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| Spatially Coherent System (SCS) |
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Spatially Coherent System (SCS), Cabasse's key principle for many innovations
Coherence of direct sounds |
Sound
is formed by a series of waves of variable length (frequency) and
height (intensity), just like waves on the surface of water. To get
wide, high waves, you need big stones which could displace a lot of
water, whereas very light pebbles will make small waves or ripples
which are very short and close to each other. Likewise, in air, you
need a loudspeaker with a large diameter for bass frequencies and a
small one for trebles. In music, you need a large, heavy cord for the
deep tones of a double bass and a small light one for the high notes of
a violin.
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To faithfully reproduce a sound field from the
deepest sounds to the highest notes, the most obvious solution is to
use several drivers, each one designed for its own range of use
(treble, midrange, bass). However a note, produced by a piano, for
instance, is not made up of a pure frequency, but of the note’s own
fundamental frequency (440 Hz for the note A used to bring an orchestra
into tune) and a host of other multiple frequencies of varying
intensities.
This is what
makes it possible to distinguish between a piano and a trumpet, for
starters, then a Stradivarius and a student violin, and finally, a
Steinway and its twin. This note, recorded at a single point in space
by a microphone, should come to your ear via an acoustic speaker
without being distorted.
If some of the harmonics making up
this note are delayed because they are coming from another drive unit
within the loudspeaker's enclosure located further away from your ear,
the note will be modified. If the harmonic is located in the range of
frequencies common to both sources, it may vary in intensity due to the
time delay.
In the first case, the positioning in space and
the filtering will make it possible to sufficiently reduce the delay,
at least at a given listening distance. In the second case, the
distance between the axes of the two sources must be very small, and
all the more so when they share a wide frequency range. In the case of
2-channel stereo, two sets of sources must make the same set of
frequencies reach the ears of one or several listeners in order to
respect the timbres of each instrument. This homogeneity of direct
sound will also influence the quality of the stereophonic image, since
the positioning of the instrument in space is determined by the
difference in sound levels and the time delay between the left and
right speakers. The better the differences present in the recording are
respected in the reproduction, the finer and more accurate the sound
image will be.
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Coherence of direct sounds and reflected sounds
Under
home listening conditionings, the sound field in the listening area is
made up of only 20 to 30% of the sounds coming directly from the
speakers and of 70 to 80% of sounds reflected by the room and its
furnishings (indirect sound). Therefore, indirect sounds play an
important role in the quality of reproduction. When you move speakers
around a room to find the optimal place for them, you can vary the
distribution of direct sounds and reflected sounds, in order to obtain
the best spectral balance and best stereophonic image at the listening
point.
Reflected sounds are mainly emitted by loudspeakers
outside of their axis all around the speaker’s cabinet. Depending on
the frequencies, the distribution of the power emitted over 360° by any
type of transducer will vary, as shown by the diagram above. To make
indirect sounds as coherent as possible, you must find off-axis the
same lack of delay between sounds emitted by the acoustic speaker’s
various transducers, whether these sounds are reflected laterally by
the walls or vertically by the floor and ceiling. The balance between
direct sounds and reflected sounds must also strike the right
proportions, since too many indirect sounds will create extreme
spatialization with an image which is unstable and inaccurate.
If
there is too much directivity, the timbres may be right, but the image
will be flat, with neither substance nor depth at its center. This is
where we reach the limits of the analogy with waves or ripples on
water, since the role of the speaker is to recreate the wave front
emitted by the orchestra towards the listener, without artificially
increasing the waves reflected by the wall behind the speakers.
On
the polar curves of the diagram here, representing the way the bass,
midrange and treble frequencies of one of our speakers are dispersed,
the shapes follow physical laws. The absence of irregularity shows
exemplary evenness even outside of the axis, which provides optimal
coherence between direct sounds and reflected sounds at the listening
point. Even though a room’s acoustic characteristics have an influence
on the quality of reflected sounds, and therefore color the sound in a
way, it is important that the sound emitted off-axis be neutral and
transparent. On the one hand, it is practically impossible to obtain
total clarity by mixing colors, and on the other, our brains assimilate
the coloration of the room we are in. Thanks to this faculty, which
microphones lack, we can recognize the timbres of an instrument or a
voice in any room.
