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Glossary of Terms

This glossary defines relevant engineering terms and user interface terms for Data Physics products



A/D Converter A device that changes an analogue signal into a digital signal
Absolute Accuracy The accuracy uncertainty of an instrument reading compared to that of a primary national traceable standard
Absolute displacement Displacement of an object relative to a fixed point in space. A signal from an accelerometer integrated to provide a displacement signal, provides an indication of absolute displacement. By contrast, an eddy current probe fastened to a housing, measuring displacement on a shaft is measuring relative displacement.
Accelerance (Inertance) Acceleration divided by force.
Accelerated Life Testing An activity during development of a new product. Prototypes are subjected to stress levels (including vibration, usually random) that are much higher than those anticipated in the field. The purpose is to identify failure-prone, marginally-strong elements by causing them to fail. Those elements are strengthened and tests are continued at higher levels. Sometimes called Test, Analyze & Fix (TAAF) testing.
Accelerated Stress Testing A post-production activity on a sampling (100% at first) of units. The intent is to precipitate hidden or latent failures caused by poor workmanship and to prevent flawed units from reaching the next higher level of assembly or the customer. Intensity is typically half that achieved in accelerated life testing.
Acceleration The rate of change of velocity with time
Accelerometer A transducer for measuring vibration in acceleration units.
Acoustic Sound, the sense of hearing, or the science of sound. SignalCalc Analyzers provide a number of optional packages that are tailored to acoustic applications. SignalCalc systems provide a number of options for acoustic analysis.
Accuracy The degree of closeness to a true value. Usually expressed as a percentage or ratio relative to the reference value.
Active Redundancy Redundancy in which all redundant items operate simultaneously.
ADC Indicators In SignalCalc Dynamic Signal Analyzers: A control group that sets and indicates the Full Scale range of each Input Channel in Volts (zero-to-peak).
Aggravated Test A test in which one or more conditions are set at a more stressful level that the test item will encounter in the field, in order to reduce test time or assure a margin of safety.
Algorithm A specific procedure for solving mathematical problems. An FFT is an algorithm.
Aliasing A phenomenon caused by sampling analogue data at too-low a frequency. It results in a digital reconstruction of the original signal at a false, lower frequency. This causes higher frequency signals to appear in a spectrum at lower frequencies (Aliasing terms). When analogue signals are digitized, the analogue input must be sampled at a rate at least twice the bandwidth of the signal to avoid loss of data (Nyquist Theorem).
Alignment A procedure in which the axes of components of a machine are adjusted so as to be perfectly true with each other
Ambient environment The surrounding conditions (e.g. temperature and humidity)
Amplification Factor (Q) (Q) (Sharpness of resonance) A measure of the sharpness of a resonance or frequency. The mechanical gain at resonance.
Amplitude The y-axis of the vibration time waveform. The maximum value of a quantity. The measurement of energy or movement in a vibrating object.
Analog Relating to a mechanism in which data is represented by continuously variable physical quantities. Quantities in two separate physical systems having consistently similar relationships to each other are called analogous. One is then called the analogue of the other. The electrical output of a transducer is an analogue of the vibration input of the transducer as long as the transducer is not operated in the non-linear (overloaded) range. This is in contrast to a digital representation of the vibration signal, which is a sampled and quantized signal consisting of a series of numbers, usually in binary notation.
Analog to Digital Conversion The process of sampling an analogue signal produces a series of numbers, which is the digital representation of the same signal. The sampling frequency must be at least twice as high as the highest frequency present in the signal to prevent aliasing errors.
Analog Spectrum Analyzer A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. An analog spectrum analyzer uses either a variable band pass filter whose mid-frequency is automatically tuned (shifted, swept) through the range of frequencies of which the spectrum is to be measured or a super heterodyne receiver where the local oscillator is swept through a range of frequencies. In general analog spectrum analyzers are used for very high frequency analysis.
Angular rate sensor A sensor that measures rotational velocity (degrees or radians per second) around its sensitive axis
Anti-aliasing filter A low pass filter designed to stop frequencies higher than the required Fspan
Append In SignalCalc Dynamic Signal Analyzers: To add additional measurements to an exiting Run or Export folder. During an Append, signal processing controls are locked to match the initial data.
ASCII An acronym that stands for American Standard Code for Information Interchange. ASCII defines a standard for representing characters on computers.
Attenuate Reduce.
Auto Correlation Auto correlation is a time-domain function that is a measure of how much a signal shape, or waveform, resembles a delayed version of itself. It is closely related to the Cepstrum, q.v. The numerical value of auto correlation can vary between zero and one. A periodic signal, such as a sine wave has an auto correlation that is equal to one at zero time delay, zero at a time delay of one-half the period of the wave, and one at a time delay of one period; in other words, it is a sinusoidal waveform itself. Random noise has an auto correlation of one at zero delay, but is essentially zero at all other delays. Auto correlation is sometimes used to extract periodic signals from noise. Certain dual-channel FFT Analyzers are able to measure auto correlation.
Auto Power Spectrum A Test type providing single channel time/frequency functions including frequency domain power averages.
Auto Spectral Density (ASD) The measure of acceleration per Hz of analysis bandwidth. See Power Spectral Density (PSD). The area under ASD curve is defined as the grms of acceleration.
Auto spectrum (power spectrum) A spectral display of the power (voltage squared) at each frequency. Phase is ignored.
Auto ranging The capability of an instrument to switch among ranges automatically.
Average Responding A measurement proportional to the average of the absolute values of all input waveforms within a specified frequency range.
Averaging Summing and dividing several like measurements to improve accuracy or to reduce asynchronous components.
A-weighting In acoustic analysis, a filter to approximately compensate for the non-flat frequency response of human hearing, in order to get numbers approximating human response. See Equal Loudness Curves.
Axial The axis along the centerline of a shaft

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Balancing For a mechanical system: Adjusting the distribution of mass in a rotating element, to reduce vibratory forces generated by rotation.
Band-pass filter A wave filter that has a single transmission band extending from a lower cut-off frequency greater than zero to a finite upper cut-off frequency.
Bandwidth The frequency range selected or available for measurements.
Baseline spectrum A vibration spectrum taken when a machine is in good working condition (new or just overhauled), used as reference for future monitoring or analysis.
Bearing (Rolling Element) A Rolling element bearing has four parts: an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation of the rolling elements.
Bearing (Sleeve) A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. Contact between the shaft and sleeve is prevented by a lubrication film.
Beat Frequency Where two cyclic components are close together in frequency they combine in such a way that their sum will vary in amplitude at a rate equal to the difference in frequency between the two components. This phenomenon is known as beating, and its frequency is the beat frequency.
Bins (Lines) In an FFT spectrum, the individual frequencies at which the amplitudes are calculated
Bit Short for binary digit. A number expressed in binary notation utilizes the digits 1 and 0, and these are called bits. Any number can be expressed with combinations of them.
Blade-passing frequency The frequency on a bladed machine such as a turbine or fan at which blades pass a given point. (number of blades multiplied by shaft speed)
Block Size The (binary) number of time samples acquired in each frame or capture window
Bode plot Magnitude and phase of vibration in a machine plotted against speed.
Bow A shaft condition (rotating machinery) in which the shaft centre line is not straight.
Breakpoint In vibration control: One of the frequencies at which amplitude is defined in order to specify the shape of the reference spectrum.
Broadband Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.
Buffer A memory location in a computer or digital instrument that is set aside for temporarily storing digital information while it is waiting to be processed.
Bump Test: Repeated shock applied to a product using a bump test machine or Shaker system.
Burn-in To assist in preventing early life field failures, electronic products are continuously powering a product, often at constant elevated temperature for a specified period before shipment.
Burst Random Signal A burst of (true) Random signal gated to be less than Tspan in duration, useful as a stimulus for Modal Testing.

