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

Acidic
Describes a solution with a high concentration of H⁺ ions. See also: Acid.

Acidic Solution
A solution in which the hydrogen ion activity is higher than that of the hydroxide ion, when the solvent is water. See also: Acid.

Acidulant
A substance added to food or beverages to lower pH and to impart a tart, acid taste. Phosphoric acid is an acidulant added to cola drinks. See also: Acid.

Ackermann Angle
The toe-out or toe-in of a vehicle with Ackermann steering when the wheels are positioned straight ahead. See also: Ackermann Steering, Toe-In, Toe-Out.

Ackermann Steering
A double-pivoting steering system where the outer ends of the steering arms are bent slightly inward so that when the vehicle is making a turn, the inside wheel will turn more sharply than the outer wheel. This is done to compensate for the greater distance the outside wheel must travel.As a rule of thumb the links connecting the track rod ends to …

Aclinic Line
The line through those points on the earth's surface at which magnetic dip is zero. The aclinic line is a particular case of an isoclinic line. Also called dip equator, magnetic equator. See also: Agonic Line, Earth.

Acme Thread
A screw thread having a 29 degree included angle. Used largely for feed and adjusting screws on machine tools.The Acme thread is easier to machine and is stronger than the square thread.

Acorn Valve
An acorn valve is a form of thermionic valve used at ultra-high frequencies, in which, by adopting very small dimensions and special constructions, the inter-electrode capacitances, lead inductances and transit time effects are greatly reduced. See also: Valve.

Acoustic Admittance
This is the ratio of volume velocity to pressure, the reciprocal of Acoustic Impedance. Also known as acoustic mobility. See also: Acoustic Impedance.

Acoustic Agglomeration
High intensity sound waves are used in a fluid to group suspended particles into largeraggregates.

Acoustic Barriers
An acoustic barrier may be an acoustic wall such as that close to a motorway designed to reduce noise propagation or a partition between two volumes/rooms to reduce noise transmission.The acoustic performance of barriers come into play in many different systems:Exterior walls, windows and doors of buildings to reduce noise from outside.Interior wal…

Acoustic Centre
The point in space of the origin of sound. For a sound emitting transducer (e.g., a loudspeaker), the point from which the spherical waves appear to diverge as observed at remote points. See also: Acoustic Origin.

Acoustic Coupler
An acoustic coupler is means of connecting external devices to a telephone handset avoiding direct electrical connection; most commonly used for low-speed data terminals.

Acoustic Emission
A measure of integrity of a material determined by sound emission when a material is stressed. Sometimes these sounds are audible such as wood cracking or rock particles abrading against one another. More often the acoustic emissions are not audible, due to their low amplitude or high frequency, or both.For rolling element bearing analysis, it is t…

Acoustic Excitation
The process of inducing vibration in a structure by exposure to sound waves. See also: Acoustics.

Acoustic Feedback
Acoustic feedback is the return of acoustic energy from the output of a sound reproducing equipment, such as a microphone-amplifier-loudspeaker combination, to the input or to an intermediate stage, thereby causing the system to generate sustained oscillations which are manifested as continual howling.

Acoustic Filter Elements
There are a number of different types of acoustic filter elements that are used in many different applications.Absorptive DuctAbsorption within ducts is used to break up standing waves and attenuate high frequency noise such as flow generated noise.Expansion ChamberThe expansion chamber can be considered as a simple low pass filter.Helmholtz Resona…

Acoustic Impedance
The total reaction of a medium to the transmission of sound through it,expressed as the ratio of sound pressure to particle velocity at agiven point in the medium. The acoustic impedance Z is useful in describingthe acoustic radiation from sources such as surfaces and ducts.whereZ = acoustic impedance [rayls or kgm^-2s^-1]p = ac…

Acoustic Mirrors
Acoustic concrete mirrors were indeed an early form of radar and some of them can still be seen today. They were strung along the south east coast of England to detect the approach of hostile aircraft during the 1920s and 1930s.The pre-radar acoustic mirrors 'collected' sound in the same way as the satellite dishes we see today collect radio waves.…

Acoustic Ohm
The unit of acoustic impedance, equal to the impedance of a medium in which a sound pressure of one dyne per square centimeter produces a velocity of one centimeter per second. See also: Acoustic Impedance, Acoustics, Dyne.

Acoustic Origin
The point in time at which the signal originates. See also: Acoustic Centre.

Acoustic Pack
Name given to all of the soft absorptive parts used to reduce noise in a vehicle. See also: Absorbent Material.

