ENERGY STORAGE
Any part of the speaker that absorbs energy will re-radiate it later in time in a highly distorted manner. Although not loud enough to be
consciously heard, stored energy causes significant detrimental effects. The music’s subtle detail is obscured, causing both a reduction in
clarity and loss of spatiality as well as noticeable colorations of voice and other midrange sounds. The main storage mechanisms are the
driver diaphragms and cabinet walls, especially the baffle. The lack of cabinet wall vibrations is one advantage of membrane speakers and
why they have an “unboxy” sound. However the problem of cabinet vibrations in dynamic loudspeakers is not inherent but rather can be
reduced as much as is affordable.
One method of attempting to reduce the problem of stored energy is to apply damping to the offending component. The idea is to damp
motion with a viscous material so that the stored energy can be dissipated as heat instead of mechanical vibration which produces unwanted
sound. This method has limited benefit for two reasons. First, energy can only be dissipated as heat if there already exists unwanted
mechanical vibration to convert. Secondly, even though some of the absorbed energy is transformed into heat, it is still absorbed from the
desired sonic output and therefore the distortion mechanism still exists. A much better approach, in our opinion, is to reduce the energy
absorbed.
The primary cabinet problem is baffle vibration because movement of the drivers can directly excite the baffle and the resulting
extraneous energy is radiated directly toward the listener. The CS1.5 employs a 2" thick baffle in the driver area to reduce unwanted
vibration. Also, the walls of the CS1.5 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, the CS1.5 woofer incorporates a chassis of cast
magnesium rather than stamped steel or plastic.
As previously discussed, much attention has been given to the reduction of diaphragm vibrations to reduce the amount of energy
absorbed through resonances.
Figure 11 is the Energy-Time curve of the CS1.5. It shows how the speaker’s output
energy to a pulse input is distributed in time. First, it shows that most of the energy is focused
in the first 150 microseconds, a result of very good time coherence. It also shows that the
speaker’s output has already decayed to -20 dB after only 400 microseconds and has fallen to
-40 dB after only 1.2 milliseconds. This rapid decay provides very clean reproduction with
very good inter-transient silence.
DISTORTION
The primary sources of distortion are the drivers’ magnetic motor systems and the electrical components of the crossover network. We
have taken unusual steps in the design of the CS1.5 to greatly reduce distortion contributed by these sources.
Crossover components
The usual type of capacitor for speakers is electrolytic. This type has the advantage of very low cost but also causes audible distortion
due to dielectric absorption and other mechanisms. There are only two electrolytics used in the CS1.5 and neither of these is in the signal
path. Both are used in zobel networks to correct the drivers’ impedance and are bypassed with high quality polypropylene types to provide
performance closer to the polypropylene type than the electrolytic. The capacitor which is most important sonically is the tweeter feed
capacitor. In this case we bypass the polypropylene unit with a very high quality custom–made polystyrene and tin foil unit. All other
capacitors are polypropylene.
All the CS1.5 inductors are air-core, which are completely free of distortion caused by magnetic saturation and hysteresis and are
wound of high purity, low oxygen copper. Also, the speaker’s internal wiring uses custom–made, solid conductor, high purity copper with
teflon insulation.
Woofer motor system
Unlike other 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 the major source of distortion and can therefore
be considered a dynamic range limit. The CS1.5 incorporates several unusual features in its woofer 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 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 CS1.5 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 CS1.5 woofer incorporates 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.
5
Time – msec
0.5
5
0
-5
-10
-15
-20
-25
-30
-35
1.0
Output — dB
1.5
2.0
2.5
Figure 11
