Acoustic Terms and Definitions: Decibel or dB
The decibel (dB) is a logarithmic unit used to measure sound level. For example, a whisper is about 30 dB and normal conversation is about 60 dB.
Decibels can be used to express a change in value (e.g., +1 dB or −1 dB) or an absolute value. In the latter case, the number of decibels 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. 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") according to NIST.
Different definitions are used for expressing a ratio in decibels, depending on the nature of the quantities: power and root-power. When expressing a power ratio, the number of decibels is ten times its logarithm in base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing root-power quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two so that the related power and root-power levels change by the same number of decibels in linear systems, where power is proportional to the square of amplitude.
The definition of the decibel originated in the measurement of power in telephony of the early 20th century in the Bell System in the United States. One decibel is one tenth (deci-) of one bel, named in honor of Alexander Graham Bell; however, the bel is seldom used. Today, the decibel 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.
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 root-power type quantities.