Acoustic Source Location
There are a number of different ways by which the location of a noise source may be found. The best starting point is a binaural system based on that used within the human hearing system.The human hearing system uses a number of different cues to locate a noise source:Interaural Time Differences these become more difficult to calculate at higher fr…

Acoustic Sources
MonopoleFor a simple monopole the pressure at a distance r is,wherep(r,t) = pressure at a distance r and at time time [Pa]?₀ = density of air [kgm^-3]c = speed of sound in air [ms^-1]? = frequency [rads^-1]t = time [s]k = wavenumber [m^-1]r = distance from centre of source [m]Q = source strength [m…

Acoustic Streaming
Unidirectional flow currents in a fluid that are due to the presence of sound waves.

Acoustic Tile
A descriptive term often used to describe commercially available room acoustic treatment. These tiles are often used in false ceilings where the acoustics within the room in question needs to be improved. However, the location of the absorption is as important as the amount that is applied.

Acoustic Wedges
The acoustic treatment used on all surfaces of an anechoic chamber. The distance from wedge tip to the surface of the wall needs to be a quarter of a wavelength for the lowest frequency of interest for optimum absorption. The wedge shape offers a much greater surface area for a given wall surface area. An air gap between the wall and the back surfa…

Acoustics
Acoustics (Greek., akouein, 'to hear'), term sometimes used for the science of sound in general, as in those qualities of an enclosure that together determine its character with respect to distinct hearing. It is more commonly used for the special branch of that science, architectural acoustics, that deals with the construction of enclosed areas so…

Acre
An old British unit of area traditionally used for measuring the area of a field.Conversions1 acre=4840 yd²1 acre=4.046856x10³ m²1 acre=10 square chains1 acre=160 square rods1 square mile=640 acres1 in²=6.4516 cm²1 square mile=2.58999 km²1 acre=0.4046856 hectarettle='Area';xiunt='acr…

Acre Foot
A volume one foot deep covering an area of one acre. Used to measure the capacity of reservoirs.Conversions1 acre foot=43560 cubic feet1 acre foot=1233.482 m³1 acre foot=0.123 348 hectare metre1 acre foot=12 acre inchesttle='Acre Foot';xiunt='m³';yiunt='acre foot';mconv=1233.482;cconv=0.0; See also: Acre, Acre Inch, Foot, Volu…

Acre Inch
A volume one inch deep covering an area of one acre.Conversions1 acre inch=3630 cubic feet1 acre inch=102.7902 m³1 acre foot=12 acre inchesttle='Acre Inch';xiunt='m³';yiunt='acre foot';mconv=102.7902;cconv=0.0; See also: Acre, Acre Foot.

Acrylic
Synthetic resin made from acrylic acid or a derivative thereof; acrylics possess the property of transparency and offer flame resistance. See also: Plastic.

Actinic
Pertaining to electromagnetic radiation capable of initiating photochemical reactions, as in photography or the fading of pigments.Because of the particularly strong action of ultra violet radiation on photochemical processes, the term has come to be almost synonymous with ultraviolet, as in actinic rays. See also: Actinic Ray.

Actinic Ray
A ray of light of short wavelengths, as ultraviolet or violet, that produces photochemical effects. See also: Actinic, Wavelength.

Actinide
Elements 89-102 are called actinides. Electrons added during the Aufbau construction of actinide atoms go into the 5f subshell. Actinides are unstable and undergo radioactive decay. The most common actinides on Earth are uranium and thorium.HHeLiBeBCNOFNeNaMgAlSiPSClArKCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKrRbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXeCsBaLaHfTaWRe…

Actinium
A radioactive element. The name derives from the Greek, aktis or akinis for 'beam or ray' because it is a powerful source of alpha radiation. Symbol AcDiscoveredDiscovered by the French chemist Andre-Louis Debierne in 1899 and independently discovered by German chemist Friedrich Oskar Giesel in 1902, who called it emanium.

Actinometer
An actinometer is an instrument for measuring heat radiation. See also: Actinometry, Heat.

Actinometry
The science of measurement of radiant energy, particularly that of the sun, in its thermal, chemical, and luminous aspects. See also: Actinometer, Sun.

Action Level
An action level is basically a noise exposure level at which employers are required to take certain steps to reduce the harmful effects of noise on hearing. There are two action levels for continuous Noise:The first action level is set at an 8-hour average noise exposure level (or daily personal noise exposure level, LEP,d) of 85 dB(A), at which th…

Activated Charcoal
A porous form of carbon that acts as a powerful adsorbent, used to decolourise liquids, recover solvents, and remove toxins from water and air.

Activated Complex
State of highest energy during a reaction. When reactants form the activated complex, bond breaking and bond formation is occurring. Also known as transition state.

Activation Energy
For the forward reaction is the energy required to go from reactants to the transition state. The activation energy for the reverse reaction is the energy required to go from products to the transition state.

