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In space, no one can hear you scream, because there is
no air or other medium for sound to travel. Sound needs a medium;
an intervening substance through which it can tr avel from point
to point; it must be carried on something.
That something can be solid, liquid or gas. They can
hear you scream underwater— briefly. Water is a medium.
Air is a medium. Nightclub walls are a medium. Sound travels in
air by rapidly changing the air pressure relative to its normal
value (atmospheric pressure). Sound is a disturbance in
the surrounding medium.
A vibration that spreads out from the source, creating a series
of expanding shells of high pressure and low pressure ...
high pressure ... low pressure ... high pressure ...
low pressure. Moving ever outward these cycles of alternating
pressure zones travel until finally dissipating, or reflecting off
surfaces (nightclub walls), or passing through boundaries, or getting
absorbed usually a combination of all three.
Left unobstructed, sound travels outward, but not forever. The air
(or other medium) robs some of the sound’s power as it passes.
The price of passage: the medium absorbs its energy. This
power loss is experienced as a reduction in how loud it is (the
term loudness is used to describe how loud it is from moment
to moment) as the signal travels away from its source.
The loudness of the signal is reduced by one-fourth for each
doubling of distance from the source. This means that it is
6 dB less loud as you double your distance from it. [This is known
as the inverse square law since the decrease is inversely
proportional to the square of the distance traveled for example,
2 times the distance equals a 1/4 decrease in loudness, and so on.]
How do we create sound, and how do we capture
sound? We do this using opposite sides of the same electromagnetic
coin. Electricity and magnetism are kinfolk: If you pass a coil
of wire through a magnetic field, electricity is generated within
the coil. Turn the coin over and flip it again: If you
pass electricity through a coil of wire, a magnetic field is generated.
Move the magnet, get a voltage; apply a voltage, create a magnet
this is the essence of all electromechanical objects.
Microphones and loudspeakers are electromechanical objects. At their
hearts there is a coil of wire (the voice coil) and a magnet
(the magnet). Speaking causes sound vibrations to travel
outward from your mouth. Speaking into a movingcoil (aka
dynamic) microphone causes the voice coil to move within
a magnetic field.
This causes a voltage to be developed and a current to flow proportional
to the sound— sound has been captured. At
the other end of the chain, a voltage is applied to the loudspeaker
voice coil causing a current to flow which produces a magnetic field
that makes the cone move proportional to the audio signal applied
— sound has been created.
The microphone translates sound into an electrical dignal, and the
loudspeaker translates an electrical signal into sound. One capturing,
the other creating. Everything inbetween is just details. And in
case you’re wondering: yes; turned around, a microphone can
be a loudspeaker (that makes teeny tiny sounds), and a
loudspeaker can be a microphone (if you SHOUT REALLY LOUD).
SIMPLE DIVISION
Loudspeaker crossovers are a necessary evil. A different
universe, a different set of physics and maybe we could have what
we want: one loudspeaker that does it all. One speaker that reproduces
all audio frequencies equally well, with no distortion, at loudness
levels adequate for whatever venue we play. Well, we live here,
and our system of physics does not allow such extravagance.
The hard truth is, no one loudspeaker can do it all. We need at
least two — more if we can afford them. Woofers and tweeters.
A big woofer for the lows and a little tweeter for the highs. This
is known as a 2-way
system. (Check the accompanying diagrams for the following
discussions.) But with two speakers, the correct frequencies must
be routed (or crossed over) to each loudspeaker.
Passive
At the simplest level a crossover is a passive network.
A passive network is one not needing a power supply to operate —
if it has a line cord, or runs off batteries, then it is not a passive
circuit. The simplest passive crossover network consists of only
two components: a capacitor connecting to the high frequency
driver and an inductor (aka a coil) connecting
to the low frequency driver.
A capacitor is an electronic component that passes high
frequencies (the passband) and blocks low frequencies (the
stopband); an inductor does just the opposite:
it passes low frequencies and blocks high frequencies. But as the
frequency changes, neither component reacts suddenly. They do it
gradually; they slowly start to pass (or stop passing) their respective
frequencies.
The rate at which this occurs is called the crossover
slope. It is measured in dB per octave, or shortened
to dB/oct. The slope increases or decreases so many dB/oct.
At the simplest level, each component gives you a 6 dB/oct slope
(a physical fact of our universe). Again, at the simplest level,
adding more components increases the slope in 6 dB increments, creating
slopes of 12 dB/oct, 18 dB/oct, 24 dB/oct, and so on.
The number of components, or 6 dB slope increments, is called the
crossover order. Therefore, a 4th-order crossover
has (at least) four components, and produces steep slopes of 24
dB/oct. The steeper the better for most drivers, since speakers
only perform well for a certain band of frequencies; beyond that
they misbehave, sometimes badly. Steep slopes prevent these frequencies
from getting to the driver.
You can combine capacitors and inductors to create a third path
that eliminates the highest highs and the lowest lows, and forms
a mid-frequency crossover section. This is naturally called
a 3-way system. (See diagram on page 2)
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