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 DiracDelta - Science & Engineering encyclopedia
Category: Sciences > Science & Engineering
Date & country: 01/12/2007, UK
Words: 4470


Apodize
To apodize is to remove or smooth a sharp discontinuity in a mathematical function, an electrical signal or a mechanical structure. An example would be to use a Hanning Window in the FFT analyzer to smooth the discontinuities at the beginning and end of the sample time record.

Apostilb
A unit of surface luminance used when defining diffusing surfaces equal to 1 lumen m-2. The apostlib equal to the brightness produced by 1/pi candela per square meter or 10-4 lambert. See also: Illuminance.

Apothecaries' Ounce
Once used as a measurement of weight for drugs and other medical substances in Britain and America. It has now been replaced by the SI kilogram.1 oz (ap.) = 3.110347680x10-2kg

Apparent Horizon
Where the sky appears to meet the Earth. See also: Dip Angle.

Apparent Mass
Force per unit acceleration. See also: Accelerance, Dynamic Stiffness, Mobility, Receptance.

Apparent Source Width (ASW)
Discovered and developed by A. H. Marshall, ASW is a subjective parameter of spaciousness in concert halls, and is related to the level, at the listener's ears, of lateral reflections in the first 50 to 80 milliseconds after the arrival of the direct sound.Increasing the ratio of this reflected energy to the direct sound increases the sense of spac…

Applet
A small application, often downloaded from a remote server and run in a controlled environment. Typically written in a language such as Java for execution by a WWW browser.

Appold Dynamometer
This is a friction based dynamometer with a water cooled drum and automatically corrects any variations of load due to changes in the coefficient of friction. See also: Dynamometer.

Aprotic Solvent
A solvent that does not act as an acid or as a base; aprotic solvents don't undergo autoprotolysis.Examples:pentanepet ethertoluene.

Aqua Regia
A mixture of nitric and hydrochloric acids, usually 1:3 or 1:4 parts HNO3 to HCl, used to dissolve gold. Also known as 'Royal Water'.It is a highly corrosive liquid that will attack many substances unaffected by others. See also: Gold, Hydrochloric Acid, Nitric Acid.

Aquaplaning
When water stands on a road or race track it may not be dispersed effectively from the tyre contact area with the road. The result is less grip and hence 'Aquaplaning'.

Aqueduct
A channel designed to transport water from a remote source, usually by gravity. See also: Water.

Aqueous Solution
A solution in which water is the dissolving medium or solvent. See also: Solvent.

Arago Spot
A bright spot that appears in the shadow of a uniform disc being backlit by monochromatic light emanating from a point source.Poisson Spot Poisson originally predicted the existence of such a spot, and used the prediction to demonstrate how the wave theory of light must be in error to produce such a counterintuitive result. Subsequent observation o…

Arbor
A shaft or spindle for holding cutting tools; most usually on a milling machine.

Arbor Press
A hand-operated machine tool designed for applying high pressure for the purpose of pressing together or removing parts.

Arc Cutting
A group of cutting processes in which the cutting of metals is accomplished by melting with the heat of an arc between the electrode and the base metal. See also: Arc Oxygen Cutting, Arc Welding.

Arc Oxygen Cutting
An oxygen-cutting process used to sever metals by a chemical reaction of oxygen with a base metal at elevated temperatures. See also: Arc Cutting.

Arc Welding
Photograph courtesy of Draper.co.ukA group of welding processes in which fusion is obtained by heating with an electric arc or arcs, with or without the use of filler metal.Arc BlowThe deflection of an electric arc from its normal path because of magnetic forces.Arc LengthThe distance between the tip of the electrode and the weld puddle.Arc Voltage…

Arch
A curved structural member that spans an opening and is generally composed of wedge-shaped blocks that transmit the downward pressure out laterally.The top of an arch is called the extrados or back.The underside of an arch is called the intrados or soffit. See also: Corbel, Diaphram Arch.

