The
crossover is a key element in the design of a loudspeaker.
Its role and its development are very complicated,
the personality of a loudspeaker depends on it.
The
role of the Crossover
The crossover has the function of distributing the
signal from the amplifier to the various drive units
in a loudspeaker: bass, midrange and treble. The
most critical area of filtering this signal, with
out any doubt, is between the midrange and treble.
A low-pass filter rolls off the higher frequencies
of the midrange. A high-pass filter rolls off the
low frequencies from the tweeter. This crossover
point generally occurs in the region between 2-5kHz.
This underlines a fundamental point in terms of
realistic sound reproduction: “directivity”.
Mastering Directivity
The diameter of a drive unit designed to reproduce
midrange frequencies is generally between 130 and
170mm. These dimensions correspond to a frequency
of between 2 and 2.6kHz. The directivity, meaning
the angle over which a drive unit will radiate its
energy, reduces as frequency increases. For frequencies
where the wavelength is smaller than the diameter
of the cone, the sound becomes very directional.
This means that the sound will be radiated in an
increasingly narrow beam. As a consequence, the
acoustic power radiated from the loudspeaker is
not evenly balanced, principle criterion for the
reproduction of a realistic image over a three dimensional
space. Here our inverted dome tweeter offers a unique
advantage in that its design and conception allow
it work down to frequencies sufficiently low enough
to avoid the effects of midrange beaming.
Mastering Phase
In addition to the constraints imposed on the tweeter
at low frequencies, there is also another very critical
aspect: the phase behavior of the loudspeaker and
crossover combination in this critical region. The
ear and the auditive system are extremely sensitive
in the zone 2-2.5kHz. The midrange drive unit and
its crossover must display an amplitude response
that is “the exact mirror image” of
that of the tweeter and its crossover. For this
criterion to be met, the phase between the midrange
and tweeter must be perfectly matched for the two
to overlap and sum perfectly and create a balanced
tonality.
The difference in phase between the tweeter and
midrange at the crossover point must be zero. Thus,
there is total summation between the two emission
sources at the crossover frequency. Out of phase,
the loudspeaker should show a very deep and symmetrical
hole in the response curve.
OPC Technology
These aforementioned steps form the basis of OPC
technology. However, it would be naive to think
that this ideal could be reached simply through
an extensive development of crossover technology.
It is necessary to have total control over the response
and performance of the transducers themselves, as
is only possible for a few manufacturers who develop
their own drive units as Focal: a specific drive
unit is developed for each individual type of product.
It's unrealistic to think that a mechanical deffect
can be corrected electricaly.
With the OPC technology, we use an acoustic roll-off
of 36dB/Octave for the slopes between midrange and
tweeter along with a perfectly aligned phase. This
translates into a sound that is extremely coherent
and with a natural timbre. Imaging is extremely
precise and at the same time dispersion wide so
that the result can be enjoyed over a wide listening
area.
