Absolute. A term applied to calibration (e.g. of an accelerometer) based upon the primary standards of mass, length and time. (See also comparison calibration.)

Absolute Accuracy. A measure of the uncertainty of an instrument reading compared to that of a primary standard traceable to NJST.

Absolute vibration. Vibration of an object relative to a fixed point in space. Seismic sensors (accelerometers and velocity pickups) measure absolute vibration. Contrasts with relative vibration.

Absorber. A device capable of reducing, of attenuating, of "soaking up" vibration, usually converting it to heat.

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 or by substitution of weaker parts 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. Acceleration is rate of change of velocity with time (denoted as dv/dt or d2x/dt2), usually along a specified axis, usually expressed in g or gravitational units. It may refer to angular motion.

Accelerometer. A sensor or transducer or pickup for converting acceleration to an electrical signal. Two common types are piezoresistive and piezoelectric.

Accessibility. A measure of the related ease of access to various portions of an item for operation or maintenance.

Accuracy. The capability of an instrument to indicate the true value. Do not confuse with inaccuracy (sum of hysteresis + non-linearity + drift + temperature effect, etc.) nor with repeatability.

A/D Converter. A device that changes an analog signal such as voltage or current into a digital signal (consists of discrete data values).

Aggravated Test. A test in which one or more conditions are set at a higher stress more level than the test item will encounter in the field, in order to reduce test time or assure a margin of safety.

Aliasing. A spectrum analysis problem resulting from sampling data at too low a sampling frequency. It causes high-frequency signals to appear in a spectrum at low frequencies. In a sampled data system, the analog input must be sampled at a rate at least twice the bandwidth of the signal to avoid loss of data (Nyquist Theorem).

Alignment. A desired machinery condition, in which the axes of components of a machine are adjusted so as to be colinear, parallel or perpendicular, and thus creating least vibration.

Ambient environment. The conditions (e.g. temperature and humidity) characterizing the air or other medium that surrounds materiel.

Amplitude. The magnitude of variation (in a changing quantity) from its zero value. Always modify it with an adjective such as peak, RMS, average, etc. May refer to displacement, velocity, acceleration, voltage, current, force or pressure.

Angular Frequency. (Also known as circular or torsional frequency.) w is the torsional vibration frequency in radians per second. Or multiply by 2p and express in hertz (Hz) or (obsolete) cycles per second (cps).

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 some fraction of the sample rate, in order to minimize aliasing.

ASD. Auto spectral density. The measure of acceleration power per Hz of analysis bandwidth. Also known as Power spectral density, PSD. The square root of the area under the ASD curve is defined as the g RMS of acceleration.

Attenuate. Reduce.

Autoranging. The capability of an instrument to switch among ranges automatically. The ranges usually are in decade steps.

Auto spectrum (power spectrum). A spectral display of the power (voltage squared) at each frequency. Phase is ignored.

Availability. A measure of the degree to which an item is in an operable and committable state at the start of a mission, when the mission is called for at an unknown (random) time.

Average. Refer to a textbook on electrical engineering. In the exclusive case of a pure sine wave, the average value is zero.

Average Responding. A measurement proportional to the average of the absolute values of all input waveforms within a specified frequency range.

Averaging. Summing and suitably dividing several like measurements to improve accuracy or to lessen the effect of any asynchronous components.

A-weighting Emphasis given by filtering to sound measurements with the goal of compensating for the non-flat frequency response of human hearing, in order to get numbers approximating human response. See Equal Loudness Curves.

Axial. Along the centerline of a shaft.

Axial (thrust) position. Change in a rotors position or displacement along its axis, relative to some nearby fixed point.

Balancing. (mechanical) Adjusting the distribution of mass in a rotating element, to reduce vibratory forces generated by rotation.

Bandwidth. The frequency range (usually stated in hertz or Hz) within which a measuring system can accurately measure a quantity.

Barrier. Blocks airborne sound from entering passenger compartment via an aperture. Example, a piece of sticky elastomer that is pressed on so as to cover a hole.

Bias. Refers to a more or less persistent tendency for measurements, as a group, to be too large or too small.

Bounce Test: A shaking of unrestrained (loose) cargo. The cargo is repeatedly thrown a short distance into the air and then falls onto the vibrating platform.

Broadband. Vibration (or other) signals which are unfiltered. Signals at all frequencies contribute to the measured value.

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.

Blade-passing frequency. A potential vibration frequency on any bladed machine (turbine, fan, etc.), the number of blades x shaft speed.

Bode plot. The magnitude of vibration at 1x shaft speed, also its phase relative to shaft position, both plotted against running speed.

