vibration shock glossary

We offer engineers, technicians and others an extensive glossary of terms relating to vibration and shock testing, measurements, analysis and calibration.

Select the first letter of the word from the list below to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the ‘#’ link.


– A –

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

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

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 rotor’s position or displacement along its axis,
relative to some nearby fixed point.

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– B –

(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

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).

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– C –

(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

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. Today’s 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

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

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.

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– D –

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 automobile’s 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
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

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 jΩ, where Ω is frequency
multiplied by 2Π.

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.

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– E –

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

Excitation. The voltage or current applied
to a transducer.

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– F –

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
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

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

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

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

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

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

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– G –

g The
acceleration produced by Earth’s gravity. By international
agreement, the value for 1 gravitational unit is 9.80665 m/s²
= 386.087 in/sec² = 32.1739 ft/sec².

gal. 1 gal (seems to mainly be
used in Japan) is an acceleration of 1 cm/sec².
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

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/s²
or 386.087 in/sec² or 32.1739 ft/sec²).

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.

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– H –

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)

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 system’s response where a
change in input does not produce a change in output.

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– I –

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

Impact. A collision between masses.

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

Impulse. The integral of force
over a time interval.

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

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 jΩ, where Ω is frequency multiplied
by 2Π.

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.

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– J –

The rate of change of acceleration with time.

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– K –

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.

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– L –

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

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.

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).

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– M –

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

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.

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– N –

Natural environments.
Conditions occurring in nature, not caused by any equipment;
effects are observed whether an equipment is at rest or in

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

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).

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– O –

The interval between two frequencies differing by exactly

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).

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– P –

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

Power spectral density or PSD.
Describes the power of random vibration intensity, in mean-square
acceleration per frequency unit, as g²/Hz or m²/s³. 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

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 g²/Hz or m²/s³.

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– Q –

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.

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– R –

Radial. A
direction perpendicular to a shaft’s 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 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
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.

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– S –

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

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

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

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

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.

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– T –

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

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

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.

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– U –

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).

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– V –

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.

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– W –

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.

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– X –


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– Y –

Rotation about the vertical axis.

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– Z –

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.

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– # –

6DOF machines.
Often called Repetitive Shock Machines. That class of non stationary random
vibration producing machines that use air-driven impact hammers
to excite a vibrating plate or table onto which products are either
attached directly or fixtured. The resulting excitation is
a combination of impulse and table modal response, or an intermodulation of both.