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

Diplexer
A circuit or system that allows the ability to transmit and receive two distinct signals simultaneously.

Dipole
Acoustic DipoleThis can be considered as two equal strength monopoles that are out of phase and a small distance apart. There is no net introduction of fluid by a dipole. As one source exhales, the other source inhales and the fluid surrounding the dipole simply sloshes back and forth between the sources. It is the net force on the fluid which caus…

Dipstick
The metal rod that passes into the oil sump it is used to determine the quantity of oil in the engine. See also: Internal Combustion Engine.

Dirac Delta Function
Represented as:its value is zero everywhere except at x=x₀ where its value is infinite. Integrating over any range of x that includes x₀ is a unit area. It is, therefore, the limiting case of a pulse with unit area that is infinitely short and at the same time infinitely high.

Dirac's Constant
Equal to Planck's constant divided by 2 pi.Also, known as h bar.h bar = 1.054571596×10^-34 J sttle='Diracs Constant';consttxt='h bar';constval=1.054571596e-34;constunt='J s';alternativelyh bar = 6.58211889×10^-16 eV s

Direct Coupled Amplifier
An amplifier in which the output of one stage is coupled to the input of the next without the use of a capacitor. This type of amplifier will amplify Direct Currents and low frequency waveforms. See also: Amplification.

Direct Current Amplifier
A direct coupled amplifier that can provide gain for zero-frequency signals. See also: Amplification, Amplifier.

Direct Current
A current that does not change in direction. See also: Alternating Current.

Direct Quenching
Quenching carburized parts directly from the carburizing operation. See also: Carburizing, Interrupted Quenching, Quenching.

Direct Sound
Sound reaching the listening location without reflections, i.e., sound that travels directly to the listener.

Directivity Factor
The ratio of the sound pressure squared, radiated directly ahead of a sound source, to the sound pressure squared radiated in all directions.

Disc
A circle together with its interior. See also: Circle.

Discontinuity
A break in sequence or continuity of anything.

Discrete
A type of random variable which may take on only a limited set of values, such as 1,2,3,...,10. The list may be finite, or there may be an infinite number of values. A discrete random variable is to be contrasted with a continuous random variable.

Discrete Fourier Transform
The digital (sampled) version of the fourier transform. Any block size can be used. This technique involves long computations, and normally the Fast Fourier Transform is used instead due to its quicker computation.The discrete fourier transform of f isfor n=0,1,.....,N-1The inverse discrete fourier transform isfor k=0,1,.....,N-1The DFT transform i…

Disinfectant
A substance that kills or stops the growth of microbes.

Dislocation
A linear crystalline defect around which there is an atomic misalignment. See also: Crystal, Dislocation Line.

Dislocation Line
The line that extends along the end of the extra half-plane of atoms for an edge dislocation, and along the center of the spiral of a screw dislocation. See also: Dislocation, Edge Dislocation, Mixed Dislocation.

Dispersion
The spreading of light into a spectrum of colour.The variation in the speed of a periodic wave due to its wavelength or frequency.

Displacement
Displacement is a vector quantity that specifies the change of position of a body or particle and is usually measured from the mean position or position of rest. In general, it can be represented as a rotating vector or a translation vector, or both. Displacements can either be absolute (i.e. relative to fixed axes) or relative (e.g. a shaft relati…

Dissociation
Breaking down of a compound into its components. See also: Compound.

Dissolve
To mix a solid and a liquid to form a solution. When a substance dissolves it spreads evenly throughout the liquid. See also: Liquid, Solid.

Distance Double Law
In pure spherical divergence of sound from a point source in free space, the sound pressure level decreases 6 dB for each doubling of the distance. This condition is rarely encountered in practice, but it is a handy rule to remember in estimating sound changes with distance.

Distillate
The vapour collected and condensed from a distillation. See also: Distillation.

Distillation
A process used to separate a liquid from a solution, e.g. to get pure water from salt water. All or some portion of a substance is vapourized and then condensed and collected. See also: Distillate, Woulfe Bottle.

Distortion
Any change in the waveform or harmonic content of an original signal as it passes through a device. The result of nonlinearity within the device.

Distributed Capacitance
Capacitance that is not concentrated within a lumped capacitor, but spread over a circuit or group of components. See also: Capacitance, Distributed Inductance.