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Campbell diagram (Interference diagram) A form of display used in machine analysis to show response amplitude as a function of speed for rotational orders.
Cavitation A localized low-pressure vaporization condition (cavities or bubbles) within a liquid, as around a propeller or within a pipe or in a pump’s suction line. Bubble collapse creates unwanted sound and/or vibration. May be destructive.
Cepstrum The forward Fourier transform of a spectrum. (the spectrum of a spectrum), It is useful in many types of signal analysis. Periodicities, or repeated patterns, in a spectrum will be sensed as one or two specific components in the cepstrum. If a spectrum contains several sets of sidebands or harmonic series, they can be confusing because of overlap. But in the cepstrum, they will be separated in a way similar to the way the spectrum separates repetitive time patterns in the waveform. Gearboxes and rolling element bearing vibrations lend themselves especially well to cepstrum analysis.
Channel Parameters In SignalCalc Dynamic Signal Analyzers: A control group affecting Input channel settings and Signal Generator outputs.
Charge amplifier An amplifier used to condition piezo electric charge coupled transducers. The charge input signal is converted into an output voltage.
Chirp Signal A low crest factor transient used for (selected) broad-band testing. A rapid frequency sweep between two limits, accomplished in less than Tspan
Circle Fit A single-degree of freedom curve fitting routine that tries to fit a mode to a circle (Nyquist plot of a single-degree of freedom system). The modal coefficient is determined by the diameter of the circle and the phase by its location relative to the imaginary axis. For a real mode, it should be either completely above or completely below the imaginary axis.
Clipboard A temporary storage area used by Windows programs for holding text or graphics.
Closed Loop Control Responses are measured and fed back to the control system so as to refine or modify drive signals in order to bring responses closer to the reference or desired motions.
Coefficient of Thermal Expansion The constant value or factor of expansion of a material for a given increase in temperature, divided by the length of the material. This is different for each material.
Coherence A measure of the similarity of vibration at two locations.
Co-Incident Another name for the real part of the frequency response function.
Complex Modes The points on a structure which have varying phase relationships between them at a natural frequency. This is unlike a real mode where the phase between points is either 0° or 180°.
Compliance The reciprocal of stiffness, (displacement divided by force).
Condition monitoring (CM) The current machine condition (signature) compared with an earlier condition used as an indicator to detecting potential failure before it occurs. Also called machinery health monitoring.
Confidence level Defines the degree of spread of a series of measurements around the true value. For example, one may estimate with 90% confidence (confidence level) that an averaged power spectral density estimate is within 2 dB (the confidence interval) of the true power spectral density function.
Constant percentage filter A band-pass filter whose bandwidth is maintained at a constant proportion to the centre frequency.
Constant-bandwidth filter A band-pass filter whose bandwidth is independent of centre frequency. In an FFT analysis the effective filters are constant bandwidth.
Control Panel In SignalCalc Dynamic Signal Analyzers: An area on the user interface screen consisting of test information and command buttons for test operation.
Correlation A Test type providing Auto and Cross Correlation functions to measure cause/effect in the time domain
Critical Damping The smallest amount of damping required to return a system to its equilibrium condition without oscillating.
Critical frequency A resonant frequency at which damage or degradation in performance is likely.
Critical speeds Any rotating speed which results in high vibration amplitudes. Often these are speeds which correspond to system natural frequencies. See Campbell Diagram.
Cross Correlation Cross correlation is a measure of the similarity in two time domain signals. If the signals are identical, the cross correlation will be one, and if they are completely dissimilar, the cross correlation will be zero.
Cross-axis sensitivity Sensitivity of a transducer in a direction perpendicular to the normal measurement axis.
Crossover frequency In a swept sine vibration test, the frequency at which the units of displacement, velocity or acceleration yields the same test level.
Cross-talk Pickup in one channel coming from another channel.
Cycle The complete sequence of values of a periodic quantity that occur during a period.

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D/A converter A device that converts a digital signal into an analogue signal.
Damped Natural Frequency The damped natural frequency is the frequency at which a damped system will oscillate in a free vibration situation.
Damping The dissipation of energy with time or distance.
Damping Factor (Damping Ratio) The ratio of actual damping in a system to its critical damping
Decade The interval between two frequencies which differ by exactly 10:1.
DeciBel (dB) The decibel is a unit which denotes the magnitude of a quantity with respect to an arbitrarily established reference value of the quantity, in terms of the logarithm (to the base 10) of the ratio of quantities. For example, in electrical transmission circuits, a value of power may be expressed in terms of a power level in decibels. Ratios of quantities can expressed in deciBels ( dB). For acoustic or power quantities dB = 10 log10 P1/P2. P2 (the reference level), equals 0 dB. For voltage units such as acceleration, dB = 20 log10 V1/V2 V2, (the reference level), equals 0 dB.
Default In SignalCalc Dynamic Signal Analyzers: A selection automatically used by the system in the absence of a choice made by the user.
Default Folder In SignalCalc Dynamic Signal Analyzers: A directory of “starting point” signal processing Setup and graphical layout files initially copied into a New Test folder.
Degrees of Freedom (Mechanical) The number of degrees of freedom of a mechanical system is equal to the minimum number of independent coordinates required to define completely the positions of all parts of the system t any instant of time. In general, it is equal to the number of independent displacements that are possible.
Degrees of freedom (Statistical) A statistical term expressing the confidence in an estimated measurement. In a random signal 1 Average = 2 DOF)
Delimited A character or combination of characters used to separate one item or set of data from another. For example, in comma delimited records, a comma is used to separate each field of data.
Design limit The operational limit of a product, beyond which it not required to function properly.
Detector An electronic circuit that determines the amplitude level of a signal in accordance with certain rules. The simplest type of detector consists of a resistor and a capacitor, and it measures the average value of a fluctuating DC signal. A more complex but much more useful type of detector is an RMS detector. RMS detectors are used because they are proportional to the power or energy present in the signal or a vibration.
Deterministic A type of signal whose spectrum consists of a collection of discrete components, as opposed to a random signal, whose spectrum is spread out or “smeared” in frequency. Some deterministic signals are periodic, and their spectra consist of harmonic series. Vibration signatures of machines are in general deterministic, containing one or more harmonic series, but they always have non- deterministic components, such as background noise.
Deterministic vibration A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic example. Complex vibration is less simple (two or more sinusoids).
DF (Delta Frequency) The nominal frequency resolution (Hz) of a spectrum; the difference between adjacent frequency points; numerically equal to 1/Tspan.
DFT In mathematics, the discrete Fourier transform (DFT), sometimes called the finite Fourier transform, is a transform widely employed in digital signal processing. It is used to identify the frequencies contained in a sampled signal. The DFT can be computed efficiently in practice using a fast Fourier transform (FFT) algorithm. SignalCalc analyzers and SignalStar controllers use both FFT and DFT algorithms in various applications.
Dialog box A window that appears during the operation of the program that requires you to make a choice or enter information.
Differential Inputs Two inputs, where the measured signal is the difference between them. Any voltage common to both is rejected. Differential inputs can reduce noise picked up by the signal leads.
Differentiation Representation in terms of time rate of change. Differentiating velocity yields acceleration.
Digital Digital instrumentation consists of devices that convert analogue signals into a series of numbers through a sampling process and an analogue to digital converter. They then perform operations on the numbers to achieve such effects as equalization, data storage, data compression, frequency analysis, etc. This process in general is called digital signal processing. It is characterized by several advantages and disadvantages. One advantage is that the converted signals can be manipulated, transformed and copied without introducing any added noise or distortion. The disadvantage is that the signal representation may not be truly representative of the original signal.
Digital Spectrum Analyzer A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. A digital spectrum analyzer uses the Fast Fourier transform (FFT), a mathematical process that transforms a waveform into the components of its frequency spectrum. SignalCalc systems are an example of a modern high speed digital spectrum analyzer.
Discrete With reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous. An example of a discrete spectrum is a harmonic series. An FFT spectrum, which consists of information only at specific frequencies (the FFT lines), is actually discrete regardless of the input signal. For instance, the true spectrum of a transient is continuous, and the FFT of a transient appears continuous on the screen, but still only contains information at the frequencies of the FFT lines. The input signal to an FFT Analyzer is continuous, but the sampling process necessary to implement the FFT algorithm converts it into a discrete form, with information only at the specific sampled times.
Discrete Fourier Transform The mathematical calculation that converts, or “transforms” a sampled and digitized waveform into a sampled spectrum. The fast Fourier transform, or FFT, is an algorithm that allows a computer to calculate the discrete Fourier transform very quickly. See also Fast Fourier Transform.
Displacement 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.
Distortion Electrical (Usually expressed as Total Harmonic Distortion) the amount by which the signal deviates from its pure form.
Distortion Mechanical Structural response at harmonics or sub-harmonics of a forcing frequency.
Domain A domain is a set of coordinates in which a mathematical function resides. A waveform, for instance, has dimensions of amplitude and time, and it is said to exist in the time domain, while a spectrum has dimensions of amplitude and frequency, and is said to exist in the frequency domain.
Double-click The act of pressing the left mouse button twice in rapid succession
Drift Slow variation of a performance characteristic such as gain, frequency, or power output; for instance, due to temperature or aging. Usually, drift only is significant when measuring low-level signals (a few millivolts) over long periods of time or in difficult environmental conditions.
Driving Point Measurement A frequency response measurement where the excitation point and direction are the same as the response point and direction.
DSP Digital Signal Processor
DT (Delta Time) The time resolution (second); the time difference between adjacent signal samples.
Durability A measure of useful life (a special case of reliability).
Duration of a shock pulse is how long it lasts. For “classical” pulses, time is usually measured between instants when the amplitude is greater that 10% of the peak value.
DUT Device under test.
Dynamic Compliance See compliance.
Dynamic motion Movement, as compared with non-moving or static positin. Dynamic motion is sensed with displacement or velocity pickups or with accelerometers.
Dynamic Range The ratio (usually expressed in dB) of the maximum level to the minimum detectable value.
Dynamic Signal Analyzer DSA A system using digital signal processing and the Fast Fourier Transform (FFT) o display time, order, frequency and phase components of signals
Dynamic Stiffness The frequency response function of force/displacement.

