Apple Soundtrack Pro 3 - Manual (Page 488)

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.
Amplitude
Amplitude (or intensity) refers to the strength of a sound wave, which the human ear
interprets as volume or loudness. People can detect a very wide range of volumes, from
the sound of a pin dropping in a quiet room to a loud rock concert. Because the range
of human hearing is so large, audio meters use a logarithmic scale (decibels) to make the
units of measurement more manageable.
Phase
Phase compares the timing between two similar sound waves. If two periodic sound
waves of the same frequency begin at the same time, the two waves are said to be in
phase. Phase is measured in degrees from 0 to 360, where 0 degrees means both sounds
are exactly in sync (in phase) and 180 degrees means both sounds are exactly opposite
(out of phase). When two sounds that are in phase are added together, the combination
makes an even stronger result. When two sounds that are out of phase are added together,
the opposing air pressures cancel each other out, resulting in little or no sound. This is
known as phase cancelation.
488 Appendix B Audio Fundamentals