Archimedes (287-212 BC)
Lived in Syracuse on Sicily. Forever to be known for the Archimedean principle: 'a body plunged in a fluid loses as much weight as ...'Archimedes invented the water screw, a device for raising water using an encased screw open at both ends. The screw is set at an angle, and as the screw turns, water fills the air pockets and is transported upwards.…

Archimedes' principle
A body that is submerged in a fluid is buoyed up by a force equal in magnitude to the weight of the fluid that is displaced, and directed upward along a line through the centre of gravity of the displaced fluid.The fluid may be a gas or a liquid.The buoyant force is given by:whereF = buoyant force [N]g = standard acceleration due to gravity [ms
Are
A derived SI Unit of area introduced by the French after the French Revolution. Through common usage 100m2 has been named the Are.1 Are (a) = 100m2ttle='Are';xiunt='are';yiunt='m2';mconv=0.01;cconv=0.0;

Area
The derived SI unit of AreaThe derived SI unit of area is the m2.

Arene
A hydrocarbon that contains at least one aromatic ring.

Argon
An inert gas. Symbol ArDiscoveredIn 1895 the chemist William Ramsay and physicist Robert John Strutt (Lord Rayleigh) discovered Argon in liquified air.AbundanceMakes up about 1% of the atmosphere.

Aristotle (384-322 BC)
A Greek philosopher who stressed the importance of direct observations in securing facts and data. He drew conclusions from direct observations, and thus initiated the basis for a scientific method of solving problems. Earlier philosophers had a tendency to reach conclusions and then select data and facts that agreed with their conclusions. He theo…

Arithmetic Coding
Perhaps the major drawback to each of the Huffman encoding techniques is their poor performance when processing texts where one symbol has a probability of occurrence approaching unity. Although the entropy associated with such symbols is extremely low, each symbol must still be encoded as a discrete value. Arithmetic coding removes this restrictio…

Arithmetic Error
As error that occurs when discarding least significant bits of a fixed-point arithmetic operation.

Arithmetic Mean
The arithmetic mean of n numbers is the sum of the numbers divided by n. See also: Average, Mean.

Arko Metal
A brass made with 80% copper and 20 zinc.

Aromatic Compound
A compound containing an aromatic ring. Aromatic compounds have strong, characteristic odors. See also: Aryl.

Aromatic Ring
An exceptionally stable planar ring of atoms with resonance structures that consist of alternating double and single bonds, e. g. benzene. See also: Azobenzene, Benzene.

Arsenic
Arsenic is found in several allotropic forms and has both metallic and non-metallic properties. The grey metallic allotrope is a brittle, crystalline solid which tarnishes readily in air and burns in oxygen. It is resistant to attack by dilute acids and alkalis, but will react with hot acids and molten NaOH. Arsenic is poisonous (it is thought to h…

Arsenic III Oxide
White crystalline solid, used as an opacifier in glazes. Symbol As2O3

Arsenic V Oxide
White solid. Symbol As2O5

Articulation Class
A measure for rating the speech privacy performance of a ceiling in an open plan environment where sound is reflected off the ceiling between two adjacent spaces divided by partial-height furniture panels.A ceiling system with Articulation Class < 150 is low performance.A ceiling system with Articulation Class > 200 is high performance.

Articulation Index
A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded. An articulation index of 100% means that all speech can be understood, 0% means that no speech can be understood.Articulation Index is calculated from the 1/3 octave band levels between 200Hz and 6300Hz centre frequencies. Each …

Articulation Loss of Consonants
The measured percentage of Articulation Loss of Consonants by a listener. %ALCONS of 0 indicates perfect clarity and intelligibility with no loss of consonant understanding, while 10% and beyond is growing toward poor intelligibility, and 15% typically representing the maximum loss acceptable.