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Earthquake (earth quake) An earthquake is a tremor of the earth’s surface usually triggered by the release of underground stress along fault lines. This release causes movement in masses of rock and resulting shock waves. A valuable tool for the analysis of earthquakes is Shock Response Analysis (SRS) which is provided as an option to SignalCalc systems and as a control option for the SignalStar controllers.
Eccentricity, mechanical Variation of shaft surface radius when referenced to the shaft’s true geometric centre line. Out-of-roundness.
Eddy current probe A non-contact electrical device that measures the displacement of one surface relative to the tip of the probe. Construction consists of an electrical coil of various lengths and diameters. This coil located in the tip of the probe is energized producing an electrical field around the tip of the probe. When a conductive surface is placed in the field and the distance from the probe is noted, variations in this gap can be determined by the variations in the voltage flow to the probe tip.
Effective Mass The frequency response function of force/acceleration.
Eigenvalue The roots of the characteristic equation.
Eigenvalue Problem The mathematical formulation and solution of the characteristic equation is called the Eigenvalue problem.
Eigenvector The mode shape vectors.
Engineering Units (EU) The units in which a measurement is made; for instance acceleration may be expressed in g, velocity may be expressed in milimeters per second.
Engineering Units Table In SignalCalc Dynamic Signal Analyzers: A control table defining all available choices of input and output Engineering Units; contains unit name and definition in terms of nine SI dimensions.
Environment The aggregate of all external and internal conditions (such as temperature, humidiy, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, performance, reliability or survival of an item.
Environmental stress screening (ESS) A process in which products from the production line are subjected to thermal and / or vibration stresses to reduce the likelihood of early life field failures by forcing them to occur before final test in the factory.
Environmental testing Simulating the vibration, shock, noise, thermal, humidity etc. environment in the lab.
Equal Loudness Curves Graphs of pure tone (constant or steady) sound pressure levels (labeled as to loudness level in phons) vs. frequency, with each graph representing equal loudness.
Error The difference between the indicated and the true values of a variable being measured.
Excitation An external force (or other input) applied to a system that causes the system to respond in some way.
Exponential (Response) Window A windowing function for minimizing leakage in lightly damped structures. Typically used in Modal measurements made with an impact hammer. In a lightly damped structure, oscillations may not die out within the sampled time data block, Tspan, which results in leakage error. An exponential window adds damping to the time signal to force it to die out within the time T, thus minimizing leakage.
Exponential Averaging A continuous averaging method wherein the most recent constituent is most influential and old information decays away exponentially with time.
Export To create a file containing information in a format that can be used in a different program.
Export Folder A folder, analogous to a Run folder, containing Signals in application-specific formats including ASCII, Standard Data Format (SDF), Universal File Format (UFF), ME Scope and SMS Star modal.
Extremal control A control strategy which selects the largest response at each frequency from the measured reference spectrum at control points to generate a combined spectrum to be used in the control loop.

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Failure The event, or inoperable state, in which any item or part of an item does not, or would not, perform as previously specified.
Failure analysis Subsequent to failure, the logical systematic examination of an item, its construction, application, and documentation to identify the failure made and determine the failure mechanism and its basic course.
Failure effect The consequence(s) a failure mode has on the operation, function, or status of an item. Failure efforts are classified as local effect, next higher level, and end effect.
Failure mechanism The mechanical, chemical, physical or other process that results in failure.
Failure mode analysis A procedure aimed at determining why a failure occurred.
Failure mode and effects analysis (FMEA) A procedure by which each potential failure mode is a system is Analyzed to determine the results to effects thereof on the system and to classify each potential failure mode according to its severity.
Failure rate The total number of failures within an item population, divided by the total number of life units expended by that population, during a particular measurement interval under stated condition.
Fatigue life The amount of time under defined operational conditions that a product is expected to survive.
Fault Immediate cause of failure (e.g. maladjustment, misalignment, defect, etc.)
Feedback signal A response measurement that is used in a closed-loop control process.
FFT or Fast Fourier Transform A fast Fourier transform (FFT) is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse. SignalCalc analyzers and SignalStar controllers use both FFT and DFT algorithms in various applications.
FFT Analyzer The FFT Analyzer is a device that uses the FFT algorithm to calculate a spectrum from a time domain signal, See also Fast Fourier Transform and Dynamic Signal Analyzer.
FFT Analysis In general, FFT analysis is used to refer to any and all types of Digital Signal Analysis (DSA) which included such measurements as: linear spectrum, power spectrum, energy spectrum, cross power spectrum, transfer function, frequency response function, coherence, inverse FFT, impulse response, auto- and cross-correlation, histogram, spectrogram and others. SignalCalc analyzers and SignalStar controllers use various combinations of these to provide user friendly solutions for many applications.
FFT Library A collection of FFT algorithms and other signal processing tools that can be manipulated by the user to perform custom signal processing. The SignalCalc analyzers and SignalStar controllers have optional packages that allow user control of the Data Physics hardware for custom applications.
FFT Power Spectrum In the frequency domain, this is the square of FFT´s magnitude. The power spectrum answers the question, “Which frequencies contain the signal’s power?” The answer is in the form of a distribution of power values as a function of frequency. SignalCalc systems are an example of a modern high speed digital spectrum analyzer which can provide a power spectrum in realtime.
File extension A three letter addition to a file name that usually identifies the file type and appears after the file name, separated by a period.
Filter An electronic device to pass certain frequencies (pass band) but block other frequencies (stop band). Classified as low-pass (high-stop), high-pass (low-stop), band-pass or band-stop.
Finite Element (FEA) A computer-aided design technique for predicting the dynamic behavior of a mechanical system.
First order vibration Rotating machine vibration caused by shaft unbalance. Frequency in hertz (Hz) is calculated by shaft RPM/60. Also called 1x vibration. Additional orders, 2x, 3x …. 36x, etc. are caused by other mechanisms
Fixture The intermediate structure that attaches a device under test (DUT) to a shaker or shock test machine.
Flat top Window In signal processing, a window function is used to shape the time record to provide a correction for the basic limitations of the FFT. The flat top window is used to make accurate measurements of the amplitude of periodic signals. SignalCalc systems provide for a number of window functions.
FMEA Failure Mode and Effects Analysis. A procedure by which each potential failure mode of a system is Analyzed to determine the effects on the system and classify each potential failure mode according to its severity.
Folder Another name for a directory or sub-directory in computers with Windows.
Force Window A rectangular Window of adjustable Width used on the impact force signal in a Modal Analysis. Since the duration of the actual impact is usually very short relative to the overall digitized time sample, the frequency response function of the force signal can have a low signal to noise ratio. The force window does not alter the actual force pulse but minimizes the noise in the rest of the data block giving a much improved signal to noise ratio.
Forced Response Analysis (Forced Response Simulation) Mathematically calculating the system response to an arbitrary forcing function using modal analysis data as the system model.
Forced vibration The vibratory motion of a system caused by some mechanical excitation. If the excitation is periodic and continuous, the response motion eventually becomes steady-state.
Forcing frequency In sinusoidal vibration testing or resonance searching, the frequency at which a shaker vibrates.
Forcing function A climatic or mechanical environmental input to an item of equipment that affects its design, service life or ability to function. (Also referred to as an environmental condition or an environmental stress.
Fourier Mathematician Baron Jean Baptiste Joseph Fourier 1768-1830. (See also FFT)
Fourier Analysis Fourier analysis is another term for spectrum analysis, although it generally refers to analysis using an FFT Analyzer.
Fourier Transform See FFT
Frame A time domain “capture window” containing Block Size samples of the input signals.
Free vibration Free vibration occurs without forcing, as after a reed is plucked.
Frequency The reciprocal of the period T in seconds (of a periodic function) (1/T). Usually given in hertz (Hz), meaning cycles per second (cps).
Frequency Analysis The application of a Dynamic Signal Analyzer (DSA) to help understand the meaning of an electrical signal derived via a transducer form a physical process. SignalCalc systems are an example of a modern high speed DSA that can be used for frequency analysis.
Frequency Analyzer A frequency analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. SignalCalc systems are an example of a modern high speed digital frequency analyzer.
Frequency Domain A plot of frequency vs. amplitude, called a spectrum, and the spectrum is in the frequency domain.
Frequency range The frequency range selected or available for measurements.
Frequency Response Curve A graph of amplitude/phase vs. frequency indicating the response of a device to different frequencies. Frequency Response Functions (FRF) measurements are provided as part of all SignalCalc systems.
Frequency Response (FRF) A characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT Analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems. The frequency response function is actually a three-dimensional quantity, consisting of amplitude vs. phase vs. frequency. The so-called Bode plot consists of two curves, one of amplitude vs. frequency and one of phase vs. frequency. Another way to look at the frequency response function is to resolve the phase portion into two orthogonal components, one in-phase part (called the real part), and one part 90 degrees out of phase (called the imaginary part).
Frequency Response Matrix For an N degree of freedom system, it is an N x N symmetrical matrix whose elements are the frequency response functions between the various points on the structure. Rows correspond to response points and columns to excitation points. For example, H23 is the frequency response with excitation at point 3 and response at point 2. The matrix is redundant, that is, by knowing any row or column, the other elements of the matrix can be computed.
Frequency spectrum A description of the resolution of any electrical signal into its frequency components, giving the amplitude (sometimes also phase) of each component.
Fspan The span of frequency covered in a spectrum; numerically equal to Lines multiplied by DF.
Fundamental frequency The lowest frequency component of a complex, cyclic signal.
Fundamental mode of vibration That mode having the lowest natural frequency

