Decibel

A logarithmic unit expressing ratios of power or field quantities, equal to one tenth of a bel. Its reference value must be specified when it expresses an absolute level.

Bel: A logarithmic unit equal to ten decibels, named after Alexander Graham Bell. The decibel is one tenth of this larger unit.
Sound pressure level: A logarithmic measure of sound pressure relative to a specified reference pressure. It is a widely used acoustic level expressed in decibels.
Reference level: A stated baseline quantity against which another measurement is compared. An absolute decibel level has meaning only when its reference is known.
Neper: A logarithmic unit for ratios, defined using natural logarithms rather than base-ten logarithms. It expresses similar ratios but uses a different logarithmic base from the decibel.
Power ratio: The quotient of two power measurements, expressed as a dimensionless number. A power ratio is converted to decibels using ten times its base-ten logarithm.
A-weighting: A frequency weighting that approximates the human ear’s varying sensitivity to sound. Sound levels in dBA apply this weighting before expressing the result in decibels.
Sound intensity: The acoustic power passing through a unit area perpendicular to sound propagation. Sound intensity can be expressed logarithmically relative to a reference intensity.
Phon: A unit of perceived loudness level defined by equal-loudness comparisons with tones. Unlike a decibel level, a phon value represents perceived loudness rather than a physical ratio.
Amplitude ratio: The quotient of two field amplitudes, such as voltage or sound pressure. For equal impedances, its decibel value uses twenty times the base-ten logarithm.
Signal-to-noise ratio: The ratio of signal power to background-noise power in a system. It is commonly reported in decibels to compare signal strength with noise.

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