The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. One of these values is often a standard reference value, in which case the decibel is used to express the level of the other value relative to this reference. The number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities,〔''IEEE Standard 100 Dictionary of IEEE Standards Terms, Seventh Edition'', The Institute of Electrical and Electronics Engineering, New York, 2000; ISBN 0-7381-2601-2; page 288〕 or of the ratio of the squares of two field amplitude quantities. One decibel is one tenth of one bel, named in honor of Alexander Graham Bell; however, the bel is seldom used.
The definition of the decibel is based on the measurement of power in telephony of the early 20th century in the Bell System in the United States. Today, the unit 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. By contrast, use of the decibel complicates operations of addition and subtraction.
A change in power by a factor of 10 corresponds to a 10 dB change in level. At the half power point an audio circuit or an antenna exhibits an attenuation of . A change in voltage by a factor of 10 results in a change in power by a factor of 100, which corresponds to a 20 dB change in level. A change in voltage ratio by a factor of 2 approximately corresponds to a 6 dB change in level.
The decibel symbol is often qualified with a suffix that indicates the reference quantity that has been used or some other property of the quantity being measured. For example, dBm indicates a reference power of one milliwatt, while dBu is referenced to approximately 0.775 volts RMS.〔
In the International System of Quantities, the decibel is defined as a unit of measurement for quantities of type level or level difference, which are defined as the logarithm of the ratio of power- or field-type quantities.〔(【引用サイトリンク】url=http://www.iso.org/iso/catalogue_detail?csnumber=31888 )〕
The decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. The unit for loss was originally ''Miles of Standard Cable'' (MSC). 1 MSC corresponded to the loss of power over a 1 mile (approximately 1.6 km) length of standard telephone cable at a frequency of 5000 radians per second (795.8 Hz), and matched closely the smallest attenuation detectable to the average listener. The standard telephone cable implied was "a cable having uniformly distributed resistance of 88 ohms per loop mile and uniformly distributed shunt capacitance of 0.054 microfarad per mile" (approximately 19 gauge).
In 1924, Bell Telephone Laboratories received favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the ''Transmission Unit'' (TU). 1 TU was defined as ten times the base-10 logarithm of the ratio of measured power to a reference power level.
The definition was conveniently chosen such that 1 TU approximated 1 MSC; specifically, 1 MSC was 1.056 TU. In 1928, the Bell system renamed the TU into the decibel, being one tenth of a newly defined unit for the base-10 logarithm of the power ratio. It was named the ''bel'', in honor of the telecommunications pioneer Alexander Graham Bell.
The bel is seldom used, as the decibel was the proposed working unit.〔, Robert J. Chapuis, Amos E. Joel, 2003〕
The naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931:
In April 2003, the International Committee for Weights and Measures (CIPM) considered a recommendation for the inclusion of the decibel in the International System of Units (SI), but decided against the proposal.〔(Consultative Committee for Units, Meeting minutes ), Section 3〕 However, the decibel is recognized by other international bodies such as the International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO).〔("Letter symbols to be used in electrical technology – Part 3: Logarithmic and related quantities, and their units" ), ''IEC 60027-3 Ed. 3.0'', International Electrotechnical Commission, 19 July 2002.〕 The IEC permits the use of the decibel with field quantities as well as power and this recommendation is followed by many national standards bodies, such as NIST, which justifies the use of the decibel for voltage ratios.〔 The term ''field quantity'' is deprecated by ISO, which favors root-power. In spite of their widespread use, suffixes (such as in dBA or dBV) are not recognized by the IEC or ISO.
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