Copyright © Philip M. Parker, INSEAD. Terms of Use.

Bel

Specialty Definition: Bel

Top     

Specialty Definition: Bel

(From Wikipedia, the free Encyclopedia)

For the Mesopotamian deity, see Bel (god)

A bel (symbol B) is a unit of measure of ratios of power levels, i.e., relative power levels. It is mostly used in telecommunication, electronics and acoustics. Invented by engineers of the Bell Telephone Laboratory, it was originally called the transmission unit or TU, but was renamed in 1923 or 1924 in honour of the laboratory's founder and telecommunications pioneer Alexander Graham Bell.

The bel is a logarithmic measure. The number of bels for a given ratio of power levels is calculated by taking the logarithm, to the base 10, of the ratio. Therefore, one bel corresponds to a ratio of 10:1. Mathematically, the number of bels is calculated as B = log₁₀(P₁/P₂) where P₁ and P₂ are power levels. The neper is a similar unit which uses the natural logarithm.

The bel is too large for everyday use, so the decibel (dB), equal to 0.1 B, is more commonly used. One decibel is equivalent to a ratio of about 1.259:1. It is defined as 10 log₁₀(P₁/P₂) where P₁ and P₂ are the powers.

The decibel is not an SI unit, although the CIPM has recommended its inclusion in the SI system.

Uses

Optics

In an optical link, if a known amount of optical power, in dBm (decibel.milliwatts), is launched into a fibre, and the losses, in dB (decibels), of each component (e.g. connectors, splices, and lengths of fibre) are known, the overall link loss may be quickly calculated by simple addition and subtraction of decibel quantities.

Acoustics

The decibel is often used in acoustics to quantify sound levels relative to some 0 dB reference. The reference may be defined as a sound pressure level (SPL), commonly 20 micropascal (20 μPa). To avoid confusion with other decibel measures, the term dB(SPL) is used for this. The reference can also be defined as the sound intensity at the threshold of human hearing, which is conventionally taken to be one picowatt per square metre (1 pW/m²), roughly the sound of a mosquito flying 10 feet (3 m) away.

The reason for using the decibel is that the ear is capable of hearing 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 more than 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 more than one 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.

Psychologists have found that our perception of loudness is roughly logarithmic, see Weber-Fechner Law. In other words, you have to multiply the sound intensity by the same factor to have the same increase in loudness. This is why the numbers around the volume control dial on a typical audio amplifier are related not to the absolute power amplification, but to its logarithm.

Various frequency weightings are used for acoustical measurements to approximate the changes in sensitivity of the ear to different frequencies at different levels. These include the dB(A), dB(B), and dB(C) weightings.

Sounds above 85 dB are considered harmful, while 120 dB is unsafe and 150 dB causes physical damage to the human body. Windows break at 163 dB. Jet airplanes are 165 dB. Eardrums pop at 190 to 198 dB. Shock waves and sonic booms are 194 dB. Sounds around 200 dB can cause death to humans and are generated near bomb explosions. The space shuttle is around 215 dB. Nuclear bombs are 240 dB to 258 dB. Even louder are earthquakes, tornados, hurricanes and volcanoes.

To do: these example SPLs could do with distances from source...

Electronics

The decibel is used rather than arithmetic ratios or percentages because when certain types of circuits, such as amplifiers and attenuators, are connected in series, expressions of power level in decibels may be arithmetically added and subtracted.

In radio electronics, the decibel is used to describe the ratio between two measurements of electrical power. It can also be combined with a suffix to create an absolute unit of electrical power. For example, it can be combined with "m" for "milliwatt" to produce the "dBm". 0 dBm is one milliwatt, and 1dBm is one decibel greater than 0 dBm, or about 1.259 mW.

Telecommunications

In telecommunications, decibels are commonly used to measure signal-to-noise ratios.

Seismology

Earthquakes are measured on the Richter scale, which is expressed in bels. (The units in this case are always assumed, rather than explicit.)

Typical abbreviations

Absolute measurements:

• dBm - dB(mV/m²) - millivolts per square metre - signal strength of a radio signal
• dBμ or dBu - dB(μV/m²) - microvolts per square metre - strength of a radio signal
• dBf - dB(fW) - femtowatts - amount of power required to drive a radio receiver
• dBW - dB(W) - watts - amount of power transmitted by a low-power radio station
• dBk - dB(kW) - kilowatts - amount of power transmitted by a broadcast radio station
• dBV - dB(V) - volts - amplitude of an audio signal in a wire
• dBv or dBu - dB(0.775V) - same as dBV but referenced to 0.775 volts instead of 1 volt
• dBm - dB(mW@600Ω) - in analogue audio, milliwatts into a 600-ohm load

Relative measurements:

• dBA, dBB, or dBC - different weightings of the human ear's response to sound
• dBd - dB(dipole) - effective radiated power compared to a dipole antenna
• dBi - dB(isotropic) - effective radiated power compared to an imaginary isotropic antenna
• dBfs or dBFS - dB(full scale) - amplitude of a signal (usually audio) compared to the maximum which a device can handle before clipping occurs

Common misconception: +3 dB means "times two"

Not exactly. As stated above, decibels are defined so that +10 dB means "ten times the power". From this, we calculate that +3 dB actually multiplies the power by 10^3/10. This is a ratio of 1.9953 or about 0.25% different from the "times 2" ratio that is sometimes assumed. A level difference of +6 dB is 3.9811, about 0.5% different from 4.