Artificial Intelligence
Field of computer science concerned with the concepts and methods of symbolic inference by computer, and the symbolic representation of the knowledge to be used in making inferences. See also: Programming Languages.

Artificial Reverberation
Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., added to a signal to make it sound more lifelike. See also: Acoustics, Reverberation Time, Sound.

Aryl
A molecular fragment or group attached to a molecule by an atom that is on an aromatic ring. See also: Aromatic Compound.

Asbestos
A natural material made up of fine fibres. This description covers a number of fibrous silicate minerals such as calcium magnesium silicate. Use very restricted due to health implications.

Ash
A straight grained, very tough and elastic timber. Used where sudden shocks have to be resisted e.g. handles of tools, oars, blocks and pulleys, spokes of wheels and milk-pails. Ash bark is used tanning and the ashes of ash is used for potash.

Ashlar Masonry
Uniform, rectangular blocks of stone with parallel faces, as used in the construction of classical Greek and Roman buildings. The word 'ashlar' shares a common Latin root with 'axis,' probably relating to the fact that uniform stone blocks can be laid in courses having a straight horizontal axis.

Aspartic Acid
A nonessential amino acid that is abundant in molasses. The carboxylic acid group on the side chain is ionized under physiological conditions, making aspartic acid residues in proteins hydrophilic.

Aspect Ratio
AerodynamicsThe relationship of the wing span to the wing chord, expressed numerically by the number of times the span can be divided by the chord.Graphics deviceThe ratio of the screen dimensions, normally defined as vertical screen dimension divided by horizontal screen dimension.

Asphalt
A black sticky semi-solid material mainly bitumen with minerals, occurs naturally, but made artificially and used in the construction of roads and buildings.

Aspin Engine
A rotary valve engine developed by Frank Aspin. See also: Engine, Internal Combustion Engine.

Assay
A quantitative determination of the metal in an ore or alloy. See also: Alloy, Ore.

Assembler
A computer program that converts symbolic assembly language programs intoequivalent binary machine language programs. Each symbolic instruction is converted to asingle machine instruction.

Assembly
A unit made from many parts. May refer to a molecule, machine tool, or any grouping of objects to form a larger entity.

Astable
A circuit that has no stable state and thus oscillates at a frequency dependent on component values. Produces a square wave output from a DC voltage. See also: Bistable, Flip Flop, Monostable.

Astatine
The heaviest halogen and radioactive. Symbol At Also known as Alabamine

Asterism
A group of stars which are traditionally imagined to represent a pattern (for example, Orion) within an area, known as a constellation, on the celestial sphere. See also: Star.

Asteroids
Asteroid GaspraThese are rocky bodies, the vast majority of which orbit the Sun between Mars and Jupiter. Several thousands of asteroids are known to exist but it is thought that there must be around 100,000 in all, most of which are too small or too faint to be detected from the Earth at present.The first asteroid, Ceres, was discovered by Giusepp…

Astigmatism
An aberration, or defect, in a mirror or lens that causes the image of a point to spread out into a line. See also: Aberration.

Astrolabe
An astrolabe is an instrument that was used to determine the altitude of objects in the sky (like the sun or stars). It was first used around 200 B.C. by astronomers in Greece. The astrolabe was replaced by the sextant. See also: Altitude.

Astrometry
The branch of astronomy concerned with measuring the positions of celestial bodies, such as stars and galaxies, and their real and apparent motions.

Astronaut
A person who rides in a space vehicle.

Astronomical Unit (AU)
Defined as the mean distance of the Earth from the Sun.One light year = 63240 AU1 AU = 1.495979x1011m1 AU = 214.94 solar radii

Astrophysics
The physics of astronomical objects such as stars and galaxies. It seeks to explain why celestial objects, and the universe in general, behave in the way that they do. It relies on the assumption that the laws of physics are the same everywhere in the universe and throughout all time. See also: Astronomy.

