Lexicon DB
Dimensionless logarithmic unit of measurement for the ratio of two measures, especially two powers. The basic unit of measurement is the Bel (named after the American telephone pioneer Graham Bell), but the derived Dezi-Bel (i.e. tenths of a Bel), written dB, is used almost exclusively. 10dB correspond to a power ratio of 10, 20dB to a power ratio of 100, 30dB to 1000, and so on.
The dB is also often used to indicate voltage ratios or current ratios. In these cases, a constant Load Impedance is assumed, and since the voltage or the current is quadratically included in the power at constant impedance, a voltage or current ratio of 10 corresponds to the value 20dB (because 10 times the voltage or 10 times the current results in 100 times the power).
The dB can thus be used to indicate amplification or attenuation. For this purpose, for example, the ratio between output voltage and input voltage is formed and expressed as a dB value. 0dB corresponds to amplification 1, i.e. the output voltage is equal to the input voltage, no amplification or attenuation takes place. Positive dB values indicate amplification, negative ones attenuation.
If one assumes a known reference point, then one moves from a relative dB specification to an absolute specification of the signal strength. For this purpose, the dB designation is supplemented by additional characters that indicate the reference point used. In hi-fi technology, you will find these designations, among others: dBV: dB referred to 1V voltage. Therefore: 0dBV = 1V dBu: dB referred to 1mW at 600 Ohm (= 774,6mV). Therefore: 0dBu = 774.6mV dBFS: dB referred to the strongest possible sinusoidal signal in a digital system (FS = Full Scale) [1]. dBµV: dB referred to a voltage of 1µV. The following applies: 0dBµV = 1µV. Used e.g. for antenna signals.
Unfortunately, these terms are often used in a casual way, where the reference point has to be inferred from the context. Strictly speaking, however, the following applies: The designation dB alone does not contain a reference point and therefore indicates ratios between two measuring points, whereby one must infer which these are from the context. With addition, it is a question of absolute measured values at a point, referred to a fixed reference.
The use of a logarithmic measure may seem strange and obscure at first, but it fits quite well with the properties of hearing itself and electronics, and it allows a convenient representation of the ratios once one has become familiar with the logarithmic principle.
Footnotes:
- [1] More precisely: the strongest sinusoidal signal whose peak value can still be represented digitally.
See also: