Apple Final Cut Pro 6 - Manual (Page 808)

Chapter 1 Audio Fundamentals 23
I
Because the ear responds to sound pressure logarithmically, using a logarithmic
scale corresponds to the way humans perceive loudness. Audio meters and sound
measurement equipment are specifically designed to show audio levels in decibels.
Small changes at the bottom of an audio meter may represent large changes in signal
level, while small changes toward the top may represent small changes in signal level.
This makes audio meters very different from linear measuring devices like rulers,
thermometers, and speedometers. Each unit on an audio meter represents an
exponential increase in sound pressure, but a perceived linear increase in loudness.
Important: When you mix audio, you don’t need to worry about the mathematics
behind logarithms and decibels. Just be aware that to hear incremental increases in
sound volume, exponentially more sound pressure is required.
What Is a Decibel?
The decibel measures sound pressure or electrical pressure (voltage) levels. It is a
logarithmic unit that describes a ratio of two intensities, such as two different sound
pressures, two different voltages, and so on. A bel (named after Alexander Graham Bell)
is a base-ten logarithm of the ratio between two signals. This means that for every
additional bel on the scale, the signal represented is ten times stronger. For example,
the sound pressure level of a loud sound can be billions of times stronger than a quiet
sound. Written logarithmically, one billion (1,000,000,000 or 10
9 ) is simply 9. Decibels
make the numbers much easier to work with.
In practice, a bel is a bit too large to use for measuring sound, so a one-tenth unit
called the decibel is used instead. The reason for using decibels instead of bels is no
different from the reason for measuring shoe size in, say, centimeters instead of meters;
it is a more practical unit.
Number of decibels Relative increase in power
0 1
1 1.26
3 2
10 10
20 100
30 1000
50 100,000
100 10,000,000,000