Bow. An undesirable shaft condition (rotating machinery) in which the shaft centerline is not straight.

Bump Test: A horizontal striking of an impacting surface against cargo, or cargo motion arrested by impacting a stationary object.

Burn-in. Continuously powering a product, often at constant elevated temperature, in order to accelerate the aging process. Much less effective, in my opinion, than power-on ramping of temperature + random vibration.

Buzz A sound exemplified by loose power transformer laminations (dominated by 120 Hz where the power frequency is 60 Hz).

Calibration. (as applied to vibration sensors) An orderly procedure for determining sensitivity as a function of frequency, temperature, altitude, etc.

Campbell diagram. A mathematically-constructed diagram used to check for coincidences of vibration sources (1x, 2x, etc. shaft speed) with rotor natural frequencies, resulting in rotor resonances. It plots frequency vs RPM, with plot size growing with increasing amplitude. Sometimes called an interference diagram.

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.

Channel. A sensor (or pickup or transducer) with its associated signal conditioner and monitor (for observing the signal) and recorder (for storing the signal).

Characterization is aimed at providing the shaker control system with needed information about the testing system and the test article. Pretest characterization (at reduced intensity), repeated several times iteration is commonly performed. Increasingly, adaptive characterization modifying the control system, based upon observing responses, is being performed during the test.

Charge amplifier. An amplifier which converts a charge input signal (as from an accelerometer) into an output voltage; a charge-to-voltage converter.

Circuit card. A flat board that holds chips and other components on the top side and has printed electrically conductive paths in multiple layers for the components on its bottom side.

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. See iterative closed loop control.

Coherence. A measure of the similarity of vibration at two locations, giving insight into possible cause and effect relations.

Comparison. A term applied to calibration (e.g. of an accelerometer) in which sensitivity is tested against a standard. See also absolute calibration.

Compliance. The reciprocal of stiffness, i.e. displacement divided by force.

Condition monitoring (CM). The measurement, recording and analysis of machinery parameters (such as acceleration) to determine machinery health. Todays condition is compared with earlier condition, as when a machine was new. Also called machinery health monitoring, MHM.

Constant-bandwidth filter. A bandpass filter whose bandwidth is independent of center frequency. Filters simulated digitally by an FFT process are constant bandwidth.

Constant percentage filter. A bandpass filter whose bandwidth relates (1/3 x, 1/10 x, etc.) to center frequency. May be synthesized digitally.

Contact Bounce. The intermittent opening of relay contacts during closure.

Critical frequency. A particular resonant frequency (see resonance) at which damage or degradation in performance is likely.

Critical machines. Machines (in a plant) that are vital to continued operation. Vibration should be monitored continuously.

Critical speeds. Any rotating speed which results in high vibration amplitudes. Often these are speeds which correspond to system natural frequencies.

Cross-axis sensitivity. See transverse sensitivity.

Crossover frequency. In sinusoidal vibration testing, the unique forcing frequency at which the required displacement yields the desired acceleration and vice versa.

Cross-talk. Interference or noise in a sensor or channel, coming from another sensor or channel.

Cycle. The complete sequence of instantaneous values of a periodic event, during one period.

D/A converter. A device that converts a digital signal (discrete values) into an analog voltage.

Damp (not dampen). To reduce the Q or maximum resonant response, by extracting energy, usually by converting motion into heat. Your automobiles shock absorbers damp body motion.

Damper. One form adheres or is sprayed onto relatively thin metal panels so as to extract energy when panels flex. Lowers "Q" of each panel mode. Goal: to reduce air flow (over the panel)-induced vibration and radiated noise.

Damping. Dissipation of oscillatory or vibratory energy with motion or with time. Critical damping Cc is that value of damping that provides most rapid response to a step function without overshoot. Damping ratio is some fraction of Cc.

Decade. The interval between two frequencies which differ by exactly 10:1.

deciBel. Ratios of identical quantities are expressed in decibel or deciBel or dB units. The number of dB is ratiod against some standard or reference value in terms of the base 10 logarithm of that ratio. In measuring acoustic or vibration power (as in PSD or ASD of random vibration), the number of dB = 10 log10 P/Po. Po, the reference level, equals 0 dB. In measuring the more common voltage-like quantities such as acceleration, the number of dB = 20 log10 E/Eo Eo, the reference level, equals 0 dB.-

Degrees of freedom. In mechanics, the total number of directions of motion (of all the points being considered) on a structure being modeled or otherwise evaluated. Example: a platform experiencing heave, fore-and-aft, left-and-right motions as well as roll, pitch and yaw, is said to have six degrees of freedom. Statistical degrees of freedom: In random vibration measurement, twice the number of averages being used.

