The decibel began as a way of measuring miles of telephone cable
Before the decibel, telephone engineers measured signal loss in miles of standard cable: the fading of power over one mile of a specified wire, which happened to be about the smallest drop a listener could notice. The Bell System swapped it for a logarithmic unit in the 1920s and named it after Alexander Graham Bell.
The old unit came into use soon after cable was introduced in 1896. One mile of standard cable meant the power lost along a mile of line with set resistance and capacitance, tested at about 796 hertz. In 1924 Bell Telephone Laboratories won support from a European long-distance telephony committee for a new transmission unit, defined as ten times the base-10 logarithm of a power ratio and deliberately chosen to be nearly equal to the old mile. In 1928 Bell renamed it the decibel, one tenth of a bel.
The logarithm is what makes it useful. Every tenfold increase in power adds 10 decibels, so a hundredfold gain is 20 and a thousandfold is 30. One decibel corresponds to a power ratio of about 1.26. Engineers could simply add and subtract the decibel figures for each section of a circuit instead of multiplying ratios. For amplitudes such as voltage, a tenfold change counts as 20 decibels, because power rises with the square of amplitude; the two scales are built to agree.
Strictly, a decibel is only a ratio, but people soon began quoting absolute levels by fixing a reference, with suffixes like dBV for levels measured against one volt. That drift worried purists. Proposals for alternatives, such as the decilog and J. W. Horton's logit of 1954, never caught on. The bel itself is almost never used on its own, and nobody writes millibels; hundredths of a decibel are preferred.
Official status remains curious. When the body that oversees the SI weighed admitting the decibel in April 2003, it said no, though standards bodies such as the IEC and ISO recognise it. Amplifier gain, signal-to-noise ratios and sound levels in acoustics are all still expressed in it.
Source: Decibel
