The
inverted dome tweeter is a hallmak of Focal-JMlab.
As a result of more than twenty years of development
into a tweeter that could produce a high output,
but without requiring high energy input and without
increasing directivity. And now the company is ready
to make a major leap forward with the introduction
of the pure Beryllium version.
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 inverted 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 40000Hz.
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.
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
world’s 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.
