Apple Final Cut Pro 7 - Manual (Page 834)

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.
Relative increase in power Number of decibels
1 0
1.26 1
2 3
10 10
100 20
1000 30
100,000 50
10,000,000,000 100
Decibel Units
Audio meters are labeled with decibels. Several reference levels have been used in audio
meters over the years, starting with the invention of the telephone and evolving to present
day systems. Some of these units are only applicable to older equipment. Today, most
professional equipment uses dBu, and most consumer equipment uses dBV. Digital meters
use dBFS.
• dBm: The m stands for milliwatt (mW), which is a unit for measuring electrical power.
(Power is different from electrical voltage and current, though it is related to both.)
This was the standard used in the early days of telephone technology and remained
the professional audio standard for years.
834 Chapter 52 Audio Fundamentals