Distributed Inductance
Inductance that is not concentrated within a lumped inductor, but spread over a circuit or group of components. See also: Distributed Capacitance, Inductance.

Distributed Load
An external force which acts over a region of length, surface, or area: essentially any external force which is not a concentrated force. See also: Concentrated Force.

Distribution
A probability function which describes the relative frequency of occurrence of data values when sampled from a population. Distributions are either continuous, typically used for variables which can be measured, or discrete, typically used for data that are the result of counts.

Dither
The noise (analog or digital) added to a signal prior to quantization (or word length reduction) which reduces the distortion and noise modulation resulting from the quantization process. Although there is a slight increase in the noise level, spectrally shaped dither can minimize the apparent increase. The noise is less objectionable than the dist…

Diurnal Aberration
The component of stellar aberration resulting from the observer's diurnal motion about the center of the Earth. See also: Aberration, Annual Aberration.

Divalent
Binds to two other things: atoms, molecules, ions, or electrons.

Divergence
The divergence operation is performed on a vector and produces a scalar. The del operator followed bythe dot product operator is read as 'the divergence of' and is an operation performed on a vector. Inrectangular coordinates, ?· means the sum of the partial derivatives of the magnitudes in the x, y, and z directions with respect to the x, y, and z…

Dividend
A number or quantity that is to be divided by another number or quantity. See also: Denominator, Division, Divisor.

Dividing Head
A machine tool holding fixture which positions the work for accurately space holes, slots, flutes and gear teeth and for making geometric shapes See also: Mill.

Division
Symbol : or /The binary operation of finding the quotient of two quantities. Division is the inverse operation to multiplication. In arithmetic, the division of two numbers is not commutative, i.e. , nor associative, i.e. . The identity element for division is 1 only when it is on the right hand side, i.e. .

Divisor
A number or quantity that is to be divided into another number or quantity. See also: Dividend, Division, Fraction, Numerator.

Dodecagon
A polygon with 12 sides. Each internal angle is 150° and the sum of the internal angles is 1800°. See also: Dodecahedron, Polygon.

Dodecahedron
A solid figure with 12 faces A regular dodecahedron is a regular polyhedron with 12 faces. Each face is a regular pentagon. See also: Dodecagon, Polyhedral.

Dolby Prologic
This is a four-channel sound reproduction system consisting of left, center, right and rear channel, (the single rear channel is usually played through two speakers).

Donjon
The principal tower of a castle.

Donor
An atom which is likely to give off one or more electrons when placed in a crystal. In semiconductors a donor material is intentionally added to a pure semiconductor material in order to increase the population of free electrons in that semiconductor, resulting in a net negative charge. A semiconductor that has had donor material added to it is cal…

Door
Opening to a room, compartment or system. The door has a number of functions:Isolation of air spaceNoise controlFire control

Doping
The intentional alloying of semiconductor materials with controlled concentrations of donor or acceptor impurities. Dopants in semiconductors are either charge carrier donors or acceptors, and make the semiconductor either n-type (surplus electrons), or p-type (shortage of electrons). See also: Donor.

Doppler Effect
A change in the frequency of a periodic wave due to the motion of the observer, the source, or both.It occurs both in sound as well as electromagnetic phenomena, although it takes on different forms in each.Source toward stationary observerSource away from stationary observerObserver toward stationary sourceObserver away from stationary sourcewhere…

Doppler Sonar
An acoustic instrument that measures the change in the acoustic frequency of the scattered sound or echo from that of the transmitted pulse. The magnitude and direction of the shift in frequency is related to the relative motion of the sensor and the scatterer. See also: Doppler Effect, Geometric Scattering.

Dormer Window
A window housed in a gable or similar structure affixed to the sloping part of a roof, providing natural light and ventilation to the rooms beneath the roof. See also: Window.

Dosimeter
A device worn by a worker for determining the worker's accumulated noise exposure with regard to level and time according to a pre-determined integration formula. See also: Noise.

Dot
A description of a point in which the point has a definite size. See also: Point.

