decibel (dB) Unit for measuring the relative intensities of sounds or the relative amounts of acoustic or electric power. Because it requires about a tenfold increase in power for a sound to register twice as loud to the human ear, a logarithmic scale is useful for comparing sound intensity. Thus, the threshold of human hearing (absolute silence) is assigned the value of 0 dB and each increase of 10 dB corresponds to a tenfold increase in intensity and a doubling in loudness. The “threshold of pain” for inten¬ sity varies from 120 to 130 dB among different individuals. A related unit is the bel = 10 dB.
Nonlinear (Decibel) and Linear (Intensity) Scales
Decibels
Intensity*
Type of sound
130
10
artillery fire at close proximity (threshold of pain)
120
1
amplified rock music; near jet engine
no
10 -'
loud orchestral music, in audience
100
10 2
electric saw
90
10- 3
bus or truck interior
80
10- 4
automobile interior
70
10- 5
average street noise; loud telephone bell
60
10- 6
normal conversation; business office
50
10- 7
restaurant; private office
40
10" 8
quiet room in home
30
10- 9
quiet lecture hall; bedroom
20
10 - 1 °
radio, television, or recording studio
10
10 -"
soundproof room
0
10-' 2
absolute silence (threshold of hearing)
*ln watts per square meter.
deciduous tree Broad-leaved tree that sheds all its leaves during one season. Deciduous forests are found in three middle-latitude regions with a temperate climate characterized by a winter season and year-round pre¬ cipitation: eastern North America, western Eurasia, and northeastern Asia. They also extend into more arid regions along stream banks and around bodies of water. Oaks, beeches, birches, chestnuts, aspens, elms, maples, and basswoods (or lindens) are the dominant trees in mid-latitude deciduous forests. Other plants that shed their leaves seasonally may also be called deciduous. See also conifer, evergreen.
decision theory In statistics and related subfields of philosophy, the theory and method of formulating and solving general decision problems. Such a problem is specified by a set of possible states of the environment or possible initial conditions; a set of available experiments and a set of
possible outcomes for each experiment, giving information about the state of affairs preparatory to making a decision; a set of available acts depend¬ ing on the experiments made and their consequences; and a set of pos¬ sible consequences of the acts, in which each possible act assigns to each possible initial state some particular consequence. The problem is dealt with by assessing probabilities of consequences conditional on different choices of experiments and acts and by assigning a utility function to the set of consequences according to some scheme of value or preference of the decision maker. An optimal solution consists of an optimal decision function, which assigns to each possible experiment an optimal act that maximizes the utility, or value, and a choice of an optimal experiment. See also cost-benefit analysis, game theory.
Decius \'de-shos\, Gaius Messius Quintus Trajanus (b. c. 201, Budalia, Pannonia Inferior—d. June 251, Abrittus, Moesia) Roman emperor (249-251). Of uncertain origins, he served as senator, consul, and provincial military commander before taking the throne from Philip the Arabian. He resisted the Gothic invasion of Moesia and instituted the first organized persecution of Christians throughout the empire (250), which only served to strengthen the Christian cause. He ended the per¬ secutions in 251, shortly before he was defeated and killed by the Goths.
Declaration of Independence (July 4, 1776) Document approved by the Continental Congress that announced the separation of 13 North American British colonies from Britain. The armed conflict during the American Revolution gradually convinced the colonists that separation from Britain was essential. Several colonies instructed their delegates to the Continental Congress to vote for independence. On June 7, Richard Henry Lee of Virginia offered a resolution for independence. The congress appointed Thomas Jefferson, John Adams, Benjamin Franklin, Roger Sher¬ man, and Robert R. Livingston to draft a declaration. Jefferson was per¬ suaded to write the draft, which was presented with few changes on June 28. It began with a declaration of individual rights and then listed the acts of tyranny by George III that formed the justification for seeking inde¬ pendence. After debate and changes to accommodate regional interests, including deletion of a condemnation of slavery, it was approved on July 4 as “The Unanimous Declaration of the Thirteen United States of America.” It was signed by Congress president John Hancock, printed, and read aloud to a crowd assembled outside, then engrossed (written in script) on parchment and signed by the 56 delegates.
Declaration of the Rights of Man and of the Citizen Mani¬ festo adopted by France’s National Assembly in 1789, which contained the principles that inspired the French Revolution. One of the basic char¬ ters of human liberties, it served as the preamble to the Constitution of 1791. Its basic principle was that “all men are born free and equal in rights,” specified as the rights of liberty, private property, the inviolabil¬ ity of the person, and resistance to oppression. It also established the prin¬ ciple of equality before the law and the freedoms of religion and speech. The Declaration represented a repudiation of the pre-Revolutionary monarchical regime.
Declaratory Act (1766) Declaration by the British Parliament that accompanied repeal of the Stamp Act. It stated that Parliament’s author¬ ity was the same in America as in Britain and asserted Parliament’s author¬ ity to make laws binding on the American colonies.
declaratory judgment In law, a judgment merely declaring a right or establishing the legal status or interpretation of a law or instrument. It is binding but is distinguished from other judgments or court opinions in that it includes no executive element (an order that something be done); instead it simply declares or defines rights to be observed or wrongs to be eschewed by litigants, or expresses the court’s view on a contested question of law.
decolonization Process by which colonies become independent of the colonizing country. Decolonization was gradual and peaceful for some British colonies largely settled by expatriates but violent for others, where native rebellions were energized by nationalism. After World War II, Euro¬ pean countries generally lacked the wealth and political support neces¬ sary to suppress faraway revolts; they also faced opposition from the new superpowers, the U.S. and the Soviet Union, both of which had taken positions against colonialism. Korea was freed in 1945 by Japan’s defeat in the war. The U.S. relinquished the Philippines in 1946. Britain left India in 1947, Palestine in 1948, and Egypt in 1956; it withdrew from Africa in the 1950s and ’60s, from various island protectorates in the 1970s and ’80s, and from Hong Kong in 1997. The French left Vietnam in 1954 and
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decompression chamber ► defamation I 525
gave up its North African colonies by 1962. Portugal gave up its African colonies in the 1970s; Macau was returned to the Chinese in 1999.
decompression chamber See hyperbaric chamber
decompression sickness also called the bends or caisson dis¬ ease Harmful effects of rapid change from a higher- to a lower-pressure environment. Small amounts of the gases in air are dissolved in body tis¬ sues. When pilots of unpressurized aircraft go to high altitudes or when divers breathing compressed air return to the surface, external pressure on the body decreases and the gases come out of solution. Rising slowly allows the gases to enter the bloodstream and be taken to the lungs and exhaled; with a quicker ascent, the gases (mostly nitrogen) form bubbles in the tissues. In the nervous system, they can cause paralysis, convul¬ sions, motor and sensory problems, and psychological changes; in the joints, severe pain and restricted mobility (the bends); in the respiratory system, coughing and difficulty breathing. Severe cases include shock. Recompression in a hyperbaric chamber followed by gradual decompres¬ sion cannot always reverse tissue damage.