Activator
Substance that enhances the ability of a flux to remove oxides and other contaminants from surfaces being joined.

Active Device
A component that has gain or operates in a non-linear fashion to change the basic character of an electric signal by, for example, amplification or rectification, or a device comprised of such component, for example a transistor switch. A device whose output is dependent upon an external source of power other than the input signal. See also: Active…

Active High
The active, true, one, or asserted case of a binary signal is the high or mostpositive voltage level. See also: Active Low.

Active Low
The active, true, one, or asserted case of a binary signal is the low or lesspositive voltage level. See also: Active High.

Active Metal
A metal that is easily oxidized (corroded) in air. For example, sodium will violently react with air, aluminum will always have an air-formed oxide film on its surface, and iron is easily rusted. These metals have high negative standard electrode potentials and are high the on the electromotive series.This is in contrast with a noble metal. See als…

Active Network
A circuit that produces gain. See also: Active Device.

Active Noise Control
This is an electronic method of reducing or removing unwanted sound by the production of a pressure wave of equal amplitude but opposite sign to the unwanted sound. When the electronically produced inverse wave is added to original unwanted sound the result is sound cancellation.This method of noise control is becoming increasingly popular for a va…

Active Site
A pocket or crevice on an enzyme molecule that fits reactant molecules like a hand in a glove. The active site lowers the activation energy for reaction. See also: Enzyme.

Active Sun
The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radio frequency radiation are at a maximum. See also: Sun.

Activity
The rate at which a collection of radioactive nuclei decay. One curie corresponds to 3.7x10¹⁰ decays per second. See also: Curie, Radioactive Decay.

Actuator
A device which controls or operates another device.

Acute Angle
A positive angle that is less than 90 degrees. See also: Obtuse Angle.

Acute Triangle
A triangle in which all of the angles are less than 90 degrees.

Ada
A Pascal descended computer language, designed by Jean Ichbiah's team at CII Honeywell in 1979, made mandatory for Department of Defense software projects by the Pentagon. Ada is a large, complex, block-structured language aimed primarily at embedded applications. It has facilities for real-time response, concurrency, hardware access and reliable r…

Adaptive Learning
Learning where a system programs itself by adjusting weights or strengths until it produces the desired output.

Adaptive Optics
A technique that uses deformable mirrors on a telescope to correct the blurring caused by turbulence in the atmosphere.

Adaptive Transform Acoustic Coding
ATRAC is a lossy audio coding system based on psychoacoustic principles. It compresses compact disc audio to approximately 1/5 of the original data rate with virtually no loss in sound quality. It is the encoding scheme used in MiniDisc. See also: Adaptive Differential Pulse Code Modulation.

Adcock Aerial
An Adcock aerial is a radio aerial system consisting of two vertical open-spaced dipoles with screened horizontal connections. It thus responds almost exclusively to the vertically polarised component of a received wave and is used for radio direction finding.

Addition
Symbol : +The operation of finding the sum of two or more quantities. In arithmetic, addition is commutative, i.e. , and associative, .The identity element is 0, i.e. .

Addition Reaction
A chemical reaction in which two atoms or groups of atoms are added across a double bond.

Additive Colour System
An additive colour system is a colour reproduction system in which an image is displayed by mixing appropriate amounts of red, green and blue light, as for example in a cathode-ray tube. See also: Additive Primaries, CMYK.

Additive Identity
The additive identity is the number zero, because zero will not change a number when added to it: a + 0 = a for all a. See also: Additive Inverse.

Additive Inverse
The addition of number and it's additive inverse is zero. The additive inverse of a number a is -a, Also known as the opposite of a, such that a + (-a) = 0. For example, 1 + (-1) = 0. See also: Additive Identity.

Additive Primaries
Red, green, and blue are the primary colours of light from which all other colours can be made. See also: Additive Colour System, CMYK, Colour.

Address
The binary number that represents the collection of binary signals used by memoryhardware to determine which memory register to access. See also: Effective Address, Memory.

Adhere
To stick or be glued to something. See also: Adhesion.

Adhesion
The state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking action.egadhesion between water molecules and glass creates a meniscus

Adhesive
A substance capable of holding material together by surface attachment. See also: Abhesive, Anaerobic Adhesive, Glue.

Adhesive Force
The attractive force exerted on a liquid molecule by the molecules in the surface of the solid.

Adhesive Tape
A tape with a sticky substance on one side. See also: Adhesive.

Adiabatic Process
A process in which the system does not exchange heat with the surroundings.Very fast processes can often be considered adiabatic with respect to heat exchange with the surroundings, because heat exchange is not instantaneous.wherep = PressureV = VolumeT = Temperature? = Ratio of heat capacities?W = Work done on system

Adipic Acid
White crystalline solid used in the manufacture of nylon. Symbol C₆H₁₀O₄

Adjacent Angles
Two angles are adjacent if they share a common vertex and have a common side between them.

