![]() 3.2 Representation of multiplication operations.2.2 Field quantities and root-power quantities. ![]() 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. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. ![]() Today, the decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. One decibel is one tenth ( deci-) 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. The decibel scales differ so that the related power and field quantities change by the same number of decibels. The extra factor of two is due to the logarithm of the quadratic relationship between power and amplitude in most systems. When expressing field (root-power) quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing a power ratio, the number of decibels is ten times its logarithm to base 10. Two different scales are used when expressing a ratio in decibels, depending on the nature of the quantities: power and field (root-power). For example, if the reference value is 1 volt, then the suffix is " V" (e.g., "20 dBV"), and if the reference value is one milliwatt, then the suffix is " m" (e.g., "20 dBm"). In the latter case, it expresses the ratio of a value to a fixed reference value when used in this way, a suffix that indicates the reference value is often appended to the decibel symbol. It can be used to express a change in value (e.g., +1 dB or −1 dB) or an absolute value. The decibel (symbol: dB) is a unit of measurement used to express the ratio of one value of a physical property to another on a logarithmic scale, called the level. For other uses, see Decibel (disambiguation).Īn example scale showing power ratios x, amplitude ratios √ x, and dB equivalents 10 log 10 x.
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