Dot Product
The dot product of two vectors is obtained by adding the products of the respective components of the vectors.The dot product of two vectors:is represented and calculated as:Commutative lawDistributive lawalsoThe dot product of a vector with itself is the square of the magnitude:Note:if A=0 and only if A=0Cauchy-Schwarz InequalityThe dot product of…

Double Bond
When an atom is bonded to another atom by two sets of electron pairs.

Double Glazing
Two layers of glazing used to improve thermal and acoustic radiation. See also: Acoustic Barriers.

Double Hung Window
Two sashes which, when both are closed, are positioned one immediately above the other. Both sashes can be slid up and down in tracks, but are kept from dropping to the sill by counterweights attached by cords or chains and concealed in the window casing.By the nature of their design, double-hung windows can be opened to a maximum of 50% of the are…

Double Word
Unit of information equal to 2 short words, 4 bytes or 32 bits. See also: Bit, Byte, Word.

Dowel
A pin fitted or keyed in two adjacent parts to accurately align the parts when assembling them.

Down Quark
The second flavour of quark (in order of increasing mass), with electric charge -1/3. See also: Bottom, Charm, Quark.

Downforce
The opposite of aerodynamic lift, sometimes referred to as negative lift. The force caused by the air over the wings to push the car into the ground, increasing grip and cornering speeds.

Downsampling
Downsampling is used to reduce the sampling rate of data. Shannon's Sampling Theorem must be obeyed, i.e., before you sample, you must low-pass filter the data with a cut-off frequency less than half of the new sampling rate.Note: If you omit the filtering, the data will be aliased and cannot be corrected.Low-pass filter the data with a cut-off fre…

Dozen
Twelve pieces of something. See also: Baker's Dozen, Gross.

Drag
AerodynamicsResistance of a vehicle body to motion through the air. A smooth surface has less drag than a rough one.It may be broken down into three main components: skin friction: this is drag due to the surface texture and area.profile drag: this is drag from the three-dimensional shape of the aircraft/vehicle.service drag: this is drag from air …

Drag Coefficient
A dimensionless value that allows the comparison of drag incurred by different sized and different shaped bodies.The force on an object due to aerodynamic drag can be calculated using:whereF = aerodynamic drag force [N]C_d = drag coefficientA = frontal area [m²]? = density of fluid [kgm^-3]v = velocity of object relat…

Drawing
A deformation technique used to fabricate metal wire and tubing. Deformation is accomplished by pulling the material through a die by means of a tensile force applied on the exit side. See also: Fullering.

Dressing
Photograph courtesy of Draper.co.ukThe act of removing the glaze and dulled abrasives from the face of a grinding wheel to make it clean and sharp. See also: Abrasive Wheel.

Drift
Motion of carriers caused by an electric field.orSlow variation of a performance characteristic such as gain, frequency, or power output; for instance, due to temperature or aging.

Drill Gauge
A flat steel plate drilled with holes of various sizes, each marked with the correct size or number into which small twist drills may be fitted to determine the size of their diameters. See also: Drill Bits.

Drill Jig
A jig that holds parts of a structure and by means of bushings, guides the drill so that the holes are properly located. See also: Drill Bits.

Drill Press
Photograph courtesy of Draper.co.ukAn upright power-driven machine for drilling holes in metal, wood, or other material. It has a counterbalanced spindle which makes it possible for the operator to control accurately the rate at which the drill is fed into the work.

Drive Fit
One of several classes of fits in which parts are assembled by pressing or forcing one part into another. See also: Fit, Free Fit.

Drum Tower
A large, circular tower, usually low and squat.

Drumming Noise
Often used to describe noise in the frequency range 30 to 60Hz. See also: Acoustics, Boomy, Hearing.

Dry Bulb Thermometer
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature.

Dry Sump
A lubrication system in which the oil is pumped into the engine's sump under pressure and then pumped out again. This reduces the losses from the oil being churned around by the crank and big ends at the same time as allowing it to be cooled efficiently.

Drystone Wall
Unmortared masonry wall.

Dual Voting
Concept where two independent inputs are required before action, usually machine shutdown, is taken. Most often used with axial position measurements, where failure of a single transducer might lead to an unnecessary shutdown.

Duct
A pipe or closed conduit made of sheet metal, fibreglass board, or other suitable material used for conducting air to and from an air handling unit. See also: Duct Acoustics.

