coil
1
2
3
4
peak excursion ±mm
8%
6%
4%
2%
distortion
1
2
3
4
peak excursion ±mm
8%
6%
4%
2%
distortion
The second distortion mechanism results from the fact that almost
all woofers use a long coil/short gap motor system where the long coil
is acted upon by the “fringe” field in front of and behind the gap
region. As the coil moves forward or backward to produce bass
energy, the magnetic field acting on the coil becomes less intense
because the coil is further from its rest position where the magnetic
field is strongest. This weakening of field strength as the coil moves
away from its rest position is the primary distortion producing
mechanism in woofers.
In the CS1.5’s short coil/long gap system the coil is much shorter
than the magnetic gap and therefore, even when the coil moves a
considerable distance from its rest position, it continues to be acted
upon by an unchanging magnetic field and does not experience the
changes in magnetic field strength with position as in the conventional
system. As shown in Figure 12, the distortion produced by the short
coil motor system at normal excursion levels (
±
3 mm or less) is only
one-tenth that produced by the typical long coil system.
The third distortion mechanism is that the coil current is dependent
not only on the driving voltage and the coil resistance but also on the
coil inductance. The problem is that the coil inductance varies with the
amount of iron inside the coil and, therefore, with conventional magnet
system geometry, inductance changes during the excursions necessary
to reproduce low frequencies. As the diaphragm and coil move back,
more of the coil is around the pole, increasing the inductance and
decreasing the mid-frequency output of the driver by about 1 dB. As
the coil moves forward, less of the coil is around the pole, the
inductance decreases and the mid-frequency response increases
approximately 1 dB. By this mechanism the frequency response of the
speaker is modulated by driver excursion. This problem has been
virtually eliminated in the CS1.5 woofer. The short coil design results
in the entire coil surrounding the pole in all positions and therefore the
coil’s inductance does not change with the diaphragm position. In
addition, the problem is further reduced by the copper sleeve which
reduces the inductance of the coil to a fraction of its normal value by
acting as a shorted turn of a transformer secondary winding.
Short coil advantages
In summary, the short coil/long gap magnet system used in the CS1.5 provides several important advantages. First, it provides
extremely low distortion, both harmonic and intermodulation. This is especially important in two-way systems where intermodulation of
the upper midrange by the bass is possible. Secondly, it allows the excursion (and therefore bass capability) to be increased without
increasing coil length but by only increasing magnet size. Thirdly, it allows the coil to be kept very light for improved upper midrange
performance. The disadvantage of this approach is that more magnet weight is required which increases costs.
Stray magnetic field
For applications where the speaker is used very near a TV it is
necessary that there be a low level of stray magnetic field to avoid
distorting the picture. This is achieved in the CS1.5 woofer by
configuring the required magnet weight into two magnets, polarized
in opposite directions, one magnet placed on the rear of the driver and
the other occupying its normal position. The rear magnet cancels
most of the field that would exist outside the speaker but also
continues to contribute to the strength of the field acting on the coil
by forcing magnetic energy into the gap that would otherwise escape.
Therefore, the total magnet weight of the two magnets is no greater
than a single magnet required to achieve the same gap field strength
and stray field reduction is achieved without significant additional
cost.
All of these design details contribute to the CS1.5’s exceptional
technical performance, its accurate reproduction of musical
information, and its ability to faithfully reproduce the musical
experience.
6
Magnesium frame for rigidity
and clearer reproduction
Short coil / long gap
motor for very low
levels of distortion
Copper sleeve for low distortion
and exceptional attack
Phase plug for more
uniform response
Two magnets with a
total weight of 1.1 Kg
for exceptional bass
control and low stray
magnetic field
Aluminum diaphragm for rigidity
and transparent reproduction
CS1.5 Woofer
Figure 12