Dependability. A measure of the degree to which an item is operable and capable of performing its required function at any (random) time during a specified mission profile, given item availability at the start of the mission. (This definition is significantly different from the definition of dependability used by most other US and international organizations dealing with reliability e.g., IEC and SAE. E.g. IEC 50 Chapter 191: "The collective term used to describe the availability performance and its influencing factors: reliability performance, maintainability performance and maintenance support performance." As such, its use is restricted to general descriptions in non-quantitative terms). Dependability is related to reliability; the intention was that dependability would be a more general concept then reliability.

Design limit. The operational limit of a product, beyond which it not required to function properly.

Design ruggedization. See Accelerated Life Testing.

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).

Differentiation. Representation in terms of time rate of change. Example: differentiating velocity yields acceleration. In a computer, this is accomplished by multiplying the velocity signal by j, where is frequency multiplied by 2.

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.

Displacement specifies change of position, or distance, usually measured from mean position or position of rest. Usually applies to uniaxial, less often to angular motion.

Distortion. In mechanics, any unwanted motion. If sinusoidal motion were desired at a fundamental frequency, distortion is any motion at harmonics or subharmonics of that frequency, or any mechanical "hash" (perhaps due to parts colliding). In electronic measurements, distortion is any unwanted signal; e.g. amplifiers may generate unwanted signals.

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.

DSP - Digital Signal Processor. A microprocessor optimized for digital signal manipulations.

Duration of a shock pulse is how long it lasts. For "classical" pulses (not found in the real world), time is usually measured between instants when the amplitude is greater that 10% of the peak value.

Durability. A measure of useful life (a special case of reliability).

DUT. Device under test. See also UUT or unit under test (being tested).

Dynamic motion. Movement, as compared with nonmoving or static position. Dynamic motion is sensed with displacement or velocity pickups or with accelerometers.

Dynamic Range. The ratio of a specified maximum level of a parameter, such as power, current, voltage, or frequency, to the minimum detectable value of that parameter.

Dynamic signal analyzer DSA. Vibration analyzer using digital signal processing and the Fast Fourier Transform (FFT) to display vibration frequency components. May also display the time domain and the phase spectrum. Usually interfaced to a computer.

Eccentricity, mechanical. Variation of shaft surface radius when referenced to the shaft's true geometric centerline. Out-of-roundness.

Eddy current. Electrical current generated (and dissipated) in a conductive material (often a rotor shaft) when it intercepts the electromagnetic field of a displacement or proximity probe.

Engineering units. Units that are decided upon by an individual user or by agreement among users. Examples include inches/second, mm/s, g, Hz, Tu, etc.

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, performance, reliability or survival of an item.

Environmental engineering specialist. One whose principal work assignment lies in the technical area of natural and induced environments and their relation to military equipment. A person who has expertise in measuring and analyzing field environmental conditions, formulating environmental test criteria, specifying laboratory simulation of environments, and evaluating the effects of environments on equipment.

Environmental stress screening (ESS). A post-production process in which produced units are subjected to stresses more severe than anticipated in service. The object is to precipitate latent defects into recognizable failures, so that that particular unit does not proceed further in production nor reach the customer.

Environmental testing. Subjecting a sample of products to a simulation of anticipated storage, transport and service environments (such as vibration, shock, temperature, altitude, humidity, etc.)

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.

Essential machinery. See Critical Machinery.

Excitation. The voltage or current applied to a transducer.

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 mode made and determine the failure mechanism and its basic course.

Failure catastrophic. A failure that can cause loss.

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 in a system is analyzed to determine the resulting 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 before wearout.

Fault. Immediate cause of failure (e.g. maladjustment, misalignment, defect, etc.)

FFT or Fast Fourier Transform. A popular computer method of shifting data from the time domain to the frequency domain.

Feedback signal is a response measurement that is used in a closed-loop control process.

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 modeling or finite element analysis or FEA. A computer-aided design technique for predicting the dynamic behavior of a possible future 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.

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.

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. See Fast Fourier Transform or FFT. Term honors mathematician Baron Jean Baptiste Joseph Fourier 1768-1830.

Fragility. The maximum load an equipment can stand before failure (malfunction, irreversible loss of performance or structural damage) occurs.

Fragility test. Expensive but highly useful dynamic tests of several samples (to account for variations in tolerances, material properties and manufacturing processes) at potentially destructive frequencies, to determine fragility.

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 range - see bandwidth.

