Time – msec
0.5
5
0
-5
-10
-15
-20
-25
-30
-35
1.0
Output — dB
1.5 2.0 2.5
Figure 8 CS6 step response
off at 6 dB/octave. For example, if a typical tweeter with a low frequency roll-off of 12 dB/octave is combined with a 6 dB/octave
network, the resulting acoustical output will roll off at 18 dB/octave. Therefore, in practice, the required network circuits are much more
complex than might be thought.
The result of phase coherence (in conjunction with time coherence) is that all waveforms will be reproduced without major
alterations. The speaker’s reproduction of a step waveform best demonstrates this fact
since, like musical waveforms, a step is made up of many frequencies which have
precise amplitude and phase relationships. For a step signal to be accurately
reproduced, phase, time and amplitude response must all be accurate. Because this
waveform is so valuable, it is commonly used to evaluate the performance of
electronic components. It is not typically used for speaker evaluation because most
speakers are not able to reproduce it recognizably. Figure 8 shows the CS6’s response
to a step. That the step is reproduced so recognizably is the result of accurate phase,
time and amplitude response
ENERGY STORAGE
Any part of the speaker that absorbs energy will reradiate it later in time in a highly distorted manner. Although not loud enough to
be consciously heard, stored energy causes significant detrimental effects by obscuring music’s subtle details, causing both a reduction
in clarity and loss of spatiality. The main storage mechanisms are the driver diaphragms and cabinet walls, especially the baffle.
One method of reducing stored energy is to apply viscous damping so the stored energy can be dissipated as heat instead of
mechanical vibration which produces unwanted sound. This method has limited benefit because energy can only be dissipated as heat
after there is unwanted mechanical vibration. Also, even though some of the absorbed energy is transformed into heat, it is still absorbed
from the desired sonic output. A much better approach, in our opinion, is to reduce the energy absorbed.
The primary cabinet problem is baffle vibration because driver movement can directly excite the baffle. The CS6 employs a thick
cast concrete baffle to reduce unwanted vibration. The walls of the CS6 enclosure are constructed of 1" thick fiberboard, and extensive
internal bracing further increases wall stiffness. To increase the mechanical rigidity
and therefore reduce unwanted vibration, all CS6 drivers incorporate chassis of cast
aluminum rather than stamped steel or plastic.
Figure 9 is the Energy-Time curve of the CS6. It shows how the output energy
of the speaker is distributed in time. First, it shows that the energy is focused with a
fast risetime and a smooth decay, a result of very good time coherence. It also shows
that the speaker’s output has already decayed to -20 dB after only 600 microseconds
and has fallen to -40 dB after 1.4 milliseconds. This rapid decay provides very clean
reproduction with very good inter-transient silence.
DISTORTION
Driver motor systems
Unlike some sources of distortion, motor system distortion is very dependent on volume level, being low during quiet playback
levels but increasing rapidly as volume levels increase. At moderate to loud playback levels it is usually the major source of distortion.
The CS6 incorporates several unusual features in its drivers to decrease distortion and increase dynamic range.
The purpose of the driver’s motor system is to apply a force to the diaphragm that is directly proportional to the voltage supplied by
the amplifier as modified by the electrical network. In order for the force to be directly proportional to the voltage applied, as desired,
the magnetic field strength must be constant, the length of voice coil wire acted on by the magnetic field must be constant, and the
current in the voice coil must be directly proportional to the applied voltage. In practice, none of these three conditions actually exist but
the CS6 woofer incorporates refinements of design that greatly improve the accuracy of each of these factors.
The first distortion mechanism is that the strength of the magnet’s field is not actually constant in operation but is changed by the
current from the amplifier through the coil. This change occurs because the amplifier current through the coil generates the force to
move the diaphragm by creating its own magnetic field that “pushes” against the magnet’s field. The magnet is somewhat demagnetized
by the coil’s magnetic field when current flows in one direction and is remagnetized when current flows in the opposite direction.
Therefore, since the magnet’s field strength is not constant, the force generated is not in the desired direct proportion to the current in the
coil.
To greatly reduce this effect the CS6 drivers all incorporate a copper sleeve around the center pole. With this sleeve any changes in
the magnet’s strength induces an electrical current in the sleeve which generates a magnetic field that is opposed to and practically
cancels the original change. In addition, the CS6 woofer also incorporates a heavy copper ring around the pole to maintain the stability
of the magnetic field even under very high power conditions.
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 not only by the field within the air gap but also 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.
To eliminate this problem all three CS6 drivers use an unusual short coil/long gap system where the coil is much shorter than the
magnetic gap. Therefore, even when the coil moves a considerable distance from its rest position, it continues to be acted upon only by
the uniform magnetic field in the air gap and does not experience the changes in magnetic field strength with position as in the
6
Time – msec
0.5 1.0
Output
1.5 2.0 2.5
Figure 9 CS6 time response
