Wednesday, April 06, 2011


In radioland, the decibel is ad common a measuring unit as meters, ohms, volts or watts. But outside of radio the humble decibel is virtually unused. Bing a logarithmic unit it's as difficult to grasp to the layperson as the Richter scale. The way it's described has changed over time as well further lowering the odds of general use. A decibel is one tenth of a bel, a seldom-used unit.

This is a modern definition from Decibel, a text by Frederic P Miller from DM Publishing House. The text, mercilessly ripped off without credit in Wikipedia is as follows:
"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 quantities with the same unit, it is a dimensionless unit. A decibel is one tenth of a bel, a seldom-used unit. The decibel is useful for a wide variety of measurements in science and engineering (particularly acoustics, electronics, and control theory) 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 sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction. 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 RMS."
In the 1923 the popular unit was the TU, an acronym for transmission unit. It had been in use for 20 years. TU itself was devised to replace MSC in 1923. MSC was another acronym which stood for Miles of Standard Cable. One (1) MSC is equal to the loss of power over a 1 mile of standard telephone cable at a frequency of 795.8 Hz. This reference assumes a resistance of 88 ohms and capacitance of 54 nanofarads per loop mile. This seemingly arbitrary frequency is exactly equal to another random and archaic unit, 5000 radians per second. Some documents claim thsi is also the first unit of loss to attenuation that a normal human can detect. This is crap. Most people cant even detect a 3dB change. It was accepted because it's actually near the fundamental frequency of the diaphragms used in the standard telephone receivers of that era.  That's according to the American Institute of Electrical Engineers, at least circa 1913. In these century old measurements the people actually using the telephone were referred to in all seriousness as "telephonists."
But this unit of measurement is over 100 years old now. It was named for Alexander Graham Bell. A bel is 10 decibels, so a decibel is 0.1 bels. But the bel was only used to measure sound intensity. It describes the total acoustic energy emanating from the measured source. A decibel does not have that limitation. Interestingly Samsung uses it in some of their documentation resurrecting the archaic unit. The decibel was used initially in telephone engineering as a unit of transmission power. It was adopted formally as an international unit at the First International Acoustical Conference held in Paris, July of 1937. But in the US, it was already in use. Bell systems had been lobbying for it as a standard unit for decades. They added it to their own technical journal in an article by W.H. Martin in 1929. He compared its ascension as that of TU over MSC. It was in the NBS Standards Yearbook by 1931. They described it as follows:
"The decibel may be defined by the statement that two amounts of power differ by 1 decibel when they are in the ratio of 100.1 and any two amounts of power differ by N decibels when they are in the ratio of 10N(0.1). The number of transmission units expressing the ratio of any two powers is therefore ten times the common logarithm of that ratio."

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