Apple Soundtrack Pro 2 - Manual (Page 463)

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Audio Fundamentals
What Is Sound?
All sounds are vibrations traveling through the air as sound waves. Sound waves are
caused by the vibrations of objects and radiate outward from their source in all directions.
A vibrating object compresses the surrounding air molecules (squeezing them closer
together) and then rarefies them (pulling them farther apart). Although the fluctuations in
air pressure travel outward from the object, the air molecules themselves stay in the same
average position. As sound travels, it reflects off objects in its path, creating further
disturbances in the surrounding air. When these changes in air pressure vibrate your
eardrum, nerve signals are sent to your brain and are interpreted as sound.
Fundamentals of a Sound Wave
The simplest kind of sound wave is a sine wave. Pure sine waves rarely exist in the
natural world, but they are a useful place to start because all other sounds can be
broken down into combinations of sine waves. A sine wave clearly demonstrates the
three fundamental characteristics of a sound wave: frequency, amplitude, and phase.
Frequency
Frequency is the rate, or number of times per second, that a sound wave cycles from
positive to negative to positive again. Frequency is measured in cycles per second or
hertz (Hz). Humans have a range of hearing from 20 Hz (low) to 20,000 Hz (high).
Frequencies beyond this range exist, but they are inaudible to humans.
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Amplitude
Time