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Sound
is vibration. The reason it is perhaps so hard to grasp that
concept is because of the ways that the concept of sound has
had to be represented graphically. How do you draw vibration?
Would you see it like this?
Perhaps this would capture it for you
more accurately.
Or we could try to explain it this way.
The problem with all of these representations
(and others that you might have seen elsewhere) is that they
all imply
that there is some sort of linear movement of a wave through
space. That a sound here moves
to here and
then to here . But it doesn't, not really. So to understand how to
record this illusive "thing" we call sound, it is
useful, first, to understand something about its physics (or
acoustical properties). Then we can talk about its psychoacoustics,
or the ways that we actually hear, or experience, it. And that
will take us into the realm of actually reproducing or recording
it. First, its physics (from a layman's point of view).
We talk about sound "traveling" through the air,
but this also gives the wrong impression. What actually happens
is this. A string is plucked or bowed and it vibrates in the
air. The string has mass and its vibration strikes against
air molecules in its vicinity. Air molecules also have mass
(although we don't ordinarily "experience" them
unless we're in the midst of a windstorm or riding a bicycle
quite
fast). These molecules mutually support one another. To use
an analogy. They are actually like millions of very tiny
balls suspended in gelatin. Or as one book put it, imagine
tennis
balls connected together by springs. At any rate, each of
them is supported by all those around it. So when we push
on one
of them, it pushes on those around it. So--to return to the
vibrating string--the molecules that it strikes, in turn,
strike others. One bangs into the next, and it into the next,
and
so on. Then, having struck, the first molecule returns to
its original place where it is struck again, then returns,
is struck
again, etc. So each movement of the string causes the molecules
it connects with to strike others and a chain reaction is
set into motion. So it is not the first molecule struck which
eventually
strikes the mass of our eardrum, but actually the hundredth
thousand or the millionth molecule in this chain that pushes
against (and thus vibrates) our eardrum. Nothing by itself
moves very far, but the cumulative banging around of molecule
to molecule can carry the original vibration very great distances.
This brings us to three concepts that
are crucial to understanding sound and that are based on
this banging of molecule against molecule. These concepts
are frequency, amplitude and attenuation.
Frequency refers to the pitch of sound. Sounds can be high-pitched,
mid-pitched or low pitched. The higher the pitch of a sound
the higher its frequency. Speakers reproduce high-pitched sounds
with tweeters, while they reproduce low-pitched sound with
woofers or sub-woofers. A high-pitched, or high frequency,
sound has a very short wavelength. The length of a wave is
its distance from one peak to the next, or one mid-point (or
zero/zed point) to the next of the wave. Since all sound waves
are "s"-shaped
when represented on an oscilloscope, it is easy--with the proper
instrumentation--to calculate their frequency. Here is a reproduction
of a simple sound wave traveling through a telephone line.
If you use the mid-point of a wave and trace a complete "s" from
left to right, you will have traced one full cycle of the wave.
The number of those cycles that occur per second is what is
referred to as the waves frequency (also referred to as Hertz).
This is actually what would be called
a complex wave as opposed to a sine wave (or simple wave).
A sine wave has a single frequency and its "s" is consistent
across time. But a complex wave -- one that has been modulated
or changed by the fundamental characteristics of sound (frequency,
amplitude) -- has an "s" that varies as these characteristics
vary. In this example the pitch of the
loudness of the voice (amplitude) change as a person speaks,
so the voltage changes, too.
You will also notice that each cycle
has a plus side (above the line) and a minus side (below
the line) and that these two sides mirror each other. If
you could isolate one of these waves, cut it in half at the
mid-point, and fold the bottom half toward the top half using
the mid-point as an axis, you would create a full circle.
We begin with the basic "s" curve
of a wave. This "s" represents one cycle, and if
only one of these occurred each second, this would represent
one Hertz.
When this "s" is bisected
horizontally, and the bottom half folded left, it forms a
circle. Since a circle is 360 degrees around, we thus know
that each half of the "s" has 180 degrees (or half)
the total. The half above the mid-point is its positive (+)
phase and the half below the mid-point is its negative (-)
phase.
One more note about phases before getting
into compression and rarefaction. If you can imagine that,
where the blue line above crosses the arc of the circle on
the left is 0 degrees, then you can imagine that, when it
crosses the circle on the opposite side, it is 180 degrees.
Straight up (top of the circle) then becomes 90 degrees and
straight down (bottom of the circle) is 270 degrees. In other
words, no matter where we bisected this circle, the two points
where the bisecting line crosses at opposite sides are 180
degrees apart, or 180 degrees out of phase. This will become
important after we tackle compression and rarefaction.
Now, remember the tiny balls (molecules) suspended in gelatin?
Recapture that image.
What happens when a sound pressure wave strikes one of the
balls suspended in this gelatin?
The molecule that is pressured pushes those around it. That
molecule has been compressed. It, in turn compresses those
it contacts, and so on. And when it springs back to its original
position it rarefacts. It is the rapid cycling of compression
and rarefaction that constitutes vibration. The more rapidly
this compression/rarefaction occurs, the higher we say its
frequency is. So a high frequency sound wave (at, say 18,000
Hz) causes this process to occur rapidly, while a low frequency
sound wave (at, say 50 Hz) causes it to occur very slowly.
The compression part of this cycle is its positive phase and
the rarefaction portion its negative phase as represented on
an oscilloscope.
If you're with me this far, we can now move into more detailed
discussion of sound pressure level (SPL), or amplitude, the
qualities of different frequencies, human apprehension of
sound (or psychoacoustics), the richness of multiple
frequency apprehension
(or the role of harmonics) and the question of fidelity.
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