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Editing Sound
Sound is created by some physical vibration of an object that's
transmitted through the air as tiny and very fast changes in air pressure called
longitudinal waves. The picture below represents hard rubber balls strung on a
tight wire, separated by springs. Movement of the leftmost ball compresses the
spring to it's right, then when that spring decompresses it transfers the energy
to the next ball and so forth until the energy has been transmitted the entire
length.
Our eardrums receive air pressure changes by matching the
vibration much as an inert tuning fork will match the vibration of an active
tuning fork when they are brought together, then our brains interpret the
vibrations as sound .
If we were to bang the left ball against the springs twice in
the period of one minute the frequency of our banging would be 2 cycles per
minute. If we did it twice every second the frequency would be 2 cycles per
second or 2 Hz.
In sound terms, each event from the first movement of the left
ball to the last movement of the right ball is called a sample. In the below
illustration, time is represented by the horizontal axis and the frequency (or
pitch) is represented by the samples.
To properly chart the action of sound we also need to add a
scale for the vertical axis to represent the amplitude or volume of the wave.
This scale will represent decibels abbreviated db.
Unfortunately, sound is not quite as simple as our graph because
sound doesn't have a regular amplitude or frequency or time and it comes at us
with both high and low amplitudes mixed together and called timbre.
Timbre, which is often called tone color, is said to be rich or full when there
are many different frequencies contained in the sound and dull when it has few.
Altogether, the different frequencies at varied amplitudes of a sound are known
as the spectral content of the waveform.
Below is a picture of a digital sound
file that's a bit over 6,000,000 samples long. The actual length of the sound is
determined by the sample rate which is a count of the number of samples in a
second. The higher the sample rate, the more accurate the sound. This particular
sound file was sampled at a frequency of 22 kHz per second. We'll explore that
further in a minute.
At this point it might be worth noting that most digital audio
software confuses quality with accuracy assuming that more samples per second
means higher quality. In actual fact the opposite might well be true. If your
source is very bad, leaving out some data might in fact make it sound better.
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