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g The acceleration produced by the force of gravity, which varies with the latitude and elevation of the point of observation. By international agreement, the value 980.665 cm/sec’ = 386.087 in/sec’ = 32.1739 ft/sec’ has been chosen at the standard acceleration due to gravity.
Gear-mesh frequency A potential vibration frequency on any machine employing gears. Multiply the number of teeth on a gear times its RPM, then divide by 60
Generalized Coordinates The minimum number of independent coordinates necessary to completely describe a systems position constitutes a set of generalized coordinates. For an N degree of freedom system, N generalized coordinates are required.
Graduation mark The marks that define the scale intervals on a measuring instrument are known as graduation marks.
Graph Attributes Dialog In SignalCalc Dynamic Signal Analyzers: A control dialog that determines the appearance of a graph window.
Graph Template A user-stored graphic format that is used when opening a New Graph; provides a desired graphic appearance with minimum control interaction.
Ground Loop A current loop created when a signal source and a signal measurement device are grounded at two separate points on a ground bus through which noise currents flow. These currents generate voltage drops between the two ground connections, which cause measurement errors.

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HALT Highly accelerated life test.
Hamming Window Named after its originator, the Hamming window is a Hanning window sitting on top of a small rectangular pedestal. Its function is similar, but has its first side lobes 42 dB down, whereas the Hanning window’s first side lobes are only 32 dB down. Thus the Hamming has better selectivity for large signals, but it suffers from the disadvantage that the rest of the side lobes are higher, and in fact fall off slowly at 20 dB per octave like those of the rectangular window. The Hamming window had some advantage in the days when FFT Analyzers only had 50 dB or so of dynamic range, but today it is essentially obsolete.
Hann Window (Hanning) The standard Window for general spectrum analysis of continuous signals and all random signals.
Hard failure A product under test ceases to work correctly. It does not resume correct operation, even when the stressing environment is eased. Differs from soft failure.
Hardware The physical parts of the computer or system that you can touch.
Harmonic A sinusoidal quantity having a frequency that is an integral multiple of a fundamental frequency.
Harmonic Distortion In the output signal of a device, distortion caused by the presence of frequencies not present in the input signal.
Harmonics also called a harmonic series, are components of a spectrum that are integral multiples of the fundamental frequency. A harmonic series in a spectrum is the result of a periodic signal in the waveform. Harmonic series are very common in spectra of machinery vibration.
HASS Highly accelerated stress screening
Help button Pops up helpful information about the current dialog box or area.
Hertz The unit of frequency. Also expressed as cps (cycles per second.)
High-Pass Filter A filter that passes signal frequencies above a specific, or cut off, frequency. They are used in instrumentation to eliminate low-frequency noise, and to separate alternating components from direct (DC) components in a signal.
Hinv The inverse of the system transfer function.
Histogram A Test type providing single channel amplitude domain functions including Probability Density (PDH) and Cumulative Density (CDF).
Hotspots Areas on the graph where you can click with the mouse to bring up the appropriate Graph Attributes dialog box.
Hysteresis Also called dead band. That portion of a measuring system’s response where a change in input does not produce a change in output.
Hysteresis Damping (Hysteretic Damping, Structural Damping) Energy losses within a structure that are caused by internal friction within the structure. These losses are independent of speed or frequency of oscillation but are proportional to the vibration amplitude squared.

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IEPE (ICP) Integral Electronics Piezoelectric transducer, using a constant current source as the conditioning medium.
Imaginary Part A plot of the imaginary part of the frequency response function versus frequency. For a single-degree-of-freedom system, the magnitude is a maximum or minimum at the damped natural frequency.
Imbalance Unequal distribution of weight or mass on a rotor. The geometric centre of the mass is not where it should be: along the shaft centerline.
Impact test In modal analysis a form of structural excitation often using an impact hammer. (A hammer with an integrated force transducer to measure the impact)
Impact Testing (Hammer Test) A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system’s response. The impact hammer is instrumented with a force transducer to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse to the structure that excites it over a broad frequency range.
Impedance, mechanical The mechanical impedance of a point on a structure is the ratio of the force applied to the point to the resulting velocity at the point. It is a measure of how much a structure resists motion when subjected to a given force, and it is the reciprocal of mobility. The mechanical impedance of a structure varies in a complicated way as frequency is varied. At resonance frequencies, the impedance will be low, meaning very little force can be applied at those frequencies.
Import To convert and use a file created by another program.
Impulse The integral of force over a time interval.
Impulse Response The response of a system to a unit impulse or Dirac’s delta function. The Fourier transform of the impulse response is the frequency response function.
Impulse Signal A sharp “spike” positioned 5% from the beginning of a sample block; produces a near-flat spectrum to Fspan.
Increment A control setting that allows a Test to produce a sequence of incrementally number Run folders, one for each run of the Test.
Induced environments Conditions generated by operating some equipment, as opposed to natural environments.
Inertance (Accelerance) Acceleration divided by force.
Inertially-referenced Motion that is referenced to free space or to a fixed point in space. A sensor (such as an accelerometer) which measures such motion.
Input Channels Parameters In SignalCalc Dynamic Signal Analyzers: Controls for each input channel:
Input Impedance The shunt resistance and capacitance (or inductance) as measured at the input terminals.
Integration Representation inversely related to time rate of change. (integrating velocity yields displacement).
Integrator Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal.
Isolation A reduction in motion severity, usually by a resilient support. A shock mount or isolator attenuates shock. A vibration mount or isolator attenuates steady-state vibration. Isolators are used on electrodynamic shakers to reduce transmission of vibration energy into the floor.
Itch Sometimes called glass itch or lace itch. An automotive sound caused by metal sliding on glass.
Iterative closed loop control pre-calculates drive signals but then modifies those signals based upon resulting motion, in order to better match measured with desired motions. Evaluation and modifications take place after each excitation, repeating until the match is acceptable.

