EMC Question of the Week: December 23, 2019
The decibel is a unit named in honor of,
- Alexander Graham Bell
- Kristen Bell
- Harry Belafonte
- Heinrich Decibel
Answer
The correct answer is "a". In the 1920s, Bell Telephone Laboratories proposed the decibel as a unit for quantifying signal loss in telegraph and telephone transmissions. The bel, named in honor of telecommunications pioneer Alexander Graham Bell, was defined as the logarithm of the ratio of two signal powers. The decibel is one tenth of a bel. The decibel was the proposed working unit, since it was close in amplitude to the previous industry standard unit for loss, the MSC (Miles of Standard Cable).
Decibels (dB) are the preferred units for expressing quantities such as signal gain, loss, crosstalk and shielding effectiveness. As long as the impedance of the two signals is the same, decibel notation makes it unnecessary to clarify whether it is the signal amplitudes or powers that are being compared. It also makes it easier to express measurement results or measurement error without ambiguity. If Signal A is 3 dB higher than Signal B, then Signal B is 3 dB lower than Signal A.
Unlike most other standard units, the bel is rarely used with prefixes other than deci. For example, 20 decibels would be written 20 dB rather than 2 B or 2 bels. That's why an internet search of the phrase "two bels" is likely to turn up an image like the one on the right.
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