Absolute dB scales
It is also possible to use the decibel as
a unit of absolute power, in addition to using it as an expression of
power gain or loss. A common example of this is the use of decibels as a
measurement of sound pressure intensity. In cases like these, the
measurement is made in reference to some standardized power level
defined as 0 dB. For measurements of sound pressure, 0 dB is loosely
defined as the lower threshold of human hearing, objectively quantified
as 1 picowatt of sound power per square meter of area.
A sound measuring 40 dB on the decibel
sound scale would be 104 times greater than the threshold of
hearing. A 100 dB sound would be 1010 (ten billion) times
greater than the threshold of hearing.
Because the human ear is not equally
sensitive to all frequencies of sound, variations of the decibel
sound-power scale have been developed to represent physiologically
equivalent sound intensities at different frequencies. Some sound
intensity instruments were equipped with filter networks to give
disproportionate indications across the frequency scale, the intent of
which to better represent the effects of sound on the human body. Three
filtered scales became commonly known as the "A," "B," and "C" weighted
scales. Decibel sound intensity indications measured through these
respective filtering networks were given in units of dBA, dBB, and dBC.
Today, the "A-weighted scale" is most commonly used for expressing the
equivalent physiological impact on the human body, and is especially
useful for rating dangerously loud noise sources.
Another standard-referenced system of
power measurement in the unit of decibels has been established for use
in telecommunications systems. This is called the dBm scale. The
reference point, 0 dBm, is defined as 1 milliwatt of electrical power
dissipated by a 600 Ω load. According to this scale, 10 dBm is equal to
10 times the reference power, or 10 milliwatts; 20 dBm is equal to 100
times the reference power, or 100 milliwatts. Some AC voltmeters come
equipped with a dBm range or scale (sometimes labeled "DB") intended for
use in measuring AC signal power across a 600 Ω load. 0 dBm on this
scale is, of course, elevated above zero because it represents something
greater than 0 (actually, it represents 0.7746 volts across a 600 Ω
load, voltage being equal to the square root of power times resistance;
the square root of 0.001 multiplied by 600). When viewed on the face of
an analog meter movement, this dBm scale appears compressed on the left
side and expanded on the right in a manner not unlike a resistance
scale, owing to its logarithmic nature.
An adaptation of the dBm scale for audio
signal strength is used in studio recording and broadcast engineering
for standardizing volume levels, and is called the VU scale. VU
meters are frequently seen on electronic recording instruments to
indicate whether or not the recorded signal exceeds the maximum signal
level limit of the device, where significant distortion will occur. This
"volume indicator" scale is calibrated in according to the dBm scale,
but does not directly indicate dBm for any signal other than steady
sine-wave tones. The proper unit of measurement for a VU meter is
volume units.
When relatively large signals are dealt
with, and an absolute dB scale would be useful for representing signal
level, specialized decibel scales are sometimes used with reference
points greater than the 1mW used in dBm. Such is the case for the dBW
scale, with a reference point of 0 dBW established at 1 watt. Another
absolute measure of power called the dBk scale references 0 dBk
at 1 kW, or 1000 watts.
- REVIEW:
- The unit of the bel or decibel may
also be used to represent an absolute measurement of power rather than
just a relative gain or loss. For sound power measurements, 0 dB is
defined as a standardized reference point of power equal to 1 picowatt
per square meter. Another dB scale suited for sound intensity
measurements is normalized to the same physiological effects as a 1000
Hz tone, and is called the dBA scale. In this system, 0 dBA is
defined as any frequency sound having the same physiological
equivalence as a 1 picowatt-per-square-meter tone at 1000 Hz.
- An electrical dB scale with an
absolute reference point has been made for use in telecommunications
systems. Called the dBm scale, its reference point of 0 dBm is
defined as 1 milliwatt of AC signal power dissipated by a 600 Ω load.
- A VU meter reads audio signal
level according to the dBm for sine-wave signals. Because its response
to signals other than steady sine waves is not the same as true dBm,
its unit of measurement is volume units.
- dB scales with greater absolute
reference points than the dBm scale have been invented for high-power
signals. The dBW scale has its reference point of 0 dBW defined
as 1 watt of power. The dBk scale sets 1 kW (1000 watts) as the
zero-point reference.
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