Decibel (dB) definition
Decibel (Symbol: dB) is a logarithmic unit that indicates ratio or gain.
Decibel is used to indicate the level of acoustic waves and electronic signals.
The logarithmic scale can describe very big or very small numbers with shorter notation. The dB level can be viewed as relative gain of one level vs. other level, or absolute logarithmic scale level for well known reference levels.
Decibel is a dimensionless unit. The ratio in bels is the base 10 logarithm of the ratio of
P 1 and P 0 :
Ratio B = log 10 (P 1 / P 0 )
Decibel is one tenth of a bel, so 1 bel is equal to 10 decibel.
1B = 10dB
The power ratio in decibels (dB) is 10 times base 10 logarithm of the ratio of P 1 and P 0 :
Ratio dB = 10·log 10 (P 1 / P 0 )
The ratio of quantities like voltage, current and sound pressure level are calculated as ratio of squares.
The amplitude ratio in decibels (dB) is 20 times base 10 logarithm of the ratio of V 1 and V 0 :
Ratio dB = 10·log 10 ([V1]^2 / [V0]^2 )
= 20·log 10 (V 1 / V 0 )