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Chapter 42 Audio Fundamentals 583
VIII
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 than the reason for measuring shoe size in, say, centimeters instead of meters;
it is a more practical unit.
Decibel Units
Audio meters measure sound level using decibels. Since decibels describe the ratio
between two signals, audio meters always display the incoming signal as if it is being
compared to an assumed reference signal.
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, dBu is used for most professional equipment,
and dBV is used for most consumer equipment. dBFS is used for digital meters.
 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 since the early days of telephone technology, and
remained the professional audio standard for years.
 dBu: This reference level measures voltage instead of power, and uses a reference
level of 0.775 volts. dBu has mostly replaced dBm on professional audio equipment.
The u stands for unloaded, because the electrical load in an audio circuit is no longer
as relevant as it was in the early days of audio equipment.
 dBV: This also uses a reference voltage like dBu, but in this case the reference is
1 volt, which is more convenient than 0.775 in dBu. dBV is often used on consumer
and semiprofessional devices.
 dBFS: This scale is very different from the others because it is used for measuring
digital audio levels. FS stands for full-scale, which is used because, unlike analog
audio signals that have an optimum signal voltage, the entire range of digital values
is equally acceptable when using digital audio. 0 dBFS is the absolute highest
possible digital audio signal you can record without distortion. Unlike analog audio
scales like dbV and dBu, there is no headroom past 0 dBFS. For more information
about digital audio metering, see “About Audio Meters” on page 593.
Number of decibels Relative increase in power
01
11.26
32
10 10
20 100
30 1000
50 100,000
100 10,000,000,000