DBu

dB	Power ratio		Amplitude ratio
100	10000000000		100000
90	1000000000		31623
80	100000000		10000
70	10000000		3162
60	1000000		1000
50	100000		316	.2
40	10000		100
30	1000		31	.62
20	100		10
10	10		3	.162
6	3	.981 ≈ 4	1	.995 ≈ 2
3	1	.995 ≈ 2	1	.413 ≈ √2
1	1	.259	1	.122
0	1		1
−1	0	.794	0	.891
−3	0	.501 ≈  1⁄2	0	.708 ≈ √ 1⁄2
−6	0	.251 ≈  1⁄4	0	.501 ≈  1⁄2
−10	0	.1	0	.3162
−20	0	.01	0	.1
−30	0	.001	0	.03162
−40	0	.0001	0	.01
−50	0	.00001	0	.003162
−60	0	.000001	0	.001
−70	0	.0000001	0	.0003162
−80	0	.00000001	0	.0001
−90	0	.000000001	0	.00003162
−100	0	.0000000001	0	.00001
An example scale showing power ratios x, amplitude ratios √x, and dB equivalents 10 log10 x.

The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity. One of these values is often a standard reference value, in which case the decibel is used to express the level of the other value relative to this reference. When used in this way, the decibel symbol is often qualified with a suffix that indicates the reference quantity that has been used or some other property of the quantity being measured. For example, dBm indicates a reference power of one milliwatt, while dBV is referenced to 1 volt RMS.

There are two different scales used when expressing a ratio in decibels depending on the nature of the quantities: field quantity ratio or power quantity ratio. (Field quantity ratio is also referred to as root-power ratio or amplitude ratio.) When expressing power quantities, the number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing field quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The extra factor of two is due to the logarithm of the quadratic relationship between power and amplitude. The decibel scales differ so that direct comparisons can be made between related power and field quantities when they are expressed in decibels.

The definition of the decibel is based on 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.

...
Wikipedia