The sound of silence. Not all sound generated by speaker drive units is good sound. The kind that emerges from the back of a working driver, into a conventional box cabinet, can bounce around and make a mess of the good sound coming out of the front. Bowers & Wilkins’ trailblazing Nautilus™ speaker found a way around boxes. Tapering tubes filled with absorbent wadding soaked up the wayward sound energy and reduced resonances to an insignificant minimum. Nautilus Tapering Tubes are fitted to nearly all Bowers & Wilkins speakers, even when they’re not visible to the eye.
Sound is channelled through a hollow pole magnet, away from the diaphragm, and disappears into the tail.. So all the sound you hear is good sound.
When a driver is loaded by a tube of a similar diameter to the diaphragm, sound propagates down the tube as a series of simple plane waves. When the sound reaches the other end of the tube it is reflected back up the tube towards the driver. If it reaches the driver it causes delayed radiation that time-smears the original signal, blurring the clarity of the sound. If, however, you fill the tube with absorbent material and the tube is long enough, you can dissipate the energy before it reaches the end of the tube. The sound from the driver then remains clean and true to the input signal.
Tapering the tube enables you to make it shorter for the same level of absorption. It acts like a horn in reverse - reducing the sound level instead of increasing it. The limit of this type of loading is reached when the wavelength gets small enough to be comparable with the diameter of the tube. Above a certain frequency, the sound ceases to propagate as a simple plane wave and a series of cross-mode resonances are set up which can re-radiate through the driver diaphragm. To maintain the effectiveness of tube loading, you must restrict the bandwidth of each driver. This is one reason why the Nautilus loudspeaker is divided into a 4-way system. A more complex type of loading is required to cover a wider bandwidth and the sphere/tube enclosure was developed for the Nautilus 800 Series.
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An island in a sea of sound – Chris Watson’s Nature DiscoDecember 08,2014