Wörterbuch
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Sabin
A measure of the sound absorption of a surface; it is the equivalent of one square foot of a perfectly absorptive surface. One square foot of open window has an absorption of 1 sabin. Note: One metric sabin is equivalent to 1 sq. metre of perfectly absorptive surface.Sabine
The originator of Sabine reverberation equation.Sag
Deflection due to gravity acting on a cantilevered or otherwise supported object. Mechanical brackets that hold alignment tools always sag a certain amount. This sag must be corrected if the machine movements are to be calculated correctly.Samples
Sets of measured data, that is, one or more measured records.Sampling Frequency
How often data is measured. Given in hertz.Sampling Interval
The period of time that data is measured for at each sample point.Sampling Theorem
A theorem stating that a signal is completely described if it is sampled at a rate twice its highest frequency component.Scaling
Peak Scaling, Peak-to-Peak Scaling, and RMS Scaling are ways of displaying the amplitude axis of a spectrum.Scan Analysis
A non-stationary signal, or part of it, is recorded in a time buffer and afterwards analysed by stepping a Hanning window along the record.Schmitt Trigger
A device used for filtering unstable tacho pulses to give precise on-off control.The scientific format (Sci.) is used to set up the spectrum values with an exponent, for example, 10 E3.
Selectivity
Indicates how well a filter separates components of very different levels. The ratio of a filter’s –3 dB Bandwidth to its –60 dB Shape Factor. Selectivity is also referred to when discussing time window weighting, as a time window is effectively a type of filter. Selectivity is a measure of the narrowness of a band-pass filter. The greater the selectivity, the narrower, or more selective, the filter.Self-test
A built-in routine to check that all the required functions are operating properly.Semantic Differential Test
A test method in which recorded sounds are listened to and judged one at a time.Semi-anechoic Field
A free field above a reflective plane.Semicircular Canals
The three sensory organs for balance that are a part of the cochlea structure.Sensation Level (SL)
The level of a sound above the threshold of hearing for the same sound expressed in decibels.Sensor
Any device that translates the magnitude of one quantity into another quantity. Three of the most common transducers used in vibration measurements are accelerometer, velocity transducer, and eddy current probe.Sensorineural Hearing Loss
Hearing loss caused by damage to the hair cells of the cochlea or the auditory nerve.Sequence (maximum length)
A mathematical sequence used in determining the well depth of diffusers.Sequence (primitive root)
A mathematical sequence used in determining the well depth of diffusers.Sequence (quadratic residue)
A mathematical sequence used in determining the well depth of diffusers.Servo Force Balance Accelerometers
For measurements in the range of a few milli-g, with DC response, servo force balance accelerometers have long been the only practical instrument. They indirectly measure the amplitude of force necessary to balance the inertial forces on a proof mass caused by acceleration. Servo accelerometers have traditionally used electromagnetic effects to provide the balancing force, and measured the electrical current necessary to maintain the mass position stationary relative to the instrument housing, as the instrument accelerates. They usually use a very soft pendulous suspension for the mass, and are thus relatively fragile.Servo System
An automatic feedback control system in which the controlled variable is the mechanical position, or any derivative of this parameter.Shadow Zone
An area below which sound waves have bent upwards because of atmospheric conditions. Sound sources will not be as loud as expected.Sharpness
A measure of the excessive high-frequency content in a signal. For example, white noise has more high-frequency level than pink noise and a higher sharpness value. It is more unpleasant to listen to. See also Aures Sharpness Calculation and Widmann Sharpness.Shielding
The attenuation of a sound achieved by placing barriers between a sound source and a receiver.Shim Machine
The machine whose position is changed during shaft alignment.Shock
Rapid transient transmission of mechanical energy.Side Lobe
When a weighting window is applied to a frequency domain function, side lobes can be seen on either side of the centre frequency. The dominance of these depends on the type of window in use.Side-lobe Fall Off Rate
The rate at which side-lobes fall off, normally quoted in dB/decade.Sideband
In frequency domain functions, pairs of frequencies with similar amplitude that appear equally spaced on either side of a centre frequency.Signal
An electric voltage or current that is an analog of the vibration or sound being measured.Signal Enhancement
If a part of a repetitive signal that has been identified by a trigger is averaged in the time domain, the signal is conserved while the additive noise cancels out. The main applications are waveform analysis, reduction of background noise, enhancement of orders, and separation of mechanical and electrical vibrations.Signal Ground Coupling
Determines whether an acquisition front-end’s channels are earthed. Grounding sets a channel for single-ended grounding, while floating disconnects the screens and ground connections.Signal Groups
User-defined collections of signals. A group can be treated as a single entity allowing the same measurement criteria and functions to be applied to each signal in the group in a single operation.Signal Names
A signal name is the name given to a user-defined analysis channel. This name is user-definable, allowing descriptive names that represent the measurement situation to be used.Signal-to-noise Ratio
The difference between the nominal or maximum operating level and the noise floor in dB.Signals
To analyse a physical input, a signal must be defined on the (physical) channel concerned. A signal allows a number of different types of analyses to be made on the same channel with the same pre-processing applied.Simple Harmonic Motion
A periodic motion whose displacement varies as a sinusoidal function of time.Simulation Prediction
In structural dynamic testing, the ultimate goal of system identification is to create a dynamic mathematical model. This, in turn, may be used in simulations or to predict answers to specific questions.Sine Wave
A signal that follows the path of a sine function. This can be a swept or have a fixed frequency. A sweep can be either linear or logarithmic. A periodic wave related to simple harmonic motion.Single Degree of Freedom (SDOF)
A system where only one input can vary and is sufficient to describe the state of the system. SDOF systems are a very important concept for modelling structures, and often provide a sufficient model for a limited frequency band around a mode.This describes a model where only one degree-of-freedom is included. This is used in both dynamic and measurement models.
