Decibel

The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities. A decibel is one tenth of a bel, a seldom-used unit.

The decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. The decibel confers a number of advantages, such as the ability to conveniently represent very large or small numbers, and the ability to carry out multiplication of ratios by simple addition and subtraction.

The decibel symbol is often qualified with a suffix, that indicates which reference quantity or frequency weighting function has been used.

For example, dBm indicates a reference level of one milliwatt, while dBu is referenced to 0.775 volts RMS.

The definitions of the decibel and bel use base 10 logarithms. The neper, an alternative logarithmic ratio unit sometimes used, uses the natural logarithm (base e).

A change in power ratio by a factor of 10 is a 10 dB change. A change in power ratio by a factor of two is approximately a 3 dB change.

Acoustics

The decibel is commonly used in acoustics to quantify sound levels relative to a 0 dB reference which has been defined as a sound pressure level of .0002 microbar. The reference level is set at the typical threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure. As with other decibel figures, normally the ratio expressed is a power ratio (rather than a pressure ratio).
The human ear has a large dynamic range in audio perception. The
ratio of the sound intensity that causes permanent damage during short
exposure to the quietest sound that the ear can hear is greater than or
equal to 1 trillion Such large measurement ranges are conveniently expressed in logarithmic units: the base-10 logarithm of one trillion (10¹²)
is 12, which is expressed as an audio level of 120 dB. Since the human
ear is not equally sensitive to all sound frequencies, noise levels at
maximum human sensitivity—somewhere between 2 and 4 kHz—are factored more heavily into some measurements using frequency weighting. (See also Stevens’ power law.)

Electronics

In electronics, the decibel is often used to express power or amplitude ratios (gains), in preference to arithmetic ratios or percentages. One advantage is that the total decibel gain of a series of components (such as amplifiers and attenuators)
can be calculated simply by summing the decibel gains of the individual
components. Similarly, in telecommunications, decibels denote signal
gain or loss from a transmitter to a receiver through some medium (free space, waveguide, coax, fiber optics, etc.) using a link budget.
The decibel unit can also be combined with a suffix to create an
absolute unit of electric power. For example, it can be combined with
“m” for “milliwatt” to produce the “dBm”. Zero dBm equals one milliwatt, and 1 dBm is one decibel greater (about 1.259 mW).
In professional audio, a popular unit is the dBu (see below for all
the units). The “u” stands for “unloaded”, and was probably chosen to be
similar to lowercase “v”, as dBv was the older name for the same thing.
It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 V_RMS.
Chosen for historical reasons, it is the voltage level which delivers
1 mW of power in a 600 ohm resistor, which used to be the standard
reference impedance in telephone audio circuits.

Optics

In an optical link, if a known amount of optical power, in dBm (referenced to 1 mW), is launched into a fiber, and the losses, in dB (decibels), of each electronic component
(e.g., connectors, splices, and lengths of fiber) are known, the
overall link loss may be quickly calculated by addition and subtraction
of decibel quantities.
In spectrometry and optics, the blocking unit used to measure optical density is equivalent to −1 B.

Video and digital imaging

In connection with video and digital image sensors,
decibels generally represent ratios of video voltages or digitized
light levels, using 20 log of the ratio, even when the represented
optical power is directly proportional to the voltage or level, not to
its square, as in a CCD imager where response voltage is linear in intensity. Thus, a camera signal-to-noise ratio or dynamic range of 40 dB represents a power ratio of 100:1 between signal power and noise power, not 10,000:1.Sometimes the 20 log ratio definition is applied to electron counts or
photon counts directly, which are proportional to intensity without the
need consider whether the voltage response is linear.
However, as mentioned above, the 10 log intensity convention prevails
more generally in physical optics, including fiber optics, so the
terminology can become murky between the conventions of digital
photographic technology and physics. Most commonly, quantities called
“dynamic range” or “signal-to-noise” (of the camera) would be specified
in 20 log dBs, but in related contexts (e.g. attenuation, gain,
intensifier SNR, or rejection ratio) the term should be interpreted
cautiously, as confusion of the two units can result in very large
misunderstandings of the value.
Photographers also often use an alternative base-2 log unit, the f-stop,
and in software contexts these image level ratios, particularly dynamic
range, are often loosely referred to by the number of bits needed to
represent the quantity, such that 60 dB (digital photographic) is
roughly equal to 10 f-stops or 10 bits, since 10³ is nearly equal to 2¹⁰.