Frequency response. The portion of the frequency spectrum over which a device can be used, within specified limits of amplitude error.

Fundamental frequency. The number of hertz or cycles per second of the lowest-frequency component of a complex, cyclic motion. (See also Harmonic and Subharmonic.)

Frequency spectrum. A description of the resolution of any electrical signal into its frequency components, giving the amplitude (sometimes also phase) of each component.

Fundamental mode of vibration. That mode having the lowest natural frequency

g The acceleration produced by Earth's gravity. By international agreement, the value for 1 gravitational unit is 9.80665 m/s = 386.087 in/sec = 32.1739 ft/sec.

gal. 1 gal (seems to mainly be used in Japan) is an acceleration of 1 cm/sec. A more or less typical earthquake measures around 250 gal near the epicenter, so 400 gal is sometimes specified for earthquake testing. 980.6 gal = 1g.

Gearmesh 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

g units or gravitational units. A way to express an acceleration, in terms of a ratio. Divide a given acceleration by the appropriate value (on Earth use 9.80665 m/s or 386.087 in/sec or 32.1739 ft/sec).

Graduation mark, The marks that define the scale intervals on a measuring instrument are known as graduation marks.

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, causing measurement errors.

HALT. Highly accelerated life test. See accelerated life test.

HASS. Highly accelerated stress screening. See environmental stress screening (ESS).

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.

Harmonic. A sinusoidal quantity having a frequency that is an integral multiple (2, 3, etc.) of a fundamental (1) frequency.

Harmonic Distortion. In the output signal of a device, distortion at those frequencies caused by the presence of frequencies not present in the input signal.

Hash. Distortion (usually non-harmonic) on a signal. May be viewed on an oscilloscope trace. (slang).

hertz. (lower-case h, but abbreviated Hz) The unit of frequency. Formerly cps for cycles per second.

Hysteresis. Also called deadband. That portion of a measuring systems response where a change in input does not produce a change in output.

Imbalance. Unequal distribution of weight or mass on a rotor. The geometric center of the mass is not where it should be: along the shaft centerline.

Impact. A collision between masses.

Impact test (bump test). A broad frequency range of structural responses is caused by a deliberate impact.

Impulse. The integral of force over a time interval.

Induced environments. Conditions generated by operating some equipment, as opposed to natural environments.

Inertance (or accelerance.) The ratio of acceleration to 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. The mechanical motion, force or energy applied to a mechanical system, e.g. the vibratory input from shaker to test item. Or an electrical signal, e.g. from a shaker controller to the power amplifier driving a shaker.

Input control signal. Originates in a control sensor; sometimes selected between or averaged between several sensors. Used to regulate shaker intensity. (May originate in a force sensor for force-controlled testing.)

Input Impedance. The shunt resistance and capacitance (or inductance) as measured at the input terminals, not including effects of input bias or offset currents.

Integrator. Circuitry which converts an acceleration signal to a velocity signal or a velocity signal to a displacement signal.

Integration. Representation inversely related to time rate of change. Example: integrating velocity yields displacement. In a computer, this is accomplished by dividing the velocity signal by j, where is frequency multiplied by 2.

Intensity. The severity of a vibration or shock. Nearly the same meaning as Amplitude, defined earlier, but less precise, lacking units.

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.

Iterative closed loop control precalculates 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.

Itch. Sometimes called glass itch or lace itch. An automotive sound caused by metal sliding on glass.

Jerk. The rate of change of acceleration with time.

Keyphasor. A signal used in rotating machinery measurements, generated by a sensor that observes a once-per-revolution event. Used in phase measurements for analysis and for balancing. Bentley-Nevada trade name.

LabView. A programming language (National Instruments) for developing data acquisition software.

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. See Patent Defect.

Lateral sensitivity. See transverse sensitivity.

Leakage. An unfortunate result of sampling with finite intervals. Results in smearing of frequency components. Improved by windowing (e.g. Hanning).

Level. The (usually base 10) logarithm of the ratio between a quantity and a reference quantity. For acoustic measurements the standard reference quantity is 20 micropascals. For acceleration measurements, the reference might be 1g or 1 micro-g.

Life Cycle Testing. Subjecting products 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 cycle history. A time history of events and conditions associated with an item of equipment from its release from manufacturing to its removal from service. The life cycle should include the various phases that an item will encounter it its life, such as: handling, shipping and storage prior to use; mission profiles while in use; phases between missions, such as standby time or storage, transfer to and from repair sites and alternate locations; and geographical locations of expected deployment.

