Decibel units, Signal-to-noise ratio – Apple Final Cut Pro 6 User Manual
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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.
 dBu: This reference level measures voltage instead of power, using 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 level is
1 volt, which is more convenient than 0.775 volts 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 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 “
Signal-to-Noise Ratio
Every electrical system produces a certain amount of low-level electrical activity called
noise. The noise floor is the level of noise inherent in a system. It is nearly impossible to
eliminate all the noise in an electrical system, but you don’t have to worry about the
noise if you record your signals significantly higher than the noise floor. If you record
audio too low, you raise the volume to hear it, which also raises the volume of the
noise floor, causing a noticeable hiss.
The more a signal is amplified, the louder the noise becomes. Therefore, it is important
to record most audio around the nominal (ideal) level of the device, which is labeled
0 dB on an analog audio meter.
The signal-to-noise ratio, typically measured in dB, is the difference between the
nominal recording level and the noise floor of the device. For example, the
signal-to-noise ratio of an analog tape deck may be 60 dB, which means the inherent
noise in the system is 60 dB lower than the ideal recording level.