At Cabasse, in fact, we validate the
speakers we produce in several rooms, including a soft room which has
been treated to reverberate very little and a modern living room with
picture windows and modern furniture, in order to correctly check the
coherence between direct and reflected sounds in different acoustic
situations.
Coinciding sources to make sounds coherent TOP |
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Reproducing
a wide and deep sound image with a sound source formed by adjacent
loudspeakers is like trying to project a 3D video image over 240° using
two systems with three tubes side by side. The three colors will
coincide on a plane in front of the projector, but the walls will be
colored whenever the image moves the axes of the three tubes too far
apart. |
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The main frontal image will itself be affected by the changing
reflections. Thanks to a single punctual source, the projection system
of cinemas like the Géode in Paris can project this sort of image over
more than 180° while maintaining shapes and colors. In the same way,
coinciding sound sources respect the recording, without adding defects
due to the lack of coherence between direct sounds and reflected ones.
This is shown in the diagrams comparing the spectral balance of a
conventional speaker and a Cabasse speaker at the listening point. |
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Coaxial coincidence
The
Cabasse team’s wager was to put all the drivers on the same axis, or
even at a single point, so that they form a portion of pulsing sphere,
without getting in each other’s way. They carried the bet in 1992 with
the tri-axial TC21 speaker, which was replaced 10 years later by the
TC22. This transducer in the up-market Artis range has regular
directivity from 80 to 22,000 Hz, with no irregularities in the
overlapping zones between the midrange-woofer, midrange and tweeter
channels. It is also a vital tool for developing and perfecting
measurement protocols and verifying that the principles of spatial
coincidence are applied without compromising the quality of Cabasse’s
own characteristics. This has enabled our laboratory to create a whole
range of satellites with coaxial coincidence, from the big Baltic
sphere to the tiny Xo. This coaxial technique is the optimal solution
to comply with source coincidence criteria, and will be represented in
the very near future by a new benchmark transducer developed by
Cabasse’s design department based on the TC22. The principle of virtual coincidence |
The
law of spatial sampling makes it possible to virtually recreate a
coinciding source with several loudspeakers on different axes, if the
distance between the axes of emission do not exceed a half-length of
the cutoff frequency between the transducers. In the case of a cutoff
frequency of 5,000 Hz which corresponds to a 6.8 cm (2.7”) wavelength
and a 17 cm (6.5”)- diameter woofer, in order to comply with the
virtual coincidence criterion, the tweeter must be placed within a
radius of 8.5 cm (3.3”) from the woofer transducer. If the cutoff
frequency is lowered to 2,500 Hz, the maximum optimal distance between
the axes of the 2 drivers goes to 13.6 cm (5.4”). According to this law
from Nyquist’s studies, the critical distance can be increased by
optimizing the loudspeakers’ directivity in their coverage zone. So,
recreating a coherent multichannel source based on adjacent sources
becomes possible.
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Virtual coherent systems
Armed
with their know-how in developing new diaphragms and their mastery of
the SCS principles in the coaxial ranges, the Cabasse team members have
perfected a new series of tweeters which comply with Nyquist's theorem
laws and Cabasse's quality criteria. These midrange-tweeters are fitted
with lightweight, rigid Kalladex diaphragms made by our automated
machines.
Their large emission surfaces allow very low
cutoff frequencies. The diaphragm/horn combination has been specially
designed for optimized control of directivity outside of the axis and
in accordance with Nyquist's formula, to increase the critical distance
so that it forms a source of virtual coherence with our woofers. The
diagrams below show our midrange-tweeters' superiority in the way
directivity develops in the overlapping zone with the woofer, and their
close resemblance to the TC22's directivity curve. They are found on
the MT3, MT4 and Altura ranges.
Numerous solutions, but only one acoustic signature
With
its coaxial and midrange-tweeter applications, the SCS principle can
highlight the qualities of each range, without compromising the
acoustics:
ease of implementation, power and traditional beauty of the MT3, MT4 and Altura ranges;
compactness and unobtrusiveness of the Xo, Gallia, Cinesound and iO set ups;
elegance and modernity of the Xi and Ki satellites;
the technically and esthetically absolute of the Artis range. |
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