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Key phasor A once-per-revolution pulse. Used in phase measurements for analysis and for balancing..

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LastH In SignalCalc Dynamic Signal Analyzers: The LastH file contains the last system transfer function used by the test for the equalization of the drive signal.
Latent defect A flaw (in a part or assembly) and/or workmanship that is dormant, not immediately apparent visually or by electrical test, yet can result in failure.
Lateral sensitivity Sensitivity of a transducer in a direction perpendicular to the normal measurement axis.
Layout File In SignalCalc Dynamic Signal Analyzers: A binary file within a Test or Run folder that defines all graphical parameters for a specific (user named) arrangement of display windows.
Layout Manager In SignalCalc Dynamic Signal Analyzers: A control dialog affecting all (named) Layouts used within a Test or Run; facilitates creation, deletion, renaming and automatic usage.
Leakage Smearing of frequency components caused during the process of computing the FFT from a digitized block of time data. Weighting or Windowing functions such as Hanning reduce or remove leakage).
Level The level of a signal is its amplitude, alternatively the amplitude expressed on a decibel scale relative to a reference value.
Life Cycle Testing Subjecting a product to stresses similar to those anticipated in actual service while collecting engineering data related to life expectancy, reliability, specification compliance, or product improvements. Usually aimed at determining the products’ mean time between failures or MTBF.
Life units A measure of use duration applicable to the item. Measures include time, cycles, distance, rounds fired, attempts to operate, etc.
Limiting In Vibration Control: The ability to limit the Control spectrum based on a measurement signal approaching a Limit spectrum. Limiting is done on a spectral line basis.
Line Spacing In an FFT spectrum, the frequency difference between two adjacent bin centers or lines.
Linear Having or being a response or output that is directly proportional to the input.
Linear System A system is linear if for every element in the system the response is proportional to the excitation.
Linearity The closeness of a calibration curve to a specified straight line, preferably passing through zero. Commonly specified as a % of full scale.
Lines The measured frequency points in a spectrum
LN2 Liquid nitrogen, often used for rapid cooling of environmental test chambers.
Logarithmic A function the raises a number by an exponent to produce a given number. For example, the logarithm of 1 00 to the base 1 0 is 2.
Loudness The human ranking of an auditory sensation, usually in terms ranging from soft to loud, expressed in sones.
Low Pass Filter A filter that passes signals with less than 3 dB attenuation up to its cut-off frequency, and attenuates the signal above that frequency. The attenuation slope is called the roll off, q.v. An anti-aliasing filter is an example of a low pass filter.

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Machinery health monitoring (MHM) The current machine condition (signature) compared with an earlier condition used as an indicator to detecting potential failure before it occurs. Also called machinery health monitoring.
Magnetostriction Slight changes in the dimensions of iron or steel components resulting from changes in the magnetic fields acting on these components.
Mass Acceleration divided by force.
Mean A value intermediate between quantities under consideration.
Mean-Time-Between-Failure (MTBF) A basic measure of reliability for repairable items: The mean number of life units during which all parts of the item perform within their specified limits, during a particular measurement interval under stated conditions.
Mean-Time-To-Failure A basic measure of reliability for non-repairable items: The total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions.
Measurement Parameters A control group controlling Triggering, Averaging and Auto-Ranging.
Mechanical impedance Force / Velocity, where the velocity is a result of that force only. See Mobility
MEMS – Micro Electro-Mechanical Structures Extremely small devices utilizing both electrical and mechanical properties.
Micron Length or displacement equal to 10-6 meter. One micron = 0.04 mil or 0.00004 inch.
Microphone A transducer used to convert acoustic dynamic pressure change into an electrical signal.
Mil Length or displacement equal to 0.001 inch.
Mils/Inch Used to describe the angle of one shaft centerline to the other. It is equivalent to milliradians. It can also be expressed as rise/run (1 unit = 17.45 mils/inch), as long as the rise is measured in mils and the run is measured in inches.
Mission Profile A time-phased description of the events and environments an item experiences from initiation to completion of a specified mission.
Mission reliability The measure of the ability of an item to perform its required function for the duration of a specified mission profile. Mission reliability defines the probability that the system will not fail to complete the mission, considering all possible redundant modes of operation.
Mobility Velocity / Force. Mobility is the inverse of mechanical impedance. It is a measure of the ease with which a structure is able to move in response to an applied force, and varies it with frequency. The vibration measured at a point on a machine is the result of a vibratory force acting somewhere in the machine. The magnitude of the vibration is equal to the magnitude of the force times the mobility of the structure. From this it follows that the amplitude of the destructive forces acting on a machine are not determined directly by measuring its vibration if the mobility of the machine is not known. For this reason, it is a good idea to measure the mobility at the bearings of a machine in order to find out the levels of the forces acting on the bearings due to imbalance or misalignment.
Modal analysis A process of determining the natural frequencies, damping factors, and mode shapes for a structure. This is usually done either experimentally through frequency response testing or mathematically using finite element analysis. Complex structural motion is reduced to individual vibration modes. The process of determining a set of generalized coordinates for a system such that the equations of motion are both inertially and elastically uncoupled.
Mode A characteristic pattern in a vibrating system. All points reach their maximum displacements at the same instant.
Mode Shape The relative position of all points on a structure at a given natural frequency.
Modulus of Elasticity E (static) The initial slope of the stress vs. strain curve, where Hooke’s Law applies, before the elastic limit is reached. Typical values are 30,000,000 pounds/square inch for steel and about 10,000,000 for aluminium.
Monotonic Sequentially increasing in frequency. Each breakpoint in the table must be at a higher frequency than the previous breakpoint.
MTBF Abbreviation for Mean (or average) time between failures.
MTTF Mean-Time-To-Failure. A basic measure of reliability for non-repairable items; the total number of life units of an item divided by the total number of failures within that population.
Multiple-Degree-of-Freedom System (MDOF) An N-degree-of-freedom system is a system whose position in space can be completely described by N coordinates or independent variables.
MUX Multiplexer. A device that selects multiple inputs into an aggregate signal.
mV/EU The factor reflecting the sensitivity of a transducer per given engineering unit.

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Narrow band Analysis See FFT.
Natural environments Conditions occurring in nature, not caused by any equipment; effects are observed whether an equipment is at rest or in operation.
Natural frequency The frequency of any of the modes of vibration of a structure or system. Damping lowers the natural frequency.
Node A point or line on a vibrating structure that remains stationary.
Noise Any unwanted signal. Can be random or periodic.
Noise Floor The level at which signals cannot be discerned due to the noise floor (electronic hash) in a measuring system.
Non linearity The deviation from a best fit straight line of true output vs. actual value being measured.
Notch In Vibration Control: The process of manually or by control techniques, reducing certain frequencies in a vibration test. Often used as a form of protection to prevent over-test of sensitive items.
Nyquist Frequency Digital signal processing requires analogue to digital (A to D) conversion of the input signal. The first step in A to D conversion is sampling of the instantaneous amplitudes of signal at specific times determined by the sampling rate. If the signal contains any information at frequencies above one-half the sampling frequency, the signal will not be sampled correctly, and the sampled version of the signal will contain spurious components. This is called aliasing. The theoretical maximum frequency that can be correctly sampled is equal to one-half the sampling rate, and is called the Nyquist frequency. In all digital signal-processing systems, including FFT Analyzers, the sampling rate is made to be significantly greater than twice the highest frequency present in the signal in order to be certain the aliasing will not occur.
Nyquist Plot A plot of the real part versus the imaginary part of the frequency response function. For a single-degree-of-freedom system, the Nyquist plot is a circle. The Nyquist plot is representation of a frequency response function by graphing the “real” part versus the “imaginary” part. In the Nyquist plot, a resonance is displayed as a circle, but without indication of its.