Life units. A measure of use duration applicable to the item. Measures include time, cycles, distance, rounds fired, attempts to operate, etc.

Linear system. A system is linear if its magnitude of response is directly proportional to its magnitude of excitation, for every part of the system.

Linearity. The closeness of a calibration curve to a specified straight line, preferably passing through zero. Departures from that straight line are commonly specified as a % of full scale.

LN2. Liquid nitrogen, often used for rapid cooling of environmental test chambers.

Longitudinal or Compressional or Axial Vibration is in the same direction as the transfer of energy. Transverse Vibration is perpendicular to that direction.

Loudness. The human ranking of an auditory sensation, usually in terms ranging from soft to loud, expressed in sones (not in deciBels).

Machinery health monitoring (MHM). See Condition Monitoring

Magnetostriction. Slight changes in the dimensions of iron or steel components resulting from changes in the magnetic fields acting on these components.

Mass. A physical property, dynamically computed as acceleration divided by force. Statically computed as W (which can be measured on a butcher scale) divided by the acceleration due to gravity. Ordinary structures are not pure masses as they contain reactive elements, i.e. springs and damping.

Mean. A value intermediate between quantities under consideration. A shaker's mean acceleration must be zero - no steady-state acceleration. But a vehicle can have steady-state motion.

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.

Mechanical impedance. The ratio of force to velocity, where the velocity is a result of that force only. Its reciprocal is called mobility.

Mechanical failure. A malfunction consisting of cracking, excessive displacement, misalignment, loosening, etc.

MEMS - Micro Electromechanical 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. An instrument which converts a relatively small dynamic pressure change into an electrical signal. See transducer.

Mission Profile. A time-phased description of the events and environments an item experiences from initiation to completion of a specified mission, to include the criteria or mission success or critical failures.

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.

Mil. Length or displacement equal to 0.001 inch or 25.4 m m.

Modal analysis. The process of breaking complex structural motion into individual vibration modes. Resembles frequency domain analysis that breaks complex vibration down to component frequencies.

Mode. A characteristic pattern in a vibrating system. All points reach their maximum displacements at the same instant.

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 aluminum.

MTBF. Abbreviation for Mean (or average) time between failures.

MTTF. Mean-Time-To-Failure. A basic measure of reliability for nonrepairable items; the total number of life units of an item divided by the total number of failures within that population.

MUX. Multiplexer. A device that selects multiple inputs into an aggregate signal.

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 an undamped system's free vibration; also, the frequency of any of the normal modes of vibration.

Nonlinearity. The deviation from a best fit straight line of true output vs. actual value being measured.

Noise. The total of all interferences in a measurement system, independent of the presence of signal.

Noise Floor. The minimum discernible signal that can be detected by a receiver.

Notch. Minimum spectral value, at a natural frequency. Also, the deliberate reducing of a portion of a test spectrum (random vibration testing).

Octave. The interval between two frequencies differing by exactly 2:1.

OEM - original equipment manufacturer.

Open Loop control provides precomputed or preconceived 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 centerline during rotation. The orbit is usually observed on an oscilloscope connected to x- and y-axis displacement sensors. Sometimes called a Lissajous pattern.

Order. See First order vibration. Vibration at multiples of a machine's shaft speed.

Order tracking. Control of a computer's data sampling rate, such that display of vibrations at multiples of the shaft's rotating speed (x1, x2, x3, etc.) are enhanced and more easily.

Oscillation. Variation with time of a quantity such as force, stress, pressure, displacement, velocity, acceleration or jerk. Usually implies some regularity (as in sinusoidal or complex vibration).

Patent defect. A flaw (in a part or assembly) and/or workmanship that has failed under test or screen. See Latent Defect.

Peak. Extreme value of a varying quantity, measured from the zero or mean value. Also, a maximum spectral value.

Peak-to-peak value. The algebraic difference between extreme values (as D = 2X).

Performance-based requirements (specifications). Requirements that describe what the product should do, how it should perform, the environment in which it should operate, and interface and interchangeability characteristics. They should not specify how the product should be designed or manufactured.

Period. The interval of time over which a cyclic vibration repeats itself.

Periodic vibration. (See also Deterministic vibration.) An oscillation whose waveform regularly repeats. Compare with probabilistic vibration.

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 reference probe. A device for giving a once-per-shaft-revolution signal.

Pickup. See transducer.

Piezoelectric (PE) transducer. One which depends upon deformation of its sensitive crystal or ceramic element to generate electrical charge and voltage. Many present-day accelerometers are PE.

Piezoresistive (PR) transducer. One whose electrical output depends upon deformation of its semiconductor resistive element, offering greater resistance change than does the wire of a strain-gage transducer, for a given deformation.

Pitch. Rotation in the plane of forward motion, about the left-right axis. In music, relates to frequency.

Plastic Reinforcement. (example: inside a box section acting as a beam) raises local stiffness without adding much weight.

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.

Power spectral density or PSD. Describes the power of random vibration intensity, in mean-square acceleration per frequency unit, as g/Hz or m/s. Acceleration spectral density or ASD is preferred abroad.

Precision. The smallest distinguishable increment (almost the same meaning as resolution); deals with a measurement system's possible or design performance.

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.

Probabilistic vibration. (As compared to Deterministic vibration), one whose magnitude at any future time can only be predicted on a statistical basis.

Probe. A machinery-mounted sensor (usually internal). May sense vibration.

Proof of screen. A process aimed at showing that a screen is effective in identifying existing defects in a product. The screen must not damage good products.

Proximity sensor. Usually a displacement sensor for measuring the varying distance between a housing and a rotating shaft.

PSD. Power Spectral Density. The power of random vibration intensity in mean-square acceleration per frequency unit, as g/Hz or m/s.

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 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.

back to top back- R -

Radial. A direction perpendicular to a shafts centerline.

Random vibration. (See Probabilistic vibration.) One whose instantaneous magnitudes cannot be predicted. Adjective "Gaussian" applies if they follow the Gaussian distribution. May be broad-band, covering a wide, continuous frequency range, or narrow band, covering a relatively narrow frequency range. No periodic or deterministic components.

Range. A statement of the upper and lower limits over which an instrument works satisfactorily.

Rate gyro. A kind of gyroscope that measures rotational velocity (degrees or radians per second) around a fixed axis.

Rattle. A sound exemplified by shaking a steel can full of steel nuts and bolts.

Real-time closed loop control resembles iterative closed loop control but continuously modifies drive signals throughout the test.

Redundancy. The existence of more than one means for accomplishing a given function. Each means of accomplishing the function need not necessarily be identical. The two basic types of redundancy are active and standby.Active Redundancy - Redundancy in which all redundant items operate simultaneously.Standby Redundancy - Redundancy in which some or all of the redundant items are not operating continuously but are activated only upon failure of the primary item performing the function(s).

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?MIL-STD-721C offers (1) The duration or probability of failure-free performance under stated conditions. If you want to be complete, add (2) The probability that an item can perform its intended function for a specified interval under stated conditions. (For non-redundant items this is equivalent to definition (1). For redundant items this is equivalent to definition of mission reliability.)Atomica (formerly Guru.net) says "The extent to which an experiment, test, or measuring procedure yields the same results on repeated trials". This definition is found in a number of dictionaries.A student dictionary just says "dependable" when defining "reliable".Here are some examples I found on the Web: Rolls-Royce cars are famous for their quality and reliability. My car is seven years old but it's still fairly reliable. The reliability of this smoke detector is guaranteed.

Reliability engineering. Reliability engineering is the doing of those things which insure that an item will perform its mission successfully. The discipline of reliability engineering consists of two fundamental aspects:(1) paying attention to detail(2) handling uncertainties

Repeatability. (1) The maximum deviation from the mean of corresponding data points taken under identical conditions. (2) The maximum difference in output for identically-repeated stimuli (no change in other test conditions). Do not confuse with accuracy.

Repetitive shock machine. A platform to which products (to be tested or screened) are attached. 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.

Response. The vibratory motion or force that results from some mechanical input.

Response signal. The signal from a "response sensor" measuring the mechanical response of a mechanical system to an input vibration or shock.

Resolution. The smallest change in input that will produce a detectable change in an instrument's output. Differs from precision in that human capabilities are involved.

Resonance. Forced vibration of a true SDoF system causes resonance when the forcing frequency equals the natural frequency, when any forcing frequency change decreases system response. (See also critical frequency) Therefore resonance represents maximum sprung mass response, if forcing frequency is varied while input force is held constant. More complex systems have many resonances.

Ringing. Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked.

Rise time. 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 in the measurand.

RMS or Root-Mean-Square value. The square root of the time-averaged squares of a series of measurements. Refer to a textbook on electrical engineering. In the exclusive case of a sine wave, s, the RMS value, is 0.707 the peak value.

rms Responding. A measurement equal to the rms value of the input signal for all waveforms within the specified frequency range and crest factor limit.

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.

Sampling rate. The number of readings an A/D converter takes per second or per minute.

Screening. The process of stressing products so that defective units can be identified, then repaired or replaced. A winnowing out.

