on delayed-response waterfall plots. These cross modes can be eliminated by filling the sphere with a large amount of damping material, but listening tests revealed that this approach resulted in a decided loss in transparency.
However, it turned out that the combination of a sphere plus a tapered tube proved superior to either on its own. Additionally, by adding small amounts of absorptive material to each, and by closing the tube at its far end, the remaining resonances were removed with little loss of transparency.
Since B&W had determined that tube loading would not be appropriate for the Nautilus 801's bass cabinet, it used a modified version of its Matrix technology. Several improvements were made, including the use of a 38mm-thick curved laminate shell for the enclosure---the curved surface adds stiffness to the skin, and provides an internal shape less conducive to internal standing waves. The downfiring, double-flared reflex port has a 3.5" outlet, which, like a golf ball, employs molded dimples to reduce the air turbulence that normally accompanies high-velocity air flow. (This creates drag in the case of a golf ball, audible "chuffing" in the case of a port.)
The bass driver itself is a shallow, 15" pulp-coned driver that operates as a piston because of its narrow operating band---it's used only up to about 400Hz. Because of its relatively large area, the cone moves correspondingly little, making it sound more effortless than a 12" driver, B&W feels. Cone mass has been increased by thickening the cone with Kevlar fibers, which also make it stiffer. The coil was lengthened for greater linearity, and a doubled, "mirrored" spider construction was used to keep the speaker's structure rigid enough that it didn't flex in response to the driver's pistonic motion.
Each drive-unit enclosure is decoupled from the others to minimize the effects of cabinet resonances on each unit. The tweeter assembly rests on a decoupling pad made of IsoPath,™ which B&W describes as "effectively a liquid suspended with complete stability in a polymer molecular matrix." This makes it compliant in terms of shear and stretch. Like liquid, it has poor compressibility when confined. It is also freely elastic at low frequencies, but becomes viscoelastic at much higher frequencies (or, as B&W helpfully points out, "it has a high tan delta product"). The midrange drive-unit is also supported on a bed of IsoPath at its base, and at the rear of its tailpipe.
IsoPath is also part of the midrange driver's innovative coupling system. Because the operating portion of the driver needs to be integrated with the edge of the spherical portion of its housing, the driver isn't surrounded by the traditional fixing flange. A thin L-section gasket on the rim of the chassis seats the rear of the driver against the enclosure---no screws are used on the front surface. A tensioned rod extending from the rear of the driver assembly is bolted to the rear of the flared tube---this has the added benefit of coupling more mass to the midrange magnet, which lowers its resonant frequency and allows the cone's impedance-matching scheme to use the chassis as a virtual ground.
The crossover uses high-quality air-cored inductors, high-voltage-capable polypropylene capacitors, and Vishay foil resistors in the midrange and tweeter sections. Crossover points are 350Hz and 4kHz, with 18dB/octave third-order electrical slopes and low-phase-shift acoustical responses. The crossover, housed separately from the driver enclosures in the speaker's base, features two sets of shrouded WBT speaker terminals at the rear.---Wes Phillips