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Octave The interval between two frequencies differing by exactly 2:1.
OEM original equipment manufacturer.
Open Loop control In Vibration Control: Pre-computed drive signals to the exciter system without modifying or refining those signals based on observation of the resulting motion. See also Closed Loop.
Operational environment The aggregate of all external and internal conditions (such as temperature, humidity, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, operational performance, reliability or survival of an item.
Operational limit The extremes beyond which a product is not expected to operate.
Orbit The path of a shaft centre line during rotation. The orbit is usually observed on an oscilloscope connected to x- and y-axis displacement sensors. Sometimes called a Lissajous pattern.
Order In Machine Vibration Analysis: Vibration at multiples of a machine’s shaft speed.
Order tracking In Machine Vibration Analysis: Analysis of machine order amplitude and phase as a function of speed. (See order)
Oscillation Variation with time of a quantity. Usually implies some regularity (as in sinusoidal or complex vibration).
Overall RMS Level A measure of the total RMS magnitude within a specified frequency range.
Overlap In Dynamic Signal Analysis: The maximum percent (of Tspan) of adjacent measurement overlap that is permitted when the process runs faster than real-time; the amount of measurement redundancy desired.
Overlap Processing In Dynamic Signal Analysis: The windowing applied to a time signal stored in a buffer before being processed to form the spectrum causes data at the beginning and end of the time records to be represented at the wrong amplitude values, creating errors in the spectral amplitude levels. Overlap processing allows processing of the entire contents of the input buffer, more frequently than the time taken for the buffer to completely refresh (Tspan). Overlap processing is desirable when using a Hanning Window because it ensures against loss of data for parts of the signal that occur near the beginning and end of the window. An overlap of 66.7% will completely correct for amplitude errors caused by the Hanning window.
Overwrite The process of writing new data over existing data. This option causes you to lose the information that has been written over.

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Parameter A value or option associated with a specific field in the dialog box.
Parameter Estimation The process of evaluating and curve fitting frequency response functions in order to estimate modal parameters.
Patent defect A flaw (in a part or assembly) and/or workmanship that has failed under test or screen. See Latent Defect.
Payload In Vibration Testing, the total weight of all components mounted on the shaker table, including the derive under test (DUT), fixture and head expander (extender).
Peak maximum value of a varying quantity. Peak is also a form of spectral averaging where the maximum level at each frequency is stored and presented as the spectrum.
Peak Hold An “averaging” method that retains the maximum positive value encountered at each time or frequency point; a “roof extrema”.
Peak Pick A parameter estimation technique where the peak value of the imaginary part of the frequency response function is used to estimate the mode shape value at that point. The phase is given by its sense (positive or negative). This method is also known as quad picking since the value is being picked off the imaginary or quadrature part of the frequency response function.
Peak-to-peak value The algebraic difference between extreme values
Period The interval of time over which a cyclic vibration repeats itself.
Periodic A signal is periodic if it repeats the same pattern over time. The spectrum of a periodic signal always contains a series of harmonics.
Periodic vibration (See also Deterministic vibration.) An oscillation whose waveform regularly repeats. Compare with probabilistic vibration.
Perpendicular At right angles (90°) to a given line or plane.
Phase (Of a periodic quantity), the fractional part of a period between a reference time (such as when displacement = zero) and a particular time of interest; or between two motions or electrical signals having the same fundamental frequency.
Phase (time lag or lead) The difference in time between two events such as the zero crossing of two waveforms, or the time between a reference and the peak of a waveform. The phase is expressed in degrees as the time between two events divided by the period (also a time), times 360 degrees.
Phase reference probe A transducer giving an output once per shaft revolution.
Picket Fence Effect The FFT spectrum is a discrete spectrum, containing information only at the specific frequencies that are decided upon by setting the FFT Analyzer analysis parameters. The true spectrum of the signal being Analyzed may have peaks at frequencies between the lines of the FFT spectrum, and the peaks in the FFT spectrum will not be at exactly the correct frequencies. This is called Resolution Bias Error, or the Picket Fence Effect. The name arises because looking at an FFT spectrum is something like looking at a mountain range through a picket fence.
Piezoelectric (PE) transducer A transducer using deformation of a crystal or ceramic element to generate electrical charge and voltage.
Piezoresistive (PR) transducer A transducer using deformation of its semiconductor resistive element to generate electrical output.
Pitch Rotation in the plane of forward motion, about the left-right axis.
Platform Per MIL-STD-810, any vehicle, surface or medium that carries an equipment. For example, an aircraft is the carrying platform for internally-mounted avionics equipment and externally-mounted stores. The land is the platform for a ground radar set, and a man for a hand-carried radio.
Point and Direction A book-keeping facility to identify the degrees-of-freedom involved in every Modal measurement.
Power Spectrum In the frequency domain, this is the square of FFT´s magnitude. The power spectrum answers the question, “Which frequencies contain the signal’s power?” The answer is in the form of a distribution of power values as a function of frequency. SignalCalc systems are an example of a modern high speed digital spectrum analyzer which can provide a power spectrum in realtime.
Power Spectral Density (PSD) The power of random vibration intensity, in mean-square acceleration per frequency unit, as g2/Hz or m2/s3.
Power Spectral Density (PSD) A function of frequency whose integral over any frequency interval represents the spectral content over that frequency interval. The limiting mean-square value
Preview Average In SignalCalc Dynamic Signal Analyzers: A facility to view the most recently captured signals before they are (conditionally) added to an average.
Probabilistic vibration Vibration such as random, where magnitude at any future time can only be predicted on a statistical basis.
Probability distribution An evaluation of the magnitude of events, stresses, strengths, etc. They don’t all have the same value. Often they are distributed in some sort of “bell shaped” graph of percentage vertical vs. value horizontal. Most are clustered around the mean value.
Proof of screen A process aimed at showing that a screen is effective in identifying existing defects in a product yet that the screen does not damage good products.
Proximity sensor A displacement sensor for measuring the varying distance between two points
Pseudo Random Noise A single frame of Random Noise that is exactly repeated every Tspan. Used to separate non-linear effects from noise contamination.

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Q (Sharpness of Resonance) (Amplification factor) A measure of the sharpness of a resonance or frequency. The mechanical gain at resonance.
Quadrature motion (Or side or lateral motion or crosstalk), any motion perpendicular to the reference axis. Shakers are supposed to have zero quadrature motion.
Quadrature Response Another name for the imaginary part of the frequency response function.
Quadrature sensitivity (Or side or lateral motion or crosstalk sensitivity) of a vibration sensor is its sensitivity to motion perpendicular to the sensor’s principal axis. Commonly expressed in % of principal axis sensitivity.
Quasi-Periodic A quasi-periodic signal is a deterministic signal whose spectrum is not a harmonic series, but nevertheless exists at discrete frequencies. The vibration signal of a machine that has non-synchronous components resembles a quasi-periodic signal. In most cases, a quasi-periodic signal actually is a signal containing two or more different periodic components.

