The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level. In Physics, power (symbol P) is the rate at which work is performed or energy is transmitted or the amount of energy required or expended for Since it expresses a ratio of two (same unit) quantities, it is a dimensionless unit. In Dimensional analysis, a dimensionless quantity (or more precisely a quantity with the dimensions of 1) is a Quantity without any Physical units A decibel is one tenth of a bel (B).
The decibel is useful for a wide variety of measurements in science and engineering (e. Engineering is the Discipline and Profession of applying technical and scientific Knowledge and g. , acoustics and electronics) and other disciplines. Acoustics is the interdisciplinary science that deals with the study of Sound, Ultrasound and Infrasound (all mechanical waves in gases liquids and solids Electronics refers to the flow of charge (moving Electrons through Nonmetal conductors (mainly Semiconductors, whereas electrical It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of, for example, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.
The decibel is not an SI unit. However, following the SI convention, the d is lowercase, as it represents the SI prefix deci-, and the B is capitalized, as it is an abbreviation of a name-derived unit (the bel). deci- (symbol d) is a prefix in the SI system of units denoting a factor of 10−1 (1/10 The full name decibel follows the usual English capitalization rules for a common noun. English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States
The decibel symbol is often qualified with a suffix, which indicates which reference quantity or frequency weighting function has been used. A-weighting is the most commonly used of a family of curves defined in the International standard IEC616722003 and various national standards relating to the measurement of sound For example, "dBm" indicates that the reference quantity is one milliwatt, while "dBu" is referenced to 0. 775 volts. The practice of attaching a suffix in this way, though not permitted by SI, is widely followed.
The definitions of the decibel and bel use base-10 logarithms. For a similar unit using natural logarithms to base e, see neper. For Neper as a mythological god see Neper (mythology, for the lunar crater named Neper see Neper (crater, and for the Scottish mathematician phycisist and
The bel was originally devised by engineers of the Bell Telephone Laboratories to quantify the reduction in audio level over a 1 mile (approximately 1. Bell Laboratories (also known as Bell Labs and formerly known as AT&T Bell Laboratories and Bell Telephone Laboratories) is the Research organization A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States 6 km) length of standard telephone cable. Basic principle A traditional landline telephone system also known as "plain old telephone service" (POTS, commonly handles both signaling and audio information It was originally called the transmission unit or TU, but was renamed in 1923 or 1924 in honor of the Bell System's founder and telecommunications pioneer Alexander Graham Bell. The Bell System which was named for Alexander Graham Bell, the technologist popularly credited with the invention of the Telephone, was a Trademark and WikipediaWikiProject Aircraft. Please see WikipediaWikiProject Aircraft/page content for recommended layout In many situations, however, the bel proved inconveniently large, so the decibel has become more common.
In April 2003, the International Committee for Weights and Measures (CIPM) considered a recommendation for the decibel's inclusion in the SI system and decided not to adopt that recommendation. The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures ( CIPM, sometimes written in English 
When referring to measurements of power or intensity, a ratio can be expressed in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference level. In Mathematics, the logarithm of a number to a given base is the power or Exponent to which the base must be raised in order to produce Thus, if L represents the ratio of a power value P1 to another power value P0, then LdB represents that ratio expressed in decibels and is calculated using the formula:
Naturally, P1 and P0 must have the same dimension (that is, must measure the same type of quantity), and must as necessary be converted to the same units before calculating the ratio of their numerical values. In mathematics the dimension of a Space is roughly defined as the minimum number of Coordinates needed to specify every point within it Note that if P1 = P0 in the above equation, then LdB = 0. If P1 is greater than P0 then LdB is positive; if P1 is less than P0 then LdB is negative.
Rearranging the above equation gives the following formula for P1 in terms of P0 and LdB:
Since a bel is equal to ten decibels, the corresponding formulae for measurement in bels (LB) are
When referring to measurements of amplitude it is usual to consider the ratio of the squares of A1 (measured amplitude) and A0 (reference amplitude). This is because in most applications power is proportional to the square of amplitude. Thus the following definition is used:
The formula may be rearranged to give
Similarly, in electrical circuits, dissipated power is typically proportional to the square of voltage or current when the impedance is held constant. An electronic circuit is a closed path formed by the interconnection of Electronic components through which an Electric current can flow Electrical tension (or voltage after its SI unit, the Volt) is the difference of electrical potential between two points of an electrical Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal Alternating current (AC Taking voltage as an example, this leads to the equation:
where V1 is the voltage being measured, V0 is a specified reference voltage, and GdB is the power gain expressed in decibels. A similar formula holds for current.
Note that all of these examples yield dimensionless answers in dB because they are relative ratios expressed in decibels.
It is seen that there is a 10 dB increase (decrease) for each factor 10 increase (decrease) in the ratio of the two power levels, and approximately a 3 dB increase (decrease) for every factor 2 increase (decrease). In exact terms, the factor is 103/10, or 1. 9953, about 0. 24% different from exactly 2. Similarly, an increase of 3 dB implies an increase in voltage by a factor of approximately √2, or about 1. 41, an increase of 6 dB corresponds to approximately four times the power and twice the voltage, and so on. (In exact terms the power factor is 106/10, or about 3. 9811, a relative error of about 0. 5%. )
The use of the decibel has a number of merits:
The decibel is commonly used in acoustics to quantify sound levels relative to some 0 dB reference. Sound pressure is the local Pressure deviation from the ambient (average or equilibrium pressure caused by a Sound Wave. Acoustics is the interdisciplinary science that deals with the study of Sound, Ultrasound and Infrasound (all mechanical waves in gases liquids and solids Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies The reference level is typically set at the threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure. Sound pressure is the local Pressure deviation from the ambient (average or equilibrium pressure caused by a Sound Wave.