Suffixes are commonly attached to the basic dB unit in order to
indicate the reference level against which the decibel measurement is
taken. For example, dBm indicates power measurement relative to 1
milliwatt.
In cases such as this, where the numerical value of the reference is
explicitly and exactly stated, the decibel measurement is called an
“absolute” measurement, in the sense that the exact value of the
measured quantity can be recovered using the formula given earlier. If
the numerical value of the reference is not explicitly stated, as in the
dB gain of an amplifier, then the decibel measurement is purely
relative.
The practice of attaching a suffix to the basic dB unit, forming
compound units such as dBm, dBu, dBA, etc., is not permitted for use
with the SI. However, outside of documents adhering to SI units, the practice is very common as illustrated by the following examples.

Voltage

Since the decibel is defined with respect to power, not amplitude,
conversions of voltage ratios to decibels must square the amplitude, as
discussed above.

dBV

dB(1 V_RMS) – voltage relative to 1 volt, regardless of impedance^]

dBu or dBv

dB(0.775 V_RMS) – voltage relative to 0.775 volts. Originally dBv, it was changed to dBu to avoid confusion with dBV.
The “v” comes from “volt”, while “u” comes from “unloaded”. dBu can be
used regardless of impedance, but is derived from a 600 Ω load
dissipating 0 dBm (1 mW). Reference voltage

In professional audio, equipment may be calibrated to indicate a “0” on the VU meters some finite time after a signal has been applied at an amplitude of +4 dBu. Consumer equipment will more often use a much lower “nominal” signal level of -10 dBV.
Therefore, many devices offer dual voltage operation (with different
gain or “trim” settings) for interoperability reasons. A switch or
adjustment that covers at least the range between +4 dBu and -10 dBV is common in professional equipment.

dBmV

dB(1 mV_RMS) – voltage relative to 1 millivolt across 75 Ω. Widely used in cable television
networks, where the nominal strength of a single TV signal at the
receiver terminals is about 0 dBmV. Cable TV uses 75 Ω coaxial cable, so
0 dBmV corresponds to −78.75 dBW (−48.75 dBm) or ~13 nW.

dBμV or dBuV

dB(1 μV_RMS) – voltage relative to 1 microvolt. Widely used in television and aerial amplifier specifications. 60 dBμV = 0 dBmV.

Acoustics

Probably the most common usage of “decibels” in reference to sound loudness is dB SPL, sound pressure level referenced to the nominal threshold of human hearing:

dB(SPL)

dB (sound pressure level) – for sound in air and other gases, relative to 20 micropascals (μPa) = 2×10⁻⁵
Pa, the quietest sound a human can hear. This is roughly the sound of a
mosquito flying 3 meters away. This is often abbreviated to just “dB”,
which gives some the erroneous notion that “dB” is an absolute unit by
itself. For sound in water and other liquids, a reference pressure of 1 μPa is used.

One Pascal is equal to 94 dB(SPL). This level is used to specify
microphone sensitivity. For example, a typical microphone may put out
20 mV at one pascal. For other sound pressure levels, the output voltage
can be computed from this basis, except that noise and distortion will
affect the extreme levels.

dB(PA)

dB – relative to 1 Pa, often used in telecommunications.

dB SIL

dB sound intensity level – relative to 10⁻¹² W/m², which is roughly the threshold of human hearing in air.

dB SWL

dB sound power level – relative to 10⁻¹² W.

dB(A), dB(B), and dB(C)