Duct Acoustics
The acoustics associated with ducts or interconnecting pipes.pressure wave propagating in the duct in the x directionassuming rigid walls where there are no losses.acoustic volume flow isThe acoustic impedance of the pipe at any point iswhere S is the cross-sectional area of the duct.For a typical air intake duct of internal radius 0.03m the impeda…

Ductile Fracture
A mode of fracture that is attended by extensive gross plastic deformation. See also: Brittle Fracture.

Ductile-to-Brittle Transition
The transition from ductile to brittle behaviour with a decrease in temperature exhibited by Body Centred Cubic (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests. See also: Izod Test.

Ductility
A measure of a material's ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation or percent area reduction from a tensile test.

Dulcification
Any process in which a caustic substance is rendered less corrosive.

Duodecimal Number System
The system of numeration with base 12.

Duplex Theory of Localization
This is the combination of the two methods that describe human binaural sound localization first proposed by Lord Rayleigh around 1900.Rayleigh observed that sound arriving at a listener from sources located away from the median (mid-sagittal) plane would result in differences in the signals observed at a listener's ears. He noted that the sound re…

Duty Cycle
The ratio of operating to non-operating time for a device.

Dwell
In an engine, the time allowed for current to build in the primary circuit of the ignition coil for each spark generation.

Dye Penetrant
Penetrant with dye added to make it more readily visible under normal lighting conditions.

Dynamic Equilibrium
Equilibrium which includes inertial forces. See also: Static Equilibrium.

Dynamic Headroom
The ability of an audio device to respond to musical peaks. For example, an amplifier may only be capable of a sustained 100 watts, but may be able to achieve peaks of 200 watts for the fraction of a second required for an intense, quick sound. In this example the dynamic headroom would equal 3 dB.

Dynamic Mass
Ratio of applied force to resulting acceleration during simple harmonic motion. See also: Mass.

Dynamic Modulus
Ratio of stress to strain under vibratory conditions. See also: Young's Modulus.

Dynamic Range
The power range over which a component or system functions properly.Number of bitsDynamic range [dB]84812721696241443219264385The difference in signal level between the loudest and quietest parts of a programme, expressed in decibels. See also: Decibel.

Dynamic Sealing
It is important when assessing seals to consider the integrity of the seal under dynamic conditions. Vehicle door seals must function when the aerodynamic load on the door pulls the door away from the seals. The aerodynamic load may also act directly on the seals resulting in a possible air leak or even whistle under extreme conditions.

Dynamic Stiffness
Ratio of force to unit displacement during simple harmonic motion. Dynamic stiffness of an elastomer varies with the frequency and amplitude of the input signal. See also: Accelerance, Apparent Mass, Mobility, Stiffness.

Dynamic Unbalance
The condition which exists when unbalanced masses on a shaft or its attachments create a couple on the shaft when it rotates. See also: Critical Speeds.

Dynamic Vibration Absorber
A dynamic vibration absorber is an auxiliary mass-spring system which tends to neutralise vibration of a structure to which it is attached. The basic principle of operation is vibration out-of-phase with the vibration of such structure, thereby applying counteracting force. An absorber is only effective at its natural frequency which must be tuned …

Dynamometer
A device for determining the power of an engine. See also: Appold Dynamometer, Brake Horse Power.

Dyne
The unit of force in the cgs (centimeter grams second) system of units. A force of 1 dyne acting on a mass of 1g imparts an acceleration of 1cms^-2.ttle='Force';xiunt='dyne';yiunt='N';mconv=1e5;cconv=0.0; See also: CGS Units, Force.

Dysprosium
Dysprosium is a silvery metal of the lanthanide group. It is relatively stable in air, reacts violently with water and dissolves in acids. Applications for this element are limited, but it is used as an alloying element to produce an alloy used for magnets, and as a poison in nuclear reactors where it stops the reaction from getting out of hand. Sy…

Ear
The human ear, as a sound receiver, has to operate under a wide range of conditions. It is subject to sounds of varied complexities, frequency ranges and intensity levels. Almost all noise control measures are established to minimise the possibility of physical damage to the hearing mechanisms of the ear or to maintain acoustical environments which…

Earmuff
Photograph courtesy of Draper.co.ukHearing protector worn over the pinna of an ear. See also: Ear, Earplug, Hearing, Hearing Protector.