To contrive a more serious example, consider converting a large decibel figure into its linear ratio, for example 120 dB. This is correctly calculated as a ratio of 10¹² or one trillion. But if we use the assumption that 3 dB means "times 2", we would calculate a ratio of 2^120/3 = 2⁴⁰ = 1.0995 × 10¹², for a 10% error.

References

• Martin, W. H., "DeciBel – The New Name for the Transmission Unit", Bell System Technical Journal, January 1929.

  Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Bel."

Top     

Bel (mythology)

(From Wikipedia, the free Encyclopedia)

Bel was the name of a chief deity in Babylonian religion, the counterpart of the Phoenician Baal ideographically written as En-lil.

Since Bel signifies the "lord" or "master" par excellence, it is, therefore, a title rather than a genuine name, and must have been given to a deity who had acquired a position at the head of a pantheon. The real name is accordingly to be sought in En-lil, of which the first element again has the force of "lord" and the second presumably "might," "power," and the like, though this cannot be regarded as certain. En-lil is associated with the ancient city of Nippur, and since En-lu with the determinative for "land" or "district" is a common method of writing the name of the city, it follows, apart from other evidence, that En-lil was originally the patron deity of Nippur.

At a very early period--prior to 3000 BC--Nippur had become the centre of a political district of considerable extent, and it is to this early period that the designation of En-lil as Bel or "the lord" reverts. Inscriptions found at Nippur, where extensive excavations were carried on during 1888-1900 by Messrs Peters and Haynes, under the auspices of the University of Pennsylvania, show that Bel of Nippur was in fact regarded as the head of an extensive pantheon. Among the titles accorded to him are "king of lands," "king of heaven and earth" and "father of the gods."

His chief temple at Nippur was known as E-Kur, signifying "mountain house," and such was the sanctity acquired by this edifice that Babylonian and Assyrian rulers, down to the latest days, vied with one another in embellishing and restoring Bel's seat of worship, and the name itself became the designation of a temple in general.

Grouped around the main sanctuary there arose temples and chapels to the gods and goddesses who formed his court, so that E-Kur became the name for an entire sacred precinct in the city of Nippur. The name "mountain house" suggests a lofty structure and was perhaps the designation originally of the staged tower at Nippur, built in imitation of a mountain, with the sacred shrine of the god on the top. The tower, however, also had its special designation of "Im-Khar-sag," the elements of which, signifying "storm" and "mountain," confirm the conclusion drawn from other evidence that En-lil was originally a storm-god having his seat on the top of a mountain.

Since the Euphrates valley has no mountains, En-lil would appear to be a god whose worship was carried into Babylonia by a wave of migration from a mountainous country—in all probability from Elam to the east.

When, with the political rise of Babylon as the centre of a great empire, Nippur yielded its prerogatives to the city over which Marduk presided, the attributes and the titles of En-lil were transferred to Marduk, who becomes the "lord" or Bel of later days. The older Bel did not, however, entirely lose his right to have any considerable political importance, while in addition the rise of the doctrine of a triad of gods symbolizing the three divisions--heavens, earth and water--assured to Bel, to whom the earth was assigned as his province, his place in the religious system. The disassociation from his local origin involved in this doctrine of the triad gave to Bel a rank independent of political changes, and we, accordingly, find Bel as a factor in the religion of Babylonia and Assyria to the latest days.

It was no doubt owing to his position as the second figure of the triad that enabled him to survive the political eclipse of Nippur and made his sanctuary a place of pilgrimage to which Assyrian kings down to the days of Assur-bani-pal paid their homage equally with Babylonian rulers.

This entry was originally from the 1911 Encyclopedia Britannica.

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Bel (mythology)."

Top     

Bell

(From Wikipedia, the free Encyclopedia)

A small hand-held bell

A bell is a simple sound-making machine, a metallic drum which resonates upon being struck by a clapper. The bell is a percussion instrument and an idiophone.

In its most classical form it is a church bell, which is hung within a tower and sounded by having the entire bell swung by ropes, whereupon an internal hinged tongue strikes the body of the bell. Such a bell produces a very loud, clear tone. The traditional metal for these bells is a bronze of about 20% tin. Known as bell metal, this alloy is also the traditional alloy for the finest Turkish and Chinese cymbals.

