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.
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.
0
–
+
1 ms
Amplitude
Time
Chapter 1
Audio Fundamentals
19
I
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.
Phase cancelation can be a problem when mixing similar audio signals together, or
when original and reflected sound waves interact in a reflective room. For example,
when the left and right channels of a stereo mix are combined to create a mono mix,
the signals may suffer from phase cancelation.
In phase
Separate signals
Mixed signals
Out of phase
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Part I
Audio Mixing