Admittance
The ratio of current to voltage, the reciprocal of impedance. The unit of admittance is the siemens (S). Admittance is typically abbreviated as 'y' or 'Y.' See also: Siemens.

Adsorb
To collect molecules of a substance on a surface. See also: Adsorbent, Adsorbtion Chromatography.

Adsorbent
A substance that collects molecules of another substance on its surface. See also: Absorbent Material, Adsorb, Adsorbtion Chromatography.

Adsorption Chromatography
A technique for separating or analyzing mixtures that contain at least one component that is preferentially adsorbed by the stationary phase as it moves over it. See also: Adsorb, Adsorbent.

Advection
The transfer of matter such as water vapour or heat through the atmosphere as a result of horizontal movement of an air mass.

Aeolipile
A round vessel caused to rotate by the force of tangentially escaping steam: an early example of jet propulsion.It consists of a hollow metallic ball which rotates about its vertical axis, with horizontal arm-like tubes projecting radially, and having their free ends bent round in a tangential direction. When the water in the globe is heated, and s…

Aeolotropy
Aeolotropy is the antithesis of isotropy. It is the state of those bodies or substances which do not exhibit the same qualities in all directions.

Aeon
In astronomical terms, 1,000 million years. It was proposed in 1957 for use in geology, but was never approved by the SI and is not used very much.

Aeration
Preparation of a saturated solution of air gases by either spraying the solution in air or by bubbling air through it.

Aero Metal
A metal used in casting made from aluminium, zinc and copper.

Aerobic
Requiring or occurring in the presence of oxygen. See also: Anaerobic, Oxygen.

Aerodynamic Diameter
The diameter of a sphere with unit density that has aerodynamic behavior identical to that of the particle in question; an expression of aerodynamic behavior of an irregularly shaped particle in terms of the diameter of an idealized particle. Particles having the same aerodynamic diameter may have different dimensions and shapes.

Aerodynamic Noise
Sound generated by turbulent flow is just as if the field were generatedby a distribution of quadrupole sources (Lighthill, 'On sound generatedaerodynamically, I. General theory.', 1952, Proc. Roy. Soc. London A 211, 564).Lighthill rearranged the basic equations of fluid dynamics to show this exact analogy.The frequency content is roughly determine…

Aerodynamics Calculations
Drag CoefficientA dimensionless value that allows the comparison of drag incurred by different sized and different shaped bodies.Kinematic ViscosityThe dynamic viscosity of a fluid divided by the fluid density.ViscosityA measure of the internal friction within a fluid.

Aerodynamics Conversions
Flow RateVolume per unit of time.Kinematic ViscosityThe dynamic viscosity of a fluid divided by the fluid density.Mass Flow RateDefines the volumetric rate with which fluids flow, maintaining the equation of continuity.PressureDefined as the force exerted per unit area.ReynBritish unit of dynamic viscosity.ViscosityA measure of the internal frictio…

Aerofoil
An aerofoil is shaped so that air flows faster over the top than under the bottom. There is, therefore, a greater pressure below the aerofoil than above it. This difference in pressure produces the lift.The lift generated by a wing is based on the principle that the pressure in a fluid decreases as its velocity increases (Bernoulli's Principle):

Aerolite
A stony meteorite, composed primarily of silicates. About 93 percent of all known falls are aerolites. They include the carbonaceous chondrites, other chondrites, and achondrites.

Aeronautics
The term 'aeronautics' originated in France, and was derived from the Greek words for 'air' and 'to sail'.The study of flight and the science of building and operating an aircraft. See also: Aerodynamics.

Aerosol
A colloid in which solid particles or liquid droplets are suspended in a gas. Smoke is an example of a solid aerosol; fog is an example of a liquid aerosol.Aerosol paint products have not contained chlorofluorocarbons (CFC's) since 1978.

Aerostat
Denotes a lighter than air craft.

Aether
In Aristotelian physics, the fifth element, of which the stars and planets are made.In Classical physics, an invisible medium that was thought to suffuse all space.

Affinity
Chemical attraction. A thermodynamic measurement of the strength of binding between molecules, say between an antibody and antigen. Each antibody/antigen pair has an association constant, Ka, expressed in l mol^-1.

Aft
The back of a vessel.

After Image
An image seen after the eye's retina has been exposed for a time to an intense or stationary light source. It may be negative or positive, or appear in complementary colours.

Afterburner
Thrust augmentation feature of a gas turbine engine. See also: Gas Turbine.