Bells can be of all sizes: from tiny dress accessories to church bells literally weighing tons. Some bells are used as musical instruments, such as carillons, or ensembles of bell-players using hand-held bells of varying tones.

Bells are common as alarms; church bells formerly functioned as warnings of fire and invasion. The word is also used for the ringer in telephones, on bicycles, and in door bells. Such bells are struck rapidly and repeatedly by a mechanical or electrical clapper.

Bells are also associated with clocks, indicating the hour by ringing. Indeed, the word clock comes from the Latin word cloca, meaning bell.

In the case of clock towers and grandfather clocks, a particular sequence of tones may be played to represent the hour. One common pattern is called the "Westminster Quarters," a sixteen-note pattern named after the Palace of Westminster which popularized it.

Another method of indicating the hour by means of bells is ship's bells.

Finally, old-fashioned alarm clocks use mechanically chiming bells to indicate a preset hour, usually to rouse a person from sleep.

A variant on the bell is the tubular bell or chimes, composed of several metal tubes which are struck manually with hammers. In the case of wind or aeolian chimes, the tubes are blown against one another by the wind.

The Bell telephone company was founded by Alexander Graham Bell.

In the 19th century Bell was acquired by AT&T, often called Ma Bell, which maintained a telephone monopoly in the United States until 1984. The breakup of AT&T led to the creation of seven regional phone companies, called Baby Bells. The name persists in companies such as Pacific Bell and Southwestern Bell, which are now brands of SBC Communications. See Regional Bell operating company. See also Bell Canada.

See also telephone, AT&T, Bell Labs, and Lucent. The bel (one "L") and its derivitative the decibel or dB is a measure named after Alexander Graham Bell and used in several fields, notably audio. See bel.

The Bell Aircraft Corporation, now the Bell Helicopter Division of the conglomerate Textron, manufactured a number of important early aircraft, such as the P-39 Airacobra.

They also made a series of research aircraft, including the Bell X-1, the first aircraft to exceed the speed of sound (Mach 1), and many famous helicopters, including the Bell Model 47 (US army "Sioux"), the Bell Model 204 (aka UH-1 "Huey") and the JetRanger (model 206 or OH-55 Kiowa in army service).

Bell is also the name of some places in the United States of America:

• Bell, California
• Bell, Florida
• Bell Township, Pennsylvania

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Bell."

Top     

Abbreviations & Acronyms: Bel

Top     

Top     

Synonyms within Context: Bel

Top     

Top     

Modern Usage: Bel

Top     

Commercial Usage: Bel

Top     

Image Slideshow: Bel

Photos: Bel More pictures...

Top     

Photo Album: Bel

Thumbnail	Description & Credit	Thumbnail	Description & Credit
	Dis-donc, bel homme / Pigal. Lith. de Langlumé. Credit: National Library of Medicine.		Diorama by Norman Bel Geddes, depicting the attack by USS Enterprise (CV-6) and USS Yorktown (CV-5) dive bombers on the Japanese aircraft carriers Akagi, Kaga and Soryu in the morning of 4 June 1942. The diorama was created during World War II on the basis of information then available. It is therefore somewhat inaccurate in scope and detail. This angle of view depicts Soryu (attacked by Yorktown aircraft) in the middle distance, with Kaga and Akagi (both attacked by Enterprise aircraft) as the closer two burning ships. Credit: NAVY.
	"Old Saint John's" located in Kingsville on Highway no. 1 between Bel Air and Baltimore ... Credit: Library of Congress.		Geddes, Norman Bel, Mr., portrait photograph. Credit: Library of Congress.
	Portrait of Barbara Bel Geddes. Credit: Library of Congress.
Source: pictures compiled by the editor from various references; see picture credits.

Top     

Digital Photo Gallery: Bel
 

"Bel Motel" by Greg Schmigel Commentary: "Vintage signage from the Bel Alton Motel. Shot on E10, converted to black and white in Pshop. Enjoy. See more of my works at www.27cm.com."

Source: photographs selected by the editor, with permission from the photographers.

Top     

Use in Literature: Bel

Top     

Non-Fiction Usage: Bel

Top     

Usage Frequency: Bel

Source: compiled by the editor from several corpora; see credits.

Top     

Name Usage Frequency: Bel

Top     

Usage in Company Names: Bel

Source: compiled by the editor from Icon Group International, Inc.

Top     

Expressions: Bel

Top     

Frequency of Internet Keywords: Bel

Top     

Modern Translation: Bel

Top     

Bible Trace: Bel

Top     

Matched Bible Translations: Bel

Top     

Derivations: Bel

Top     

Rhyming with "Bel"

Top     

Anagrams: Bel

Top