The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio. The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 10 0.1 and any two amounts of power differ by N decibels when they are in the ratio of 10 N(0.1). The new transmission unit is widely used among the foreign telephone organizations and recently it was termed the 'decibel' at the suggestion of the International Advisory Committee on Long Distance Telephony. This unit was employed up to 1923 when a new unit was adopted as being more suitable for modern telephone work. The introduction of cable in 1896 afforded a stable basis for a convenient unit and the 'mile of standard' cable came into general use shortly thereafter. Since the earliest days of the telephone, the need for a unit in which to measure the transmission efficiency of telephone facilities has been recognized. The naming and early definition of the decibel is described in the NBS Standard's Yearbook of 1931: The bel is seldom used, as the decibel was the proposed working unit. It was named the bel, in honor of the telecommunications pioneer Alexander Graham Bell. 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. The definition was conveniently chosen such that 1 TU approximated 1 MSC specifically, 1 MSC was 1.056 TU. 1 TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power. In 1924, Bell Telephone Laboratories received a 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). A standard telephone cable was 'a cable having uniformly distributed resistance of 88 ohms per loop-mile and uniformly distributed shunt capacitance of 0.054 microfarads per mile' (approximately corresponding to 19 gauge wire). 1 MSC corresponded to the loss of power over one mile (approximately 1.6 km) of standard telephone cable at a frequency of 5000 radians per second (795.8 Hz), and matched closely the smallest attenuation detectable to a listener. Until the mid-1920s, the unit for loss was Miles of Standard Cable (MSC). The decibel originates from methods used to quantify signal loss in telegraph and telephone circuits.

In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. Instead, the decibel is used for a wide variety of measurements in science and engineering, most prominently for sound power in acoustics, in electronics and control theory. The bel was named in honor of Alexander Graham Bell, but the bel is seldom used. The definition of the decibel originated in the measurement of transmission loss and power in telephony of the early 20th century in the Bell System in the United States. The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude. When expressing 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, it is defined as ten times the logarithm in base 10. Two principal types of scaling of the decibel are in common use. For example, for the reference value of 1 volt, a common suffix is ' V' (e.g., '20 dBV').

In the latter case, the numeric value expresses the ratio of a value to a fixed reference value when used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value. The unit expresses a relative change or an absolute value. Two signals whose levels differ by one decibel have a power ratio of 10 1/10 (approximately 1.26) or root-power ratio of 10 1⁄ 20 (approximately 1.12). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel ( B). For other uses, see Decibel (disambiguation). For use of this unit in sound measurements, see Sound pressure level. This article is about the logarithmic unit.

Log base of decibel scale