Seismic. Having to do with earth motion, as earthquakes. A kind of sensor that depends upon the inertia of an internal mass to generate a signal, as an accelerometer or velocity pickup.

Sensitivity. Of a mechanical-to-electrical sensor or pickup, the ratio between electrical signal (output) and mechanical quantity (input)..

Sensor. (See Transducer.)

Self-induced vibration. Also called self-excited vibration, results from conversion of non-oscillatory energy into vibration, as wind exciting telephone wires into mechanical vibration.

Shock machine. Or shock test machine, a device for subjecting a system to controlled and reproducible mechanical shock pulses.

Shock pulse. An event that transmits kinetic energy into a system in a relatively short interval compared with the system's greatest natural period. A natural decay of oscillatory motion follows. The event is usually displayed as a time history, as on an oscilloscope.

Shock pulse, classical. Classical shock pulses (mechanical shock machine). Unless the procedure requires the use of a classical shock pulse, the use of such a pulse is not acceptable unless it can be demonstrated that measured data is within the tolerances of the classical shock pulses. Only two classical shock pulses are defined for testing in the method the terminal peak sawtooth pulse, and the trapezoidal pulse. The terminal peak sawtooth pulse along with its parameters and tolerances are provided on Figure 516.6-10, and is an alternative for testing in Procedure I - Functional Shock and Procedure V - Crash Hazard Shock Test.

The trapezoidal pulse along with its parameters and tolerances is provided on Figure 516.6-11, and is an alternative for testing in Procedure II - Materiel to be Packaged, and Procedure III - Fragility.

Shock pulse, haversine. A practical variation on the obsolete half-sine pulse, whose abrupt transitions at beginning and end cannot be achieved in test labs. Practical testing requires some rounding, and the result in called a haversine pulse. Another definition: an inverted cosine offset by half its amplitude. A continuous haversine resembles a sine wave.

Shock response spectrum (or SRS). A plot of maximum responses of imagined SDoF systems vs. their natural frequencies, as they respond to an applied shock.

Signal conditioner. An amplifier following a sensor, which prepares the signal for succeeding amplifiers, transmitters, readout instruments, etc. May also supply sensor power.

Simple harmonic motion. Periodic vibration that is a sinusoidal function of time.

Single-Ended Input. An analog input measured with respect to a common ground.

Slew rate. The maximum rate at which an instrument's output can change by some stated amount.

Sneak circuit analysis. An analytical procedure for identifying latent paths that cause occurrence of unwanted functions or inhibit desired functions, assuming all components are operating properly.

SNR. Signal-to-Noise Ratio. The ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time. The larger the number the better.

Solid-state sensor. One which has no moving parts.

Sone. A unit of loudness.

Soft failure. A product under test ceases to operate correctly, but resumes correct operation when the stressing environment is eased. Differs from hard failure.

Sound. (1.) An oscillation in pressure, capable of evoking the sensation of hearing. (2.) 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 20 log10 of the ratio between a given sound pressure and 20 micropascals.

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.

Specifications. Documents (the USA military service once had as many as 28,000 specifications) that describe products or services, for the purpose of procurement. Differs from standards.

Spectral map. A three-dimensional plot of amplitude (Z axis) vs. time (or shaft speed) (Y axis) vs. frequency (X axis)

Spectrum analyzer. An instrument which displays the frequency spectrum of an input signal, usually amplitude vertical vs. frequency horizontal.

Squeak. A sound resembling that of an unlubricated hinge. A sound made by PVC or other material rubbing on glass, i.e. a windshield.

Spectrum. See frequency spectrum.

Standards. Documents (the USA military services had as many as 1,700 standards) that prescribe engineering disciplines, impose particular management practices, reporting and auditing requirements. Differ from specification.

Standard deviation. A statistical term: s, the square root of the variance s2, i.e., the square root of the mean of the squares of the measured deviations from the mean value.

Standing wave. A wave that is characterized by lack of vibration at certain points (nodes), between which are areas of maximum vibration (antinodes). Standing waves are produced at certain forcing frequencies when the resulting resonant vibratory response is confined within boundaries, as in the vibrating string of a musical instrument or the wing of an airplane or the whip antenna on your automobile. Also called "stationary wave".

Stationarity. A property of probabilistic vibration if the PSD (or ASD) and the probability distribution remain constant.

Steady state vibration. Periodic vibration for which the statistical measurement properties (such as the peak, average, RMS and mean values) are constant.

Stepped sine testing. Sine shaking in a series of dwells. Frequency is incrementally increased or decreased.

Step stressing. Increasing stresses in a series of preselected increments.

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. In an unbonded wire strain-gage accelerometer, inertia affects a mass supported by nichrome wires; the wires change resistance in proportion to acceleration. The term may include piezoresistive accelerometers.

Stress. Intensity of applied load, usually at the site of a failure.

Stress Screening. A modern electronics production tool for precipitating latent defects such as poorly-soldered connections. Utilizes random vibration + rapid temperature ramping.

Subharmonic. A sinusoidal quantity having a frequency that is an integral submultiple (1/2, 1/3, etc.) of a fundamental (1) frequency.

Subsychronous. Components of a vibration signal whose frequency is less than 1x shaft speed.

Supply current. The typical current that must be supplied to a sensor (along with the supply voltage) to make it function properly.

Swept-sine testing. Sine shaking whose frequency is smoothly and continuously varied. Commonly required for sequentially identifying resonances. By contrast, see stepped sine testing.

Synchronous. Vibration components (on rotating machinery) that are related to shaft speed.

Synchronous sampling. Control of a computer's rate of data sampling to achieve order tracking.

System. A composite of equipment and skills, and techniques capable of performing or supporting an operational role, or both. A complete system includes all equipment, related facilities, material, software, services, and personnel required for its operation and support to the degree that it can be considered self-sufficient in its intended operational environment.

System effectiveness. (a) For repairable systems and items: the probability that a system can successfully meet an operational demand within a given time when operated under specified conditions. (b) For "one-shot" devices and non-repairable items: the probability that the system will operate successfully when called upon to do so under specified conditions.

TAAF. Test, analyze and fix. See Accelerated Life Testing.

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.

Temperature range. The temperatures between which a sensor will operate accurately.

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 predetermined 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.

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

Thrust position. Location in direction of a shaft centerline. See axial position.

Tracking filter. A narrow bandpass filter whose center frequency follows an external synchronizing signal.

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.

Transducer (or pickup or sensor). A device which converts some mechanical quantity into an electrical signal. Less commonly, the reverse conversion.

Transient vibration Short-term vibration of a mechanical system.

Transmissibility. In steady-state vibration, Tr is the non-dimensional ratio of response motion/input motion: two displacements, two velocities or two accelerations. The maximum Tr value is the mechanical "Q" of a system. At resonance, Tr is maximum.

Transverse sensitivity. The unfortunate sensitivity of a sensor (or pickup or transducer) in a direction perpendicular to the advertised or stated sensitive axis. Also called cross-axis or lateral sensitivity.

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

Unbalance (imbalance). Unequal mass distribution on a rotor. The mass centerline does not coincide with the rotation or geometric centerline.

UUT. Unit under test. See also DUT or device under test (being tested).

Velocity. Rate of change of displacement with time, usually along a specified axis; it may refer to angular motion as well as to uniaxial motion.

Vibration. Mechanical oscillation or motion about a reference point of equilibrium.

Vibration machine (or exciter or shaker). A device which produces controlled and reproducible mechanical vibration for the vibration testing of systems, components and structures.

Vibration meter. An apparatus (usually an electronic amplifier, detector and readout meter) for measuring electrical signals from vibration sensors. May display displacement, velocity and/or acceleration.

Vibromyography. Also called mechanomyography or acoustic myography. The measurement of muscle vibration when a muscle contracts.

Waterfall plot. A series of spectral maps taken at regular intervals of time, regularly spaced shaft speeds, etc. Early maps move down the display, followed by later maps, something like the flow of a waterfall.

Wave. Nature's mechanism for transporting energy without transporting matter.

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.

Wavelet, as used for shock testing on shakers, are half-sine windowed (amplitude modulated) sinusoids of specific frequencies, with the window chosen so the resulting waveforms have an odd number of half cyles. Usually, ten to thirty wavelets, each with different basis frequencies, e.g., 10 Hz to 2000 Hz, and different numbers of half cycles, e.g., three to 31 or so, are superimposed to synthesize a transient oscillatory signal (pulse) with a prescribed Shock Response Spectrum that can be used as a reference waveform for a shaker shock test.

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

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. That broad-band random vibration in which the PSD (ASD) is constant over a broad frequency range.

\Yaw. Rotation about the vertical axis.

Zwicker loudness. A sound measurement methodology. Loudness is measured in linear units called sones as opposed to decibels.

Zero-G drift. The amount that a sensor's no-acceleration signal shifts over some temperature range.

Zero-G output. The accelerometer output that is read when the sensor is not accelerating.

Zero-to-peak. See peak, as in peak value. Half of the peak-to-peak value.