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Radial A direction perpendicular to a shaft’s centerline.
Random Noise A random Signal Generator output exhibiting uniform spectral density over Fspan, Gaussian amplitude distribution and a crest factor of 5.
Random vibration Non periodic or deterministic vibration where magnitude cannot be predicted. Random vibration is usually termed Gaussian if it follows a Gaussian distribution.
Random-on-Random Broad-band random mixed with random narrowband tones.
Range (EU) The (zero-to-peak) Full Scale for an Input Channel, expressed in the engineering units of the transducer (signal source).
Rate gyro A kind of gyroscope that measures rotational velocity (degrees or radians per second) around a fixed axis.
Real or Normal Modes In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time.
Real Part A plot of the real part of the frequency response function versus frequency. For a single degree of freedom, the magnitude is zero at the damped natural frequency.
Rectangular Window In an FFT Analyzer, the rectangular window is actually no window at all. It is also called rectangular weighting, or uniform weighting, and is used when the signal to be Analyzed is either a transient or is exactly symmetrical within the Tspan.
Reliability The likelihood or probability that an equipment will “do its job” for a specified length of time (say 1000 hours) under specified circumstances (such as cycling in a specified manner over certain temperature limits, experiencing a particular vibration spectrum, etc.). Reliability defined in this way can be determined experimentally. Take 1000 units. Operate them under specified conditions. At the end of 1000 hours, how many are still operating correctly?
Repeatability The consistency (or variation) of readings and results between consecutive sets of measurements. It has nothing to do with accuracy.
Repetitive shock machine A machine in which products (to be tested) are attached to a platform (table). Often this platform forms the bottom surface of a thermal test chamber. Pneumatic vibrators are attached to the bottom of the platform, causing it to vibrate, usually simultaneously in several axes.
Replication Testing that reproduces a specified desired motion history (time domain) or waveform.
Residual Terms Terms added to a curve fit algorithm to take into account the effects of modes outside the range being fitted. These terms consist of a mass term on the low frequency end and a stiffness term on the high.
Resolution Usually a measure of the number of samples in a block of data or the number of frequency points in an FFT analysis.
Resolution Bias Error See Picket Fence Effect.
Resonance A vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system. The frequency of any of the natural modes of vibration of a structure or system.
Response The vibratory motion or force resulting from a mechanical or electrical input.
Response Spectrum The frequency response function, also called the response spectrum, is a characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT Analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems.
Response Window A decaying exponential Window of adjustable Time Constant used on the motion signal in an impact Modal test.
Ringing Continued oscillation after an external force or excitation is removed.
Rise time (For a vibration transducer) The time required for the output of a transducer to rise from 10% to 90% of its final value, as it responds to a step change.
RMS (Root-Mean-Square) The root of the mean of the square of a quantity. In a sine wave the RMS value is 0.707, for a square wave the value is 1.
Roll Rotation about the axis of linear motion.
Rolling element (anti-friction) bearings A shaft rotates on rollers or balls.
Root cause analysis Determining what actually caused a failure, as opposed to what appears to have been the cause. Usually requires the services of a specialized failure analysis lab.
Roots The roots of the characteristic equation are complex and have a real and imaginary part. The real part describes the damping (decay rate) of the system and the imaginary part describes the oscillations or damped natural frequency of the system.

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Sampling Parameters A control group that sets the interrelated parameters Fspan, Lines, dF, Tspan, Block size and dF.
Sampling rate The number of measurements an A/D converter takes per second.
Screening A process in which products from the production line are subjected to thermal and / or vibration stresses to reduce the likelihood of early life field failures by forcing them to occur before final test in the factory.
Seismic Measurements and activity concerning earth motion, such as earthquakes.
Selectivity Selectivity is a measure of the narrowness of a band pass filter. The greater the selectivity, the narrower (or more selective) the filter.
Self-induced vibration Also called self-excited vibration, results from conversion of non-oscillatory energy into vibration.
Sensitivity Of a mechanical-to-electrical sensor or pickup, the ratio between electrical signal (output) and mechanical quantity (input)..
Sensor Any device that translates the magnitude of one quantity into another quantity. Three of the most common transducers used in vibration measurements are accelerometer, velocity transducer, and eddy current probe.
Session Trigger In SignalCalc Dynamic Signal Analyzers: A single trigger condition awaited from an input channel; once acquired, data is collected in Free Run without need for further triggers.
Shaker A vibration machine or device for subjecting a mechanical system to controlled and reproducible mechanical vibration.
Sharpness of Resonance (Q) (Amplification factor) A measure of the sharpness of a resonance or frequency. The mechanical gain at resonance.
Shock machine Or shock test machine, a device for subjecting a system to controlled and reproducible mechanical shock pulses.
Shock pulse, classical Classical shock pulses include half-sine, square, triangular, trapezoid and sawtooth.
Shock pulse, haversine An inverted cosine offset by half its amplitude.
Shock Response Spectrum (SRS), A spectrum consisting of the maximum responses of an array of SDOF systems vs. their natural frequencies, as they respond to an applied shock.
Shock test machine A mechanical device for generating mechanical shock pulses.
SI (Systeme International) An internationally accepted system of measurement based upon nine precisely defined metric units.
SI Exponents Integer exponents of (S1) mass, length, time, etc. that define the dimensions of an EU entered to the Engineering Unit Table.
SI Units Kilogram (mass), meter (length), second (time), Ampere (current), Kelvin (temperature), candela (light intensity), mole (molecular atomic weight), radian (plane angle), steradian (solid angle)
Sidebands Spectral components that are the result of amplitude or frequency modulation. The frequency spacing of the sidebands is equal to the modulating frequency, and this fact is used in diagnosing machine problems by examining sideband families in the vibration spectrum. For instance, a defective gear will exhibit sidebands spaced apart at the gear rpm around the gear mesh frequency.
Signal In vibration analysis, a signal is an electric voltage or current which is analogue of the vibration being measured.
Signal Amplifier A signal amplifier is an electronic device that uses a small amount of energy to control a larger amount of energy generally for audio applications. SignalStar amplifiers can be used with any new or existing shaker. The relationship of the input to the output of a signal amplifier — usually expressed as a function of the input frequency — is called the transfer function, more correctly the Frequency Response Function (FRF) of the amplifier. All SignalCalc analyzers provide FRF measurements as part of the basic measurement package.
Signal Conditioner An amplifier which prepares a transducer signal for use by other instrumentation such as FFT analysis, vibration control etc.
Signal Generator A device or system output from a Dynamic Signal Analyzer producing: Sine, Swept Sine, Chirp, Random, Pseudo Random, Burst Random, Thump and Impulse signals.
Signal Map In SignalCalc Dynamic Signal Analyzers: A control window that determines what signals are stored where and in what format.
Signal-to-Noise Ratio. (SNR) The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time.
Simple harmonic motion Periodic vibration that is a sinusoidal function of time.
Sine Output A Signal Generator output signal of selectable frequency, amplitude and phase.
Single-Degree-of-Freedom System (SDOF) A system whose position in space can be completely described by one coordinate.
Single-Ended Input An analogue input measured with respect to a common ground.
Six Degree of Freedom machines Repetitive Shock Machines. That class of non stationary random vibration producing machines that are air driven impact hammers o excite a vibrating plate onto which products are either attached directly or fixtured. The resulting excitation is a combination of impulse and table modal response, or an inter-modulation of both
Slew rate The maximum rate at which an instrument’s output can change by some stated amount.
Soft failure A product under test ceases to operate correctly, but resumes correct operation when the stressing environment is eased. Differs from hard failure.
Solid-state sensor One which has no moving parts.
Sone A unit of loudness.
Sound An oscillation in pressure, capable of evoking the sensation of hearing.
Sound intensity In a specified direction, the average rate of sound energy flow through a unit area perpendicular to that direction.
Sound level The quantity in dB measured by a standardized Sound Level Meter. The reading is 20log10 of the ratio between a given sound pressure and 20 micro pascals.
Source A D/A converter output channel
Source follower A device for converting a high impedance electrical signal to low impedance. Also referred to as an “impedance converter.” Generally has a voltage gain of unity.
Source Impedance The combination of resistance and reactance that a source presents to the input terminals of a measuring instrument.
Spall A flake or chip of metal; removed from one of the races of a rolling-element bearing. That bearing is nearing the end of its useful life.
Spectra The plural of spectrum.
Spectral map A three-dimensional plot of amplitude (Z axis) vs. time (or shaft speed) (Y axis) vs. frequency (X axis)
Spectrum The spectrum is the result of transforming a time domain signal to the frequency domain. It is the decomposition of a time signal into a collection of sine waves.
Spectrum Analyzer A spectrum analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveform. Often, it measures the power spectrum. SignalCalc systems are an example of a modern high speed digital spectrum analyzer.
Squeak In Automotive sound quality, an annoying sound, A sound resembling that of an unlubricated hinge.
Squeak & Rattle A term given to annoying noises such as those in a vehicle caused by plastic parts rubbing etc.
Stable Averaging (Linear) An averaging method wherein all constituents are equally weighted; the amplitude is continuously normalized to the number of constituents currently captured.
Standard deviation The square root of the mean of the squares of the measured deviations from the mean value.
Standards Documents that prescribe engineering disciplines, impose particular management practices, reporting and auditing requirements.
Standing wave A wave that is characterized by lack of vibration at certain points (nodes), between which are areas of maximum vibration (antinodes).
Stationarity A property of probabilistic vibration if the PSD and the probability distribution remain constant.
Stationary Signal A stationary signal is a signal whose average statistical properties over a time interval of interest are constant, and it may be deterministic or not. In general, the vibration signatures of rotating machines are stationary.
Steady state vibration Periodic vibration for which the statistical measurement properties (such as the peak, average, RMS and mean values) are constant.
Step stressing Increasing stresses in a series of pre-selected increments.
Stepped Sine Used for measuring the stability of closed loop servo systems and is one of many different techniques for structural excitation and frequency response function measurement.
Stepped sine testing Sine shaking in a series of dwells. Frequency is incrementally increased or decreased.
Stiffness The ratio of force (or torque) to deflection of a spring-like element.
Strain-gage transducer A changing-resistance sensor whose signal depends upon sensitive element deformation.
Stress Intensity of applied load, usually at the site of a failure.
Stress Screening A process in which products from the production line are subjected to thermal and / or vibration stresses to reduce the likelihood of early life field failures by forcing them to occur before final test in the factory.
Structural Analysis Structural analysis comprises the set of physical laws and mathematics required to study and predict the behavior of structures. SignalCalc systems are used to analyze the dynamic behavior of mechanical systems.
Structural Modification Mathematically determining the effect of changing the mass, stiffness, or damping of a structure and determining its new modal parameters. A modal analysis provides, in essence, a mathematical model of the structure. This model can be manipulated to determine the effect of modifications to the structure. The modal model can be generated either experimentally or using a finite element program.
Sub synchronous Frequencies in a vibration spectrum that are lower than the fundamental frequency. Components of a vibration signal whose frequency is less than once per rev shaft speed.
Sub-harmonic A sinusoidal quantity having a frequency that is an integral sub-multiple of a fundamental frequency.
Supply current In ICP (IEPE) Transducers: The typical current that must be supplied to a sensor (along with the supply voltage).
Swept Sine A sine signal of linearly varying frequency.
Swept-sine testing Sine vibration testing where frequency is smoothly and continuously varied with a constant or profiled controlled amplitude.
Synchronous (Rotating machinery) Vibration components related to shaft speed.
Synchronous Average A type of signal averaging where successive records of the time waveform are averaged together. This is also know as time domain averaging. The important criterion is that the start of each time record must be triggered from a repetitive event in the signal, such as once per rev. The triggering assures that the phase of the waveform components that are synchronized with the trigger are the same in each record. Then in the averaging process, these in-phase components will overwrite in the input buffer and will remain in memory, while the rest of the signal components will gradually average out because of their random relative phases. The technique is excellent for extracting signals from noisy environments.
Synchronous sampling Control of a computer’s rate of data sampling to achieve order tracking.

