B&W Nautilus 801 loudspeaker The Evolution of a Speaker
Because the B&W Nautilus 801 combines properties of the Nautilus speaker with those of the venerable (20 years old this year) 801 model, it might be appropriate to glance at the new speaker's antecedents before looking at the new technologies involved.
Along with KEF's R105, the original 801 established the concept of separating the bass and midrange enclosures while mounting the tweeter to the top of the midrange "head." This allowed B&W to optimize each enclosure to the frequency range of the driver, and improved the off-axis response. The 1" metal-dome tweeter employed a "phase ring" to optimize the unit's dispersion. However, many listeners blamed this ring for reducing the speaker's sense of openness. The 5" woven-Kevlar midrange driver wasn't the first time the material had been employed---I believe that honor goes to B&W's DM6 of 1975---but B&W's use of it in their flagship model signaled a commitment to the material that continues to this day. The long-throw woofer's 12" cone was made from a PVC compound, and, by the series II version, B&W's Matrix™ enclosure-bracing system was used to reduce the level of cabinet radiation. At high SPLs, the cone and its surround were found to deform, which compromised bass-transient performance.
In its three iterations, the 801 was one of---if not actually the---best-selling high-end speakers of all time. Its use as a monitor was nearly universal in studios catering to classical recording, but its relatively low sensitivity (87dB) and somewhat ponderous bass response, especially when pushed, has limited the speaker's appeal in recent years, particularly in studios that specialize in synth-heavy pop and rock.
The Nautilus is certainly one of the most striking-looking speakers ever designed---you're practically obliged to love it or hate it on sight. Surprisingly, the futuristic-looking tapered tubes trailing behind the drive-units are functional, not cosmetic.
A common source of distortion in loudspeakers is the buildup of high pressures behind the drivers, which excites the internal acoustic resonances of the cabinet, and in turn affects the movement of the driver itself. A tapered tube behind the driver---an inverted horn---provides a controlled change in acoustic impedance, which, B&W claims, all but eliminates internal acoustic resonances, especially when used in conjunction with an absorbent packing material.
Because the original Nautilus bass driver's tube needed to be coiled in order to save space, the resultant loading caused it to have an overdamped alignment---the rolloff was gradual, but started at a relatively high frequency, necessitating compensation in an active crossover.
The Nautilus' four drive-units operated in very narrow frequency bands in order to keep its drive elements pistonic throughout their range. The Nautilus achieved its goals of uniform off-axis response and pistonic action by crossing over the drivers before they began to beam.
B&W's decision to meld as much Nautilus technology as possible into the 801 template presented several problems. First, they wanted to keep the Nautilus 801 a three-way system rather than a four-way, so the midrange unit would need to cover a range broader than could be managed by a single pistonic unit using a tapered tube. They also knew that using a tapered tube at low frequencies was impractical---they needed to keep the speaker compact, and wanted to avoid using an active, EQ'd crossover.
The tweeter was capable of using the Nautilus' tapered tube. The tube's exponential profile was designed, B&W says, to absorb all the energy within its operational bandwidth, while the use of absorptive wadding allowed it to be shorter than the tube used with the Nautilus. The wadding is packed loosely at the mouth of the tube, but is gradually compressed toward the end---the progressive density of the packing allows the sound energy radiating from the rear of the dome to pass through the pole piece (which has a hole bored in it for this purpose) into the tube without being reflected back into the dome. The tapered tube also serves as a heatsink for the tweeter, allowing the speaker to handle greater power levels than would otherwise be possible.
The tweeter features an aluminum-alloy dome, a neodymium/iron/boron magnet structure, and copper-clad aluminum ribbon wire on a Kapton former---all of which is designed to minimize mass and eddy currents while maximizing the volume of conductor present in the gap. A new suspension was developed for the dome, which uses compressed foam rather than a rolled surround. This, B&W claims, reduces the moving mass and avoids resonance problems. No focusing ring is required for this tweeter; it is held in place by mating the curved front cap of the tweeter enclosure to the rest of the housing, which makes for an uncluttered profile and superior dispersion. B&W claims the tweeter has an inherent 96dB SPL sensitivity, low THD, and excellent dispersion.