Thiel Coherent Source CS3.7 Owner'S Information Download Page 16

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A Solution

What is needed to solve both these problems simultaneously

is a midrange diaphragm that is ten times as stiff per weight as the
previous extremely stiff composite diaphragm while also being
flat rather than cone shaped. But these requirements work against
each other. The flatter the diaphragm’s shape the weaker it
becomes. And even with a normal shape, how can we increase the
stiffness this dramatically? Even exotic materials like beryllium
are not nearly that stiff, even in the traditional cone shape.

The solution we developed relies on geometry rather than a

more exotic material to achieve both goals. There are several
elements to the design:

• A large voice

coil is used so that
its force is applied
at a point equally
spaced between the
diaphragm’s inner
and outer edges,
rather than at the
inner edge. This
reduces the
distance from the
driving point to the
diaphragm’s edge
and therefore
greatly increases
stiffness.

• Reliance on

the great stiffness in the circular direction of the voice coil
former’s cylindrical shape so that the diaphragm itself is only
required to provide stiffness in the radial direction. This allows
the diaphragm to be formed with shapes that greatly increase
strength in the direction from the center to the edge, even if
decreasing strength around the circumference.

• An undulating, radially ribbed contour is used for the

diaphragm which provides light weight and great stiffness in the
radial direction while still maintaining a basically flat shape.

CS3.7 midrange diaphragm

Results

The

top graph
above
shows the
response
of the new
midrange
driver. As
you can
see the
response
is very
uniform,
without resonant peaks, to beyond the range of audibility. This is
unprecedented high frequency performance, and in fact is better
and more resonant-free response than is exhibited by many
tweeters!

The next graph shows the response of the tweeter when

mounted in the center of the midrange. Here you can see that the
tweeter’s performance is extremely uniform. This would be great
performance even if not mounted in a midrange driver. When
comparing this response to that of the tweeter alone (a previous
graph) there is very little difference. Compared to the graph of the
response in a normal midrange, the large improvement is obvious.

Other Details

There are of course many other elements to a driver than its

diaphragm. Even with the two problems we have discussed solved,
there are other design elements that are required to provide clean,
dynamic and clear reproduction.

In order to provide very clear (low distortion) output all THIEL

drivers employ copper-stabilized, short coil motor systems that
produce only one-tenth the distortion of conventional motor
systems. The penalty of this approach is that a much larger magnet
is required to power the much longer magnetic gap. This creates a
greater difficulty with this driver because the voice coil diameter is
so large that the required magnet, which must be outside the coil,
would be larger than the whole driver! In addition, such a large
magnet would make it impossible to provide a good rear venting
path for the diaphragm. For these reasons the CS3.7 midrange is
powered by a neodymium rare earth magnet that is 10 times as
strong per size as a normal magnet. Although this is expensive, it
provides high efficiency with a large magnetic gap while allowing
excellent venting.

The driver also utilizes a custom, die-cast, eight-ribbed chassis

that mounts the motor system from the outside and also provides a
rigid mount for the tweeter.

Other details of the driver include a copper pole sleeve to

reduce coil inductance modulation, a dual surround suspension, and
an aluminum voice coil.

CS3.7 midrange diaphragm viewed edge-on from the center

Please see next page for photo of the complete driver

Small distance from

driving voice coil to edge

of diaphragm ensures freedom

from diaphragm resonances

to a very high frequency.

Tweeter in the same plane as the