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It takes a small diaphragm, very quick and light on
its feet, to move forward, stop, move back, stop, and
repeat this 15,000 times per second. Although a small,
light cone can do this, it unfortunately cannot get hold
of nearly enough air to produce those big waves, 20
and 30 feet long, that represent the low bass notes.
Conversely, the big cone that can turn out the long,
slow-vibration waves cannot keep up with the fast
footwork necessary at the top of the scale.
That is not all. The cone, as it goes about its job of
beating the air in rhythm with the electrical frequencies,
must move back and forth a short distance. How
far does it have to move? Take a cone four inches in
diameter and set it to producing a 2,000 cycle note.
To put out a steady one watt of sound, which is much,
much louder than comfortable living room volume, the
cone must move back and forth about 25 ten-thousandths
of an inch. To allow for this motion in the
flexible mounting that holds the cone to the speaker
frame presents no problem in design or construction.
The cone can be made to feel "free" over this short
it may be wise to go even further and divide the job
into three parts: high, middle and low.
vVith the acquisition of a separate tweeter and woofer,
a new factor arises. If they are connected directly
to the amplifier, omigosh! The big bass notes will certainly
give the tweeter a monumental case of indigestion,
one from which it may never recover. The
chances are excellent that the cone will be torn from
its supports or the coils burned out by the surging currents
of the big slow waves.
The highs also would be flowing through both speakers
and the least effect of this would be that they would
be wasted on the woofer. However, although the heavy
diaphragm and coil are massively disdainful of these
tiny impulses and refuse to be pushed around much by
them, they do not escape completely unscathed. The
big cone has a tendency to ripple and b end when very
fast impulses are applied instead of moving b ack and
forth all in one piece. This bending is called "breakup"
by the engineers. It is a prime source of coarse, nasty
sound in loudspeakers. So what the highs do achieve
when they are routed tlu-ough a woofer is mostly distortion,
and that we can do without.
The course b ecomes clear. The lows must be kept
out of the tweeter and the highs out of the woofer. The
gadget that does this is the "crossover network" This
electronic traffic-cop is a collection of coils and capacitors
which separates the big waves from the little ones.
It directs the big ones to the woofer and the little ones
to the tweeter.
Choose the Crossover Frequency Carefully
These two speakers illustrate the differences
in cone structure and area discussed in the
text. The speakers are a twelve-inch woofer
(Cletron Cathedral Series, 30-5000 cycles) and
a four-inch tweeter (also Cletron Cathedral
Series, 1800-18,500 cycles).
distance and thus avoid the distortion that would result
if the mounting were stiff and held the cone tightly.
Now suppose the same cone is required to put out
one watt at 100 cycles. It would have to move % of an
inch in each direction, 300 times the distance it had to
travel at 2,000 cycles. This is too great a distance to
allow for with so small a cone. On the other hand, if
t11e cone is 16 inches across, one watt at 100 cycles
would require only 5 one-hundredths of an inch of
motion.
Why Two- and Three-Way Speaker Systems
It is obvious that the problems created by these dis- .
tances and these speeds are more readily solved b y the
use of a large cone ("woofer") for the low notes and a
small cone ("tweeter") fm the high notes. A single
speaker of careful design can do a competent, wellbalanced
musical job if the listener is satisfied with
something less than the utmost in complete frequency
range coverage at good volume. But if the listener demands
highs that are high and lows that are really low,
and wants to hear them sound forth realistically, the
speaker system must consist of at least two units, and
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The crossover network is connected to the amplifier
and the two speakers are connected to the crossover
network In a speaker system that is designed as a
unit, the proper crossover network is usually built in.
If, however, woofer and tweeter are procured separately,
then a crossover network must be added to work in
conjunction with them. It is extremely important to
get one that will perform properly with the speakers
that are to be used. Preferably, the recommendation
of the speaker manufacturer should be obtained so
there will be no error made in the selection of the crossover
network A network that does not match the
speakers can cause much aural misery.
Networks are rated in terms of a "crossover frequency."
This is the arbitrary point where the woofer and
the tweeter put out about the same volume level. If
you go higher in frequency the tweeter produces all of
the sound. If you go lower, the network prevents the
electrical waves getting into the tweeter and feeds them
only to the woofer. Crossover networks are especially
designed to prevent a "sonic hole" where no power is
being fed either speaker. Where to put this crossover
frequency, whether relatively high or low in the scale,
is the subject of much debate among audio hotbloods.
And if the system consists of more than two speakers,
the problem becomes more complicated and the debate
correspondingly more intense.
Proper design of a cross,over network requires knowledge
and skill. Good engineering will feed that speaker
right and avoid any semblance of an aural stomachache.
-END
HIFI & MUSIC REVIEW