A reason for using the decibel is that the ear is capable of detecting a very large range of sound pressures. Sound pressure is the local Pressure deviation from the ambient (average or equilibrium pressure caused by a Sound Wave. The ratio of the sound pressure that causes permanent damage from short exposure to the limit that (undamaged) ears can hear is above a million. The word million In standard English, the -lli- in million is pronounced with an l-sound followed by a 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 above one (short scale) trillion. The long and short scales are two different numerical systems used throughout the world Short scale is the English translation of the French 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. Since the human ear is not equally sensitive to all the frequencies of sound within the entire spectrum, noise levels at maximum human sensitivity — for example, the higher harmonics of middle A (between 2 and 4 kHz) — are factored more heavily into sound descriptions using a process called frequency weighting. In Acoustics and Telecommunication, the harmonic of a Wave is a component Frequency of the signal that is an Integer La or A is the sixth Note ( Submediant) in the C major scale "A" is generally used as a standard for tuning The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. A weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others for measurement or other purposes
In electronics, the decibel is often used to express power or amplitude ratios (gains), in preference to arithmetic ratios or percentages. Sound pressure is the local Pressure deviation from the ambient (average or equilibrium pressure caused by a Sound Wave. In Electronics, gain is a measure of the ability of a circuit (often an Amplifier) to increase the power or Amplitude of a Arithmetic or arithmetics (from the Greek word αριθμός = number is the oldest and most elementary branch of mathematics used by almost everyone In Mathematics, a percentage is a way of expressing a number as a Fraction of 100 ( per cent meaning "per hundred" 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. Generally an amplifier or simply amp, is any device that changes usually increases the amplitude of a signal. An attenuator is an Electronic device that reduces the Amplitude or power of a signal without appreciably distorting its Similarly, in telecommunications, decibels are used to account for the gains and losses of a signal from a transmitter to a receiver through some medium (free space, wave guides, coax, fiber optics, etc. In Classical physics, free space is a concept of Electromagnetic theory, corresponding to a theoretically "perfect" Vacuum, and sometimes A waveguide is a structure which guides waves such as Electromagnetic waves Light, or Sound waves Coaxial cable is a cable consisting of an inner conductor surrounded by a tubular insulating layer typically made from a flexible material with a high Dielectric constant, all An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length ) using a link budget. A link budget is the accounting of all of the gains and losses from the transmitter through the medium (free space cable waveguide fiber etc
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". For other uses see DBM (disambiguation dbm was the first of a family of simple Database engines originally written by Ken Zero dBm is the power level corresponding to a power of 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. In Mathematics, the root mean square (abbreviated RMS or rms) also known as the quadratic mean, is a statistical measure of the 775 VRMS. 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 almost all professional low-impedance audio circuits.
The bel is used to represent noise power levels in hard drive specifications. A hard disk drive ( HDD) commonly referred to as a hard drive, hard disk, or fixed disk drive, is a Non-volatile storage device It shares the same symbol (B) as the byte. A byte (pronounced "bite" baɪt is the basic unit of measurement of information storage in Computer science.
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. An optical link is a Communications link that consists of a single end-to-end optical circuit For other uses see DBM (disambiguation dbm was the first of a family of simple Database engines originally written by Ken An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length An electronic component is a basic electronic element usually packaged in a discrete form with two or more connecting leads or metallic pads 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. Barrier troops, also translated as blocking troops or blocking detachments (заградотряды заградительные отряды отряды заграждения In Optics, density is a unitless measure of the Transmittance of an optical element for a given length at a given Wavelength λ: In astronomy, the apparent magnitude measures the brightness of a star logarithmically, since, just as the ear responds logarithmically to acoustic power, the eye responds logarithmically to brightness; however astronomical magnitudes reverse the sign with respect to the bel, so that the brightest stars have the lowest magnitudes, and the magnitude increases for fainter stars. The apparent magnitude ( m) of a celestial body is a measure of its Brightness as seen by an observer on Earth, normalized to the value
Although decibel measurements are always relative to a reference level, if the numerical value of that reference is explicitly and exactly stated, then 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. For example, since dBm indicates power measurement relative to 1 milliwatt,
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 by SI.  However, outside of documents adhering to SI units, the practice is very common as illustrated by the following examples.
dBm or dBmW
Note that the decibel has a different definition when applied to voltage (as contrasted with power). See the "Definitions" section above.
dBu or dBv
dBμV or dBuV
dB(A), dB(B), and dB(C)
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. Minimum audibility curve is a standardised graph of the Threshold of hearing versus frequency for an average human and is used as the reference level when measuring hearing
dB Q is sometimes used to denote weighted noise level, commonly using the ITU-R 468 noise weighting
dBμ or dBu
dBFS or dBfs
dBov or dBO