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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.^{[1]} Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit. A decibel is one tenth of a bel, a seldomused unit.
The decibel is widely known as a measure of sound pressure level, but is also used for a wide variety of other measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gain of amplifiers, attenuation of signals, and signal to noise ratios are often expressed in decibels. 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 sound and light, 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 that the reference quantity is one milliwatt, while dBu is referenced to 0.775 volts RMS.^{[2]} and dBμV/m referenced to microvolts per meter for radio frequency signal strength.
The definitions of the decibel and bel use logarithms to base 10. The neper, used in electronics, uses natural logarithm to base (e).
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