Bowers & Wilkins bass drivers are backed up by an imposing magnet/voice coil system to preserve the speed and ‘slam’ of the most demanding bass lines. The construction of the cone is critical. What we’re aiming for is ‘perfect piston’ behaviour: the ability to move air with complete freedom while resisting deformation.
The cone must be as light as possible for maximum agility, but absolutely rigid in order to retain its shape as it pumps in and out.
Rohacell® is a sophisticated composite construction having a hard foam core, sandwiched between carbon fibre skins. It’s the kind of material normally used for aircraft, rockets and performance cars bodies. So high air pressures are definitely not a problem. For speed, stiffness and unshakable bass, it’s the bottom line. Our outward looking and innovative approach to materials engineering is illustrated perfectly by the application of Rohacell structural plastic foam in the construction of bass drivers.
We found Rohacell being used extensively in the high performance automotive and aerospace industries, where its unusual combination of very low weight and very high resistance to bending stress make it the ideal choice for a multitude of applications. Those very same characteristics also make it an ideal material for the structural heart of a bass driver.
If a bass driver cone is to perform at a level commensurate with our Kevlar® FST™ midrange drivers and Nautilus™ tweeters it must excel in three vital characteristics:
First, it must not bend significantly, even under huge dynamic loads. Second, it must have high self-damping so as not to display any significant resonance. And last, it must offer good acoustic attenuation so that sound energy within the speaker cabinet cannot leak out and muddy the music radiated forward.
Combining those three characteristics, which in a multitude of respects are conflicting (light materials are rarely strong, often ring like bells and almost never resist sound transmission), is a tall order. Rohacell is the key. Viewed from the front, a Rohacell driver doesn’t look significantly different from many others. The real difference, where the Rohacell foam can be found, is around the back of the cone. Rather than an overall cone thickness of perhaps one or two millimetres, typical with conventional bass drivers, a Bowers & Wilkins Rohacell cone is significantly thicker. The Rohacell foam is used as meat in a carbon-fibre skinned sandwich. The result is a cone that displays bending strength, self-damping and sound attenuation that are each an order of magnitude or more higher than conventionally manufactured cones. The potential downside of increased cone thickness is weight gain, but thanks to the low density of Rohacell, that's not a problem and low frequency cut-off and sensitivity targets can still be met. Rohacell isn’t a free lunch for bass drivers, but it’s not far off.
Of course, while the look of a Rohacell bass driver isn’t out of the ordinary, we don’t design speakers to be looked at, we design them to be listened to. Bass from a Rohacell equipped speaker system takes on a whole new quality: deep, fast, dynamic and wholly untainted by the distortions and dynamic slurring that perhaps we’ve come to accept as part of the deal with moving coil speakers. In the same way that Kevlar FST midrange drivers and Nautilus tweeters strip a layer or two of artifice from their respective segments of the audio band, so Rohacell does for bass.
Rohacell®, is a registered trademark of Evonk Industries AG or its subsidiaries.
Find out what Bowers & Wilkins customers and audio enthusiasts are talking about on our blogs, and read in-depth articles in the Sound Lab.
Moog Sound Lab: Jimi TenorAugust 19,2016