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« Why So Called "Coaxial" Speakers Aren't Uni-Q | Main | Vented Tweeter Explained »
Jun 25 2014

Better HF Through KEF's Stiffened Dome Technology

Conventional wisdom has held for years that the optimal shape for a tweeter was a flat baffle with no waveguide or element between the listener and the baffle. Using Finite Element Analysis (FEA) KEF’s engineers have concluded that, like most conventional wisdom, the conventional wisdom in this case needs to be adjusted.

The graph on the left ( Figure 1 ) is the output of a short pulse a sent through a simple dome tweeter. A high-frequency transient signal is clearly shown in the area corresponding to the tweeter’s off-axis response. The graph on the right shows the output of a short pulse reproduced in a KEF tweeter with our Optimum Dome technology and Waveguide geometry and it clearly shows that the off-axis transient has been eliminated. This translates to the listener as a wider sound field (wider high-frequency dispersion) due to the tweeter's consistent off-axis response. This means that a listener is not chained to a “sweet-spot” but can hear clear and articulate high-frequency reproduction in a wider area in the listening room. For more detail on this please visit .

Having achieved an optimum shape for our tweeter drivers, KEF’s engineers were then tasked with turning their attention to the mechanical construction of our tweeters. Rigidity is the key to consistent, distortion-free response in a tweeter dome, but fast response is required to replicate musical tones and signals in the upper frequencies.

For example, certain percussive instruments like cymbals and bells can vibrate at upwards of 16,000 times per second (16kHz). The upper range of human hearing is generally considered to be 20kHz (after years of being around loud live music I now top off at around 14kHz).

This means that a tweeter must be able to accelerate from 0 cycles per second to 16,000 cycles per second (in the case of a small cymbal or chime); this is nearly equivalent to 1000G (1000 times the force of gravity). A fighter pilot, with a proper G-suit and other aids in the cockpit, can withstand up to 9G before blacking out. This is why aluminum is used in our tweeters: it is extremely light (low density) and stiff (high rigidity), so it is able to respond at the acceleration rate needed to replicate high frequency sounds while maintaining its shape. But even aluminum has limitations under the extreme stress of such high acceleration rates.

Elliptical & Spherical: The best of both worlds. Over the years, we have learned that there are in fact two optimal shapes for a tweeter dome:

  1. Elliptical
  2. Spherical

The elliptical shape resists the acceleration forces and remains rigid, offering a mechanical solution to the problem, while a spherical shape offers the best acoustical response. As in almost all things scientific, there is a trade-off (you can’t get something for nothing) and compromises have to be made.

The challenge for us was to produce a tweeter that didn’t compromise its acoustic capabilities across its sonic range.

In 2006, we combined both shapes as we prototyped our first Stiffened Dome geometry: A brilliantly simple (and patented) solution to the mechanical and acoustic challenges faced when trying to design a tweeter that is both acoustically and physically stable. The spherical shape is superimposed over the elliptical shape so that a triangle is formed where the two shapes meet along the edges of the dome, which is typically the weakest and most prone to losing its shape to due acceleration forces.

By combining the two shapes we have eliminated deformity in the tweeter dome at frequencies up to 38kHz while maintaining the pure high-frequency response that KEF speakers are renowned for.

While others may claim to be developing this technology for the first time, we have been perfecting our Stiffened Dome technology for nearly a decade, and we're pretty pleased with the results.

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