The main
advantage of the inverted dome tweeter or the full range tweeter...
The experience acquired over the last twenty years has allowed
Focal-JMlab to push back the limits of reversed dome technology and
the work on the suspension, driver shape, adhesives and materials have
at each stage produced an improvement in linearity and bandwidth.
The research began by testing fibre domes, as a simple and less sensitive
way to validate the advantages of the reversed dome with regards to
energy response.
Then the company introduced more rigid metal domes
to extend the response in high frequencies, and with Titanium producing
the most impressive results it has proved to be the most relevant choice.
Today to exceed these limits and particularly to extend
the response to nearly 40kHz, we need to move towards
a material even lighter and more rigid. Already competitors have moved
to introduce super tweeters to satisfy the
extreme requirements of the new high-bandwidth sources. However, Focal-JMlab
does not believe the adoption of the super tweeter is to be a satisfactory
solution.
First of all, the distance between the tweeters, for frequencies where
the wavelength is of the order of cm, poses serious problems of coherence.
A filtering is imposed which destroys, by the losses and distortion
of phase, the benefits introduced by extending the frequency response.
So it is advisable to point out that simply extending the high frequency
response is not an end in itself, as the ear is most sensitive to transients.
The improvement with regards to the impulse response is only perceptible
at a tweeter output far beyond the 20Khz, limit and to far beyond the
perception of human hearing. We should not forget that Focal-JMlab's
first objective was to design tweeters offering an optimum coupling,
being able to integrate with the mid-range at sufficiently low frequencies
and before the output of the mid- range begins to tighten or beam.
Thus
the only successful solution is, therefore, to produce a tweeter with
an extended response that covers more than five octaves from 1 000Hz
to 40 000Hz.
Beryllium
- The ideal material for a dome.
An ideal dome must combine lightness, rigidity and damping.
Currently,
only one material offers the possibility of a significant improvement. Beryllium's density is 2.5 times
lower than Titanium and 1.5 times lower than Aluminium while its rigidity is 3 times higher than Titanium and 5 times superior to Aluminium. Which
means that for a dome of identical mass, a Beryllium version is 7 times
more rigid than one made of Titanium or Aluminium - which have similar
rigidity for a given mass. Moreover, the velocity of sound
in a Beryllium dome is 3 times faster than a Titanium
version and 2.5 times faster than Aluminium.
However, the manufacturing
of Beryllium remains very difficult and its production has been limited
to just three countries: the United States, France and Russia.
To date,
Great Britain, Germany and Japan do not manufacture pure Beryllium,
rather using it in the form of an alloy. Beryllium is an excellent high-tech
metal - the only metal able to scratch glass - it is only used in strategic
applications in the aeronautical and military industries and consequently,
its unique characteristics make it extraordinarily expensive, much more
than gold and nearly 100 times that of Titanium.
The only
solution for Focal-JMlab - Control the manufactruing process.
The analysis of the mechanical characteristics inevitably leads to Beryllium's
choice as the ideal material for a tweeter dome. However, a major problem
exists because no company has been able to manufacture such a dome.
To arrive at a solution, required Focal-JMlab to conceive a process for forming a pure Beryllium dome*. And after
two years research and investment, the company now posses a machine
to carry out what was considered as utopia. Technological it is a worldÕs
first and certainly exceeds anything currently found in the audio field.
And the result is in line with the company's hopes and expectations
of an outstandingly detailed loudspeaker which extends to nearly 40
Khz while maintaining a perfect impulse response.
(*) In the past
certain manufacturers have attempted to use Beryllium for the construction
of a direct radiating tweeter dome. However, they have actually only
used Beryllium as a surface coating on a dome structure leading to different
characteristics to anything achieved with pure Beryllium.
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