Definition and Measurement of Decibel
– The decibel (dB) is a relative unit of measurement equal to one tenth of a bel (B).
– It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale.
– Two signals whose levels differ by one decibel have a power ratio of 10 (approximately 1.26) or root-power ratio of 10^0.5 (approximately 1.12).
– The unit can express a relative change or an absolute value, often suffixed with letter codes indicating the reference value.
– The decibel scales differ by a factor of two, with a 10dB change in power corresponding to a 20dB change in amplitude.
History of the Decibel
– The decibel originates from methods used to quantify signal loss in telegraph and telephone circuits.
– Before the decibel, the unit for loss was ‘Miles of Standard Cable’ (MSC).
– In 1924, Bell Telephone Laboratories introduced the ‘Transmission Unit’ (TU) as a new unit definition, which later became the decibel.
– The decibel was named in honor of Alexander Graham Bell, replacing the seldom-used bel.
– The decibel was widely adopted for measuring transmission efficiency in telephone circuits.
Usage of Decibel in Various Fields
– The decibel is used for a wide variety of measurements in science and engineering.
– It is prominently used for sound power in acoustics and electronics.
– In electronics, decibels are used to express the gains of amplifiers, attenuation of signals, and signal-to-noise ratios.
– The International Electrotechnical Commission (IEC) permits the use of decibels with power, root-power, and voltage ratios.
– The decibel is recognized by international bodies like the IEC and International Organization for Standardization (ISO).
Decibel in Power Ratios
– The decibel is defined as ten times the logarithm in base 10 when expressing a power ratio.
– A change in power by a factor of 10 corresponds to a 10dB change in level.
– The decibel scale allows for direct addition or subtraction of units expressing the efficiency of different parts of a circuit.
– The decibel scale is widely used for measuring power ratios in telecommunications and other fields.
– ISO 80000-3 and IEC Standard 60027-3 provide definitions and quantities related to the decibel.
International Recognition and Limitations
– In April 2003, the International Committee for Weights and Measures (CIPM) considered including the decibel in the International System of Units (SI), but decided against it.
– However, the decibel is recognized by the IEC and ISO for various measurements.
– Suffixes like dBA or dBV are not recognized by the IEC or ISO.
– The decibel is widely used despite some concerns about confusion when used for quantities other than transmission loss.
– J. W. Horton suggested the name ‘logit’ for standard magnitudes that combine by multiplication, contrasting with ‘unit’ for magnitudes that combine by addition. Source: https://en.wikipedia.org/wiki/Decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 (approximately 1.26) or root-power ratio of 101⁄20 (approximately 1.12).
The unit expresses a relative change or an absolute value. In the latter case, the numeric value expresses the ratio of a value to a fixed reference value; when used in this way, the unit symbol is often suffixed with letter codes that indicate the reference value. For example, for the reference value of 1 volt, a common suffix is "V" (e.g., "20 dBV").
Two principal types of scaling of the decibel are in common use. When expressing a power ratio, it is defined as ten times the logarithm in base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level. When expressing root-power quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level. The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude.
The definition of the decibel originated in the measurement of transmission loss and power in telephony of the early 20th century in the Bell System in the United States. The bel was named in honor of Alexander Graham Bell, but the bel is seldom used. Instead, the decibel is used for a wide variety of measurements in science and engineering, most prominently for sound power in acoustics, in electronics and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels.