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TAAF Test, Analyze and fix. See Accelerated Life Testing.
Tailoring (Test Tailoring) Selecting or altering test procedures, conditions, values, tolerances, measures of failure, etc., to simulate or exaggerate the environmental effects of one or more forcing functions.
Test Type The mode of measurement operation
THD (Total Harmonic Distortion). For a signal, the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency.
Thermal cycling Subjecting a product to pre-determined temperature changes, between hot and cold extremes.

Thermistor An electrical device used for temperature measurement.

Thermocouple An electrical device used for temperature measurement. Two dissimilar metals joined together, making a continuous loop.

Threshold The smallest change in a measured variable that gives a measurable change in output signal.

Thrust position Location in direction of a shaft centre line. See axial position.

Thump Signal A “velocity and displacement compensated” half-sine pulse. This transient is a classic shock-test acceleration waveform.

Time constant The interval needed for an instrument’s output to move 63% of its ultimate shift as a result of a step change in its input.

Toggle A switch that causes the opposite value. For example, a switch that is “on,” when toggled, becomes “off.”

Tracking filter A narrow band-pass filter whose centre frequency follows a synchronizing signal.

Transducer A device which converts shock or vibratory motion into an electrical signal that is proportional to a parameter of the experienced motion.

Transfer Function The output to input relationship of a structure. Mathematically it is the Laplace transform of the output divided by the Laplace transform of the input.

Transform A mathematical operation that converts a function from one domain to another domain with no loss of information. For example, the Fourier transform converts a function of time into a function of frequency.

Transient vibration Short-term vibration of a mechanical system.

Transmissibility The non-dimensional ratio of response / input. The maximum Transmissibility value is the mechanical “Q” of a system.

Transverse sensitivity Sensitivity of a transducer in a direction perpendicular to the normal measurement axis.

Trigger A facility to synchronize the start of data acquisition.

Tspan The time duration (second) of each frame or capture window that is numerically equal to Block Size x dT.

Tuneable Filter A filter whose cut-off frequencies are adjustable, either manually or under remote electrical control.


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Ultrasound Ultrasound is sound with a frequency greater than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz). SignalCalc analyzers provide measurements will into the ultrasonic range, as high as 97 kHz.
Unbalance (imbalance) Unequal mass distribution on a rotor. The mass centre line does not coincide with the rotation or geometric centre line.

Undamped Natural Frequency The same as the natural frequency of a structure.

Uniform window In the FFT Analyzer, the uniform, or rectangular, window does not modify the signal amplitude at all. It is also called rectangular weighting, or uniform weighting, and is selected when the signal to be Analyzed is a transient rather than a continuous signal.

Unit Every measurement is expressed as a multiple or fraction of dome appropriate, well-defined unit quantity such as centimeter, volt, etc.

USB USB data acquisition devices are ideal for a variety of applications from simple data logging to embedded OEM systems. The SignalCalc ACE powered by Quattro is an example of a USB data acquisition product.


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Velocity Rate of change of displacement with time. A vector quantity that specifies the time rate of change of displacement with respect to a reference frame. If the reference frame is not inertial, the velocity is often designated relative velocity

Velocity Transducer An electrical/mechanical transducer who’s output is directly proportional to the velocity of the measured unit. A velocity transducer consists of a magnet suspended on a coil, surrounded by a conductive coil. Movement of the transducer induces movement in the suspended magnet. This movement inside the conductive coil generates an electrical current proportional to the velocity of the movement. A time waveform or a Fourier transform of the current will result in a velocity measurement. The signal can also be integrated to produce a displacement measurement.

Vibration Any periodic process, especially a rapid linear motion of a body about an equilibrium position. SignalCalc analyzers and SignalStar controllers require transducers to convert the mechanical vibrations to electrical signals for analysis or control.
Vibration Analysis The study of a continuing periodic change in the magnitude of a displacement with respect to a specified central reference. Various measurements for vibration analysis are provided as part of all SignalCalc systems.
Vibration meter A device for measuring electrical signals from vibration sensors. May display displacement, velocity and/or acceleration.

Vibrator A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures.

Viscous Damping Damping that is proportional to velocity. Viscous damping is used largely for system modeling since it is linear.


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Waterfall plot A spectral maps with multiple FFT traces plotted on a graph regular intervals of time, speed or other periodic parameter.

Waveform A presentation or display of the instantaneous amplitude of a signal as a function of time, as on an oscilloscope or oscillograph. In the time domain.

Weight That property of an object that can be weighed, as on a scale; the gravitational force on an object.

Weighting A “weighting function” applied to time-data prior to performing an FFT. Rectangular, Hann, Flat top, DP7, BH4, Force and Response.

Weighting (Acoustic) An artificial curve applied to acoustic measurements across the frequency spectrum. C weighting is essentially flat. A weighting attempts to compensate for the non-constant sensitivity of human hearing at certain frequencies.

Weighting Emphasis or attenuation applied to sound measurements at certain frequencies. C weighting is essentially flat. A weighting attempts to compensate for the non-constant sensitivity of human hearing at certain frequencies. See Equal Loudness Curves.

White random vibration Broad-band random vibration with constant power per bandwidth.

Window A “weighting function” applied to time-data prior to performing an FFT. Rectangular, Hann, Flat top, DP7, BH4, Force and Response.


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Yaw Rotation about the vertical axis.


Zwicker loudness

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A sound measurement in liner units (sones).