Asymmetric Waveform
A waveform that has unequal excursions above and below the horizontal axis. See also: Waveform.

Asymmetrical Support
A rotor support system that does not provide uniform restraint in all radial directions. This is typical in industrial machinery where stiffness in one plane may be substantially different than stiffness in the perpendicular plane. Occurs in bearings by design, or from preloads such as gravity or misalignment.

Asynchronous
ElectronicsThe transmission of data between a transmitting and a receiving device that occurs as a series of zeros and ones. For the data to be 'read' correctly, the receiving device must begin reading at the proper point in the series. In asynchronous communication, this coordination is accomplished by having each character surrounded by one or mo…

Atactic
A type of polymer chain configuration wherein side groups are randomly positioned on one side of the polymer backbone or the other. See also: Polymer.

Atmometer
An instrument for measuring the rate at which water evaporates, also called an evaporimeter. See also: Atmometry, Water.

Atmometry
The science of measuring the rate at which water evaporates. See also: Atmometer, Evaporation, Water.

Atmosphere
The gases around a planet or star. See also: Hydrosphere, Planet, Star.

Atmospheric Pressure
Atmospheric pressure is the force exerted by the weight of the atmosphere of the Earth at ground level. The value quoted is the standard value but will vary, with variations being measured with a barometer.1 atmosphere = 1.01325x105Nm-2 = 1.01325 bar = 1.01325x106 dynes cm2 = 14.7 lbf in-2 = 76…

Atom
The smallest particle of an element that can exist and still retain the ordinary chemical properties of that element. See also: Absorption Spectrum, Atomic weight, Aufbau Principle, Electron Affinity, Hunds Rule, Nucleus, Orbital, Valence Electrons.

Atom Percent
Concentration specification on the basis of the number of moles or atoms of a particular element relative to the total number of moles or atoms of all elements within an alloy. See also: Alloy, Atom, Mole.

Atomic Mass Unit
Defined as one twelfth of the mass of the most abundant isotope of carbon (12C)1 atomic mass unit = 1.66053873x10-27kgttle='Atomic Mass Unit';consttxt='amu';constval=1.66053873e-27;constunt='kg';Before 1960 the atomic mass unit was defined in terms of the mass of an oxygen isotope (16O) and 1 amu = 1.6599 x 10-…

Atomic Number
The atomic number (Z) of an element is the number of protons each atom of that element has in its nucleus; e.g., Z = 17 for chlorine (17Cl).The atomic number was originally defined as an index describing the position of an element in the periodic table and it was not until 1913 that it was known that it was a unit of measurement. Click o…

Atomic Orbital
A wavefunction that describes the behavior of an electron in an atom.

Atomic Packing Factor
The fraction of the volume of a unit cell that is occupied by 'hard sphere' atoms or ions. See also: Atom, Unit Cell.

Atomic weight
The weighted average mass of the atoms in a naturally occurring element. See also: Atom, Atomic Mass Unit, Atomic Number, Molecular Weight.

Atomize
To break up a liquid into extremely fine particles.

Atomus
Latin word used in medieval times to mean 'a twinkling of the eye', the smallest amount of time imaginable, defined in a precise way as 1/376 minute. See also: Time.

Attack
The beginning of a sound; the initial transient of a musical note. See also: Transient Analysis.

Attenuation
Attenuation is the reduction of a physical quantity, the opposite of amplification. Within acoustics and structural dynamics it is normally presented as the logarithm of input dividedby output.In electronics it is the the degree to which an input signal is reduced in amplitude at the output of a circuit or device. Attenuation is typically expressed…

Attenuation of Sound in Air
The attenuation of sound in air at 20°C due to viscous, thermal and rotational lossmechanisms is 1.6E-10f²dB/m. However, losses due to vibrational relaxation of oxygenmolecules are generally much greater than those due to classical processes, and theattenuation of sound varies significantly with temperature, water-vapour content andfrequency. See a…

Attenuator
A two port circuit or device that reduces the amplitude of an input signal by a desired amount typically expressed in decibels (dB). Attenuators may be comprised of fixed resistors. PIN diodes or FET's, or combinations of these. See also: Attenuation.