These symbols are often used to denote the use of different weighting filters, used to approximate the human ear’s response
to sound, although the measurement is still in dB (SPL). These
measurements usually refer to noise and noisome effects on humans and
animals, and are in widespread use in the industry with regard to noise
control issues, regulations and environmental standards. Other
variations that may be seen are dB_A or dBA. According to ANSI standards, the preferred usage is to write L_A = x dB. Nevertheless, the units dBA and dB(A) are still commonly used as a shorthand for A-weighted measurements. Compare dBc, used in telecommunications.

dB HL or dB hearing level is used in audiograms as a measure of hearing loss. The reference level varies with frequency according to a minimum audibility curve
as defined in ANSI and other standards, such that the resulting
audiogram shows deviation from what is regarded as ‘normal’ hearing.

dB Q is sometimes used to denote weighted noise level, commonly using the ITU-R 468 noise weighting

Audio electronics

dBm

dB(mW) – power relative to 1 milliwatt. No reference impedance is assumed, although 600 ohms is common in audio equipment.

dBFS

dB(full scale) – the amplitude of a signal compared with the maximum which a device can handle before clipping occurs. Full-scale may be defined as the power level of a full-scale sinusoid or alternatively a full-scale square wave.

dBTP

dB(true peak) – peak amplitude of a signal compared with the maximum which a device can handle before clipping occurs.
In digital systems, 0 dBTP would equal the highest level (number) the
processor is capable of representing. Measured values are always
negative or zero, since they are less than or equal to full-scale.

Radar

dBZ

dB(Z) – energy of reflectivity (weather radar), related to the
amount of transmitted power returned to the radar receiver; the
reference level for Z is 1 mm⁶ m⁻³. Values above 15–20 dBZ usually indicate falling precipitation.

dBsm

dBsm – decibel measure of the radar cross section
(RCS) of a target relative one square meter. The power reflected by the
target is proportional to its RCS. “Stealth” aircraft and insects have
negative RCS measured in dBsm, large flat plates or non-stealthy
aircraft have positive values.

Radio power, energy, and field strength

dBc

dBc – relative to carrier—in telecommunications,
this indicates the relative levels of noise or sideband peak power,
compared with the carrier power. Compare dBC, used in acoustics.

dBJ

dB(J) – energy relative to 1 joule. 1 joule = 1 watt per hertz, so power spectral density can be expressed in dBJ.

dBm

dB(mW) – power relative to 1 milliwatt.
When used in the radio field, the dB is usually referenced to a 50 ohm
load, with the resultant voltage being 0.224 volts. There are times when
spec sheets may show the voltage & power level e.g. −120 dBm =
0.224 microvolts.

dBμV/m or dBuV/m

dB(μV/m) – electric field strength relative to 1 microvolt per meter. Often used to specify the signal strength from a television broadcast at a receiving site (the signal measured at the antenna output will be in dBμV).

dBf

dB(fW) – power relative to 1 femtowatt.

dBW

dB(W) – power relative to 1 watt.

dBk

dB(kW) – power relative to 1 kilowatt.

Antenna measurements

dBi

dB(isotropic) – the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

dBd

dB(dipole) – the forward gain of an antenna compared with a half-wave dipole antenna. 0 dBd = 2.15 dBi

dBiC

dB(isotropic circular) – the forward gain of an antenna compared to a circularly polarized
isotropic antenna. There is no fixed conversion rule between dBiC and
dBi, as it depends on the receiving antenna and the field polarization.

dBq

dB(quarterwave) – the forward gain of an antenna compared to a
quarter wavelength whip. Rarely used, except in some marketing material.
0 dBq = −0.85 dBi

Other measurements

dB-Hz

dB(hertz) – bandwidth relative to 1 Hz. E.g., 20 dB-Hz corresponds to a bandwidth of 100 Hz. Commonly used in link budget calculations. Also used in carrier-to-noise-density ratio (not to be confused with carrier-to-noise ratio, in dB).

dBov or dBO

dB(overload) – the amplitude of a signal (usually audio) compared with the maximum which a device can handle before clipping occurs. Similar to dBFS, but also applicable to analog systems.

dBr

dB(relative) – simply a relative difference from something else,
which is made apparent in context. The difference of a filter’s response
to nominal levels, for instance.

dBrn

dB above reference noise.