Stuart Nevill trained as a mechanical engineer before joining Bowers & Wilkins. He has spent close to two decades at the Steyning Research Establishment in West Sussex, UK, dedicating himself to the pursuit of true sound.
He has been involved in developing new technologies across a broad spectrum of Bowers & Wilkins products. These include the flagship 800 Series Diamond, our partnership with Jaguar producing cutting edge in-car audio systems, and more recently the first ever Bowers & Wilkins headphones, the luxurious P5′s.
How did you approach the development of the PM1 tweeter?
Our goal with the PM1 was to make a tweeter that had a significantly higher breakup frequency than our standard aluminium ones. We were confident that this would yield a corresponding improvement to performance. We would love to put a diamond tweeter on every product simply for the improvement to performance it offers. But unfortunately it comes at a significant cost. The Carbon Braced Tweeter on the PM1 may not be diamond, but it’s the sweetest sounding non-diamond tweeter we have yet produced.
How did you come up with the idea of bracing the tweeter with carbon?
If you look at a finite element model or a laser scan of a tweeter dome at its first breakup it is fairly clear that the weakness is at the ‘skirt’ of the dome, so incorporating a stiffening ring in that area was a logical conclusion. We have actually been applying strands of carbon fibre to the skirt of Nautilus™ domes for 15 years but the process was painstaking and the carbon used not terribly stiff, (it couldn’t be stiffer or it wouldn’t have been possible to apply it!). We’ve known about pitch based carbon fibre for some time, it is incredibly stiff. We were lucky that we found someone to wind it into precision rings that fit inside the dome. This is probably an industry first.
How does pushing the breakup frequency beyond the range of human hearing benefit the sound that you can hear?
Any resonance has an audible affect on the response at more than just one frequency, it affects the nearby frequencies, too. By pushing the breakup frequency higher we are pushing all the affected frequencies further above the range of human hearing. An unbraced dome typically goes into breakup at 30kHz, the PM1 dome is pistonic to 40kHz. This results in a marked improvement to the audible sound.
The PM1 tweeter also uses the same surround material as that used on the 800 Series Diamond. What advantages does that have for the performance?
The exact choice of rubber is much more critical than most people realise, and we have clever people putting a lot of effort into characterising and refining the rubber materials to give the best performance. The rubber we use on 800 Series Diamond is the ideal compromise between compliance and damping, and was the perfect choice for using on the PM1 tweeter. The dispersion is related to the diameter of the source, which is the dome and the surround together. By controlling the behaviour of the surround we ensure that it has minimal output at high frequencies so you only hear the dome, therefore the source size is kept small and dispersion is high.
What are the benefits of placing the tweeter on top of the cabinet?
There are a number of good reasons to do this, but the two major ones are that the baffle size should always be kept small in relation with the wavelengths emitted by the speaker and that edge diffraction from a wide baffle causes the radiation of secondary sources that colour the on-axis sound. More simply put the ‘tweeter on top’ gives you enhanced soundstage depth and a much wider sweet spot. As you will experience when you listen to the PM1.