Attometre
A distance of 10-18m. See also: Decimal Prefixes, Metre.

attosecond
A time of 10-18seconds. See also: Second.

Atwood's Machine
A weight-and-pulley system devised to measure the acceleration due to gravity at the Earth's surface by measuring the net acceleration of a set of weights of known mass around a frictionless pulley.The acceleration of the masses is given by:wherea = acceleration of the masses [ms-2]m1, m2 = mass [kg]g = standard acc…

Audio Books
Sound Intensity By Frank J Fahy, Published By: Spon Press.Psychoacoustics By Eberhard Zwicker, H. Fastl, Published By: Springer-Verlag Berlin and Heidelberg GmbH & Co. KG.Fundamentals of Acoustics By Lawrence E. Kinsler, Austin R. Frey, Alan B. Coppens, James V. Sanders, Published By: John Wiley and Sons.

Audio Calculations
Acoustic CavityThe acoustics of enclosed volumes are important when considering sound propagation within the volume.A WeightingThis is the most generally used filter when making overall noise measurements.BarkThe Bark is the standard unit corresponding to one critical band width of human hearing.B WeightingThe B-weigthing curve approximately follow…

Audio Conversions
DecibelThe human ear responds logarithmically and it is convenient to deal in logarithmic units in audio systems.

Audio Frequency
The range of frequencies which can be experienced by an average human being.Generally in the range 20Hz to 20KHz. However, in practise the range is closer to 20Hz to 17kHz.The specific range of audio frequencies varies considerably from person to person -- varying especially with respect to age. See also: Acoustics, Frequency, Infrasound, Treble.

Audiogram
Graph of hearing threshold level as a function of frequency.Baseline AudiogramThe audiogram obtained from an audiometric examination administered before employment or within the first 30 days of employment that is preceded by a period of at least 12 hr of quiet. The baseline audiogram is the audiogram against which subsequent audiograms will be com…

Audiometer
An instrument for measuring hearing acuity. See also: Audiogram, Hearing.

Auditory Anomia
A neurological disorder which causes a marked inability to name otherwise familiar acoustic stimuli such as a door bell or motor vehicle. See also: Anomia.

Auditory Area
The sensory area lying between the threshold of hearing and the threshold of feeling or pain. See also: Threshold of Hearing, Threshold of Pain.

Auditory Cortex
Region of the cortex devoted to the analysis of sound information.

Auditory Nerve
Bundle of nerve fibers that carry information from the cochlea to the higher stages of the auditory system.

Aufbau Principle
An approximate procedure for writing the ground state electronic configuration of atoms. The configuration of an atom is obtained by inserting one electron into the configuration of the atom immediately to its left on the periodic table. The electron is inserted into the subshell indicated by the element's period and block. See also: Atom.

Auralization
Auralization is the process of rendering audio data by digital means to achieve a virtual three-dimensional sound space. Auralization and binaural processing of audio data is of great interest in a number of different areas.There are several approaches to the reproduction of auralized audio data. The two main categories are headphone and loudspeake…

Aurora
A faint visual phenomenon associated with geomagnetic activity,which occurs mainly in the high-latitude night sky; typical auroras are 100 to 250 kmabove the ground.The name comes from an older one, 'aurora borealis' (Latin for 'northern dawn') given because an aurora near the northern horizon (its usual location when seen in most of Europe) looks …

Auroral Kilometric Radiation
Intense radio waves whose wavelength is of the order of a kilometre, emitted from regions above the ionosphere where the aurora is (apparently) accelerated. Since the waves are even longer than those of the AM radio band, they are stopped by the ionosphere and do not reach the ground, but they are readily observed from spacecraft.