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A bel (symbol B) is a unit of measure of ratios of power levels, i.e.
relative power levels. It is mostly used in telecommunication, electronics
and acoustics. The name was coined in the early 20th century in honour of
Alexander Graham Bell, a telecommunications pioneer.
The bel is a logarithmic measure. The number of bels for a given ratio of
power levels is calculated by taking the logarithm, to the base 10, of the
ratio. Therefore, one bel corresponds to a ratio of 10:1. Mathematically,
the number of bels is calculated as B = log10(P 1/P 2) where P 1 and P 2 are
power levels. The neper is a similar unit which uses the natural logarithm.
The bel is too large for everyday use, so the decibel (dB), equal to 0.1B,
is more commonly used. 1dB is equivalent to a ratio of about 1.259:1. It is
defined as 10 log10(P 1/P 2) where P 1 and P 2 are the powers.
The decibel is not an SI unit, although the CIPM has recommended its
inclusion in the SI system.
In an optical link, if a known amount of optical power, in dBm
(decibel.milliwatts), is launched into a fiber, and the losses, in dB
(decibels), of each component (e.g. connectors, splices, and lengths of
fiber) are known, the overall link loss may be quickly calculated by simple
addition and subtraction of decibel quantities.
The decibel is often used in acoustics to quantify sound levels relative to
some 0 dB reference. The reference may be defined as a sound pressure level
(SPL), commonly 20 micropascal (20 μPa). To avoid confusion with other
decibel measures, the term dB(SPL) is used for this. The reference can also
be defined as the sound intensity at the threshold of human hearing, which
is conventionally taken to be one picowatt per square metre (1 pW/m²),
roughly the sound of a mosquito flying 10 feet (3 m) away.
The reason for using the decibel is that the ear is capable of hearing a
very large range of sound pressures. The ratio of the sound pressure that
causes permanent damage from short exposure to the limit that (undamaged)
ears can hear is more than a million. Because the power in a sound wave is
proportional to the square of the pressure, the ratio of the maximum power
to the minimum power is more than one trillion. To deal with such a range,
logarithmic units are useful: the log of a trillion is 12, so this ratio
represents a difference of 120 dB.
Psychologists have found that our perception of loudness is roughly
logarithmic, see Weber-Fechner Law. In other words, you have to multiply the
sound intensity by the same factor to have the same increase in loudness.
This is why the numbers around the volume control dial on a typical audio
amplifier are related not to the absolute power amplification, but to its
Various frequency weightings are used for acoustical measurements to
approximate the changes in sensitivity of the ear to different frequencies
at different levels. These include the dB(A), dB(B), and dB(C) weightings.
Sounds above 85 decibels are considered harmful, while 120 dB is unsafe and
150 db causes physical damage to the human body. Windows break at 163 dB.
Jet airplanes are 165 dB. Eardums pop at 190 to 198 dB. Shock waves and
sonic booms are 194 dB. Sounds around 200 db can cause death to humans and
are generated near bomb explosions. The space shuttle is around 215 dB.
Nuclear bombs are 240 to 258 dB. Even louder are earthquakes, tornados,
hurricanes and volcanoes.
The decibel is used rather than arithmetic ratios or percentages because
when certain types of circuits, such as amplifiers and attenuators, are
connected in series, expressions of power level in decibels may be
arithmetically added and subtracted.
In radio electronics, the decibel is used to describe the ratio between two
measurements of electrical power. It can also be combined with a suffix to
create an absolute unit of electrical power. For example, it can be combined
with "m" for "milliwatt" to produce the "dBm". 0 dBm is one milliwatt, and
1dBm is one decibel greater than 0 dBm, or about 1.259 mW.
In telecommunications, decibels are commonly used to measure signal-to-noise ratios.
Earthquakes are measured on the Richter scale, which is expressed in bels.
(The units in this case are always assumed, rather than explicit.)
* dBm - dB(mV/m²) - millivolts per square metre - signal strength of
a radio signal
* dBμ or dBu - dB(μV/m²) - microvolts per square metre -
strength of a radio signal
* dBf - dB(fW) - femtowatts - amount of power required to drive a radio
* dBW - dB(W) - watts - amount of power transmitted by a low-power radio
* dBk - dB(kW) - kilowatts - amount of power transmitted by a broadcast
* dBV - dB(V) - volts - amplitude of an audio signal in a wire
* dBv or dBu - dB(0.775V) - same as dBV but referenced to 0.775 volts
instead of 1 volt
* dBm - dB(mW@600Ω) - in analogue audio, milliwatts into a 600-ohm
* dBA, dBB, or dBC - different weightings of the human ear's response to
* dBd - dB(dipole) - effective radiated power compared to a dipole
* dBi - dB(isotropic) - effective radiated power compared to an imaginary
* dBfs or dBFS - dB(full scale) - amplitude of a signal (usually audio)
compared to the maximum which a device can handle before clipping