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Decibels, or ”dB” in audio is a form of measurement of loudness, much in the same way watts is a measurement of power in an amplifier.
Technically
The ‘’’decibel’’’ (’’’dB’’’) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied ‘’reference level’’. Since it expresses a ratio of two (same unit) quantities, it is a dimensionless unit. A decibel is one tenth of a ‘’’bel’’’ (’’’B’’’).
The decibel is useful for a wide variety of measurements in science and engineering (e.g., acoustics and electronics) and other disciplines. It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of, for example, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.
The decibel is not an SI unit. However, following the SI convention, the ‘’d’’ is lowercase, as it represents the SI prefix ‘’deci-’’, and the ‘’B’’ is capitalized, as it is an abbreviation of a name-derived unit (the bel). The full name ‘’decibel’’ follows the usual [[English language English capitalization rules for a common noun.
The decibel symbol is often qualified with a suffix, which indicates which reference quantity or frequency weighting function has been used. For example, “dBm” indicates that the reference quantity is one milliwatt, while “dBu” is referenced to 0.775 volts. The practice of attaching a suffix in this way, though not permitted by SI, is widely followed.
The definitions of the decibel and bel use base-10 logarithms. For a similar unit using natural logarithms to base ‘’e’’, see neper.
History
The bel was originally devised by engineers of the Bell Telephone Laboratories to quantify the reduction in audio level over a 1 mile (approximately 1.6 km) length of standard telephone cable. It was originally called the ‘’transmission unit’’ or ‘’TU’’, but was renamed in 1923 or 1924 in honor of the Bell System‘s founder and telecommunications pioneer Alexander Graham Bell. In many situations, however, the bel proved inconveniently large, so the decibel has become more common.
In April 2003, the International Committee for Weights and Measures (CIPM) considered a recommendation for the decibel’s inclusion in the SI system and decided not to adopt that recommendation.
Power
When referring to measurements of ‘’power’’ or ‘’intensity’’, a ratio can be expressed in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference level. Thus, if ‘’L’’ represents the ratio of a power value ‘’P’’ to another power value ‘’P’’, then ‘’L’’ dB represents that ratio expressed in decibels.
Merits
The use of the decibel has a number of merits:
*The decibel’s logarithmic nature means that a very large range of ratios can be represented by a convenient number, in a similar manner to scientific notation. This allows one to clearly visualize huge changes of some quantity.
*The mathematical properties of logarithms mean that the overall decibel gain of a multi-component system (such as consecutive amplifiers) can be calculated simply by summing the decibel gains of the individual components, rather than needing to multiply amplification factors. Essentially this is because the human perception of, for example, sound or light, is, roughly speaking, such that a doubling of actual intensity causes perceived intensity to always increase by the same amount, irrespective of the original level. The decibel’s logarithmic scale, in which a doubling of power or intensity always causes an increase of approximately 3 dB, corresponds to this perception.
Acoustics
The decibel is commonly used in acoustics to quantify sound levels relative to some 0 dB reference. The reference level is typically set at the threshold of perception of an average human and there are common comparisons used to illustrate different levels of sound pressure.
A reason for using the decibel is that the ear is capable of detecting a very large range of sound pressures. The ratio of the sound ‘’pressure’’ that causes permanent damage from short exposure to the limit that (undamaged) ears can hear is above a million. Because the ‘’power’’ in a sound wave is proportional to the ‘’square of the pressure’’, the ratio of the maximum power to the minimum power is above one (short scale) trillion. To deal with such a range, logarithmic units are useful: the log of a trillion is 12, so this ratio represents a difference of 120 dB. Since the human ear is not equally sensitive to all the frequencies of sound within the entire spectrum, noise levels at maximum human sensitivity — for example, the higher harmonics of middle A (between 2 and 4 kHz) — are factored more heavily into sound descriptions using a process called frequency weighting.
Electronics
In electronics, the decibel is often used to express power or amplitude ratios (gains), in preference to arithmetic ratios or percentages. One advantage is that the total decibel gain of a series of components (such as amplifiers and attenuators) can be calculated simply by summing the decibel gains of the individual components. Similarly, in telecommunications, decibels are used to account for the gains and losses of a signal from a transmitter to a receiver through some medium (free space, wave guides, coax, fiber optics, etc.) using a link budget.
The decibel unit can also be combined with a suffix to create an absolute unit of electric power. For example, it can be combined with “m” for “milliwatt” to produce the ”dBm”. Zero dBm is the power level corresponding to a power of one milliwatt, and 1 dBm is one decibel greater (about 1.259 mW).
In professional audio, a popular unit is the dBu (see below for all the units). The “u” stands for “unloaded”, and was probably chosen to be similar to lowercase “v”, as dBv was the older name for the same thing. It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 VRMS. Chosen for historical reasons, it is the voltage level which delivers 1 mW of power in a 600 ohm resistor, which used to be the standard reference impedance in almost all professional low-impedance audio circuits.
The bel is used to represent noise power levels in hard drive specifications. It shares the same symbol (’’’B’’’) as the byte.
“Absolute" and “relative” decibel measurements
Although decibel measurements are always relative to a reference level, if the numerical value of that reference is explicitly and exactly stated, then the decibel measurement is called an “absolute” measurement, in the sense that the exact value of the measured quantity can be recovered using the formula given earlier. For example, since dBm indicates power measurement relative to 1 milliwatt,
*0 dBm means no change from 1 mW. Thus, 0 dBm is the power level corresponding to a power of ‘’exactly’’ 1 mW.
*3 dBm means 3 dB greater than 0 dBm. Thus, 3 dBm is the power level corresponding to 10 times; 1 mW, or approximately 2 mW.
*−6 dBm means 6 dB less than 0 dBm. Thus, −6 dBm is the power level corresponding to 10 times; 1 mW, or approximately 250 μW (0.25 mW).
If the numerical value of the reference is not explicitly stated, as in the dB gain of an amplifier, then the decibel measurement is purely relative. The practice of attaching a suffix to the basic dB unit, forming compound units such as dBm, dBu, dBA, etc, is not permitted by SI.
However, outside of documents adhering to SI units, the practice is very common as illustrated by the following examples.
edited from Wikipedia