Slap Back
A discrete reflection from a nearby surface.Smearing
This occurs when, for example, the speed of a machine is not stable while measurements are being made.Sociocusis
Loss of hearing caused by noise exposures that are part of the social environment, exclusive of occupational-noise exposure, physiological changes with age, and disease.SONAH
An abbreviation for Statistically Optimal Near-field Acoustic Holography, which is an implementation of Acoustical Holography not using spatial Fourier transforms. See also Acoustic FRF Sone
A linear unit of loudness. The ratio of loudness of a sound to that of a 1 kHz tone 40 dB above the threshold of hearing. One sone is the loudness of a sound whose loudness level are 40 phons. Loudness is proportional to the sound’s loudness rating, for example, two sones are twice as loud as one sone. The unit of measurement for subjective loudness.Sound
Mechanical disturbance, propagated in an elastic medium, of such character as to be capable of exciting the sensation of hearing. The propagation of a sound wave can be measured in terms of the fluctuations in pressure with which it is associated. The measure of sound pressure is commonly the root mean square (RMS) value but may also be the instantaneous or peak value.Sound Absorption
The product of absorption coefficient and surface area of a material. The unit is the sabin. Designates the amount of sound absorbed by a material.Sound Absorption Coefficient
The practical unit between 0 and 1 expressing the absorbing efficiency of a material. It is determined experimentally.Sound Attenuation in Air
Approximate correction for air attenuation including the inverse square law.
Sound Concentration
The focusing of sound waves caused by reflections from a concave surface, or any other surface that focuses the sound waves in a specific direction.Sound Exposure (E)
The quantity of sound impinging on the ear over an interval of time. It is directly proportional to the A-weighted sound intensity (that is, proportional to the square of A-weighted RMS sound pressure) and directly proportional to the duration of the sound. The SI unit is Pa2.s and the practical unit for sound exposure meters is Pa2.h. For a given duration, each increase of 10 dB(A) in sound pressure level corresponds to a tenfold increase in E. A sound of 85 dB(A) lasting for 8 hours (h) produces 1 Pa2.h of sound exposure.Sound Exposure Level (LEX, ref. duration)
The A-weighted sound exposure expressed in decibels relative to a reference duration. In the special case of constant noise that lasts for the reference duration, it is numerically equal to the sound pressure level (Leq). Different terms have been used to define precisely the same quantity.Sound Exposure Meter
A small instrument designed to be worn by an individual to provide a measure of the accumulated sound exposure received by the wearer while moving about during the workday. The instrument is calibrated in Pa2.h. If the meter is worn for only a representative part of the working day, the reading must be corrected appropriately. Noise dosemeter (or dosimeter) is a more general term for instruments having a similar purpose but not necessarily measuring the physical quantity E or being calibrated in percentage of maximum permitted daily exposure and therefore can vary from country to country.Sound Fields
The above graph illustrates the fundamentals of noise generation and propagation that must be kept in mind when measuring noise emission. In the near field, the shaded area shows that noise emission cannot be measured reliably. But further away in the far field, measurements are reliable and the level decreases 6 dB per doubling of distance (spherical spreading due to inverse square law) as long as the environment is effectively free field. When the environment becomes semi-reverberant due to reflections, which add to the level of the direct sound wave, noise emission measurements again become unreliable.
Sound Intensity
The rate of sound energy transmission per unit area in a specified direction. The product of the particle velocity and pressure; it is specified in units of power per unit area and is a vector quantity describing the level and direction of the acoustic energy.Sound Intensity Probe
Used to determine the sound power. Brüel & Kjær’s sound intensity probes consist of two closely spaced microphones. This enables the pressure gradient to be measured.Sound Level
The weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B, or C as specified in ANSI specifications for sound level meters (ANSI Sl.4-1971, or the latest approved revision).Sound Level Meter (SLM)
An instrument, usually hand-held, designed to measure a frequency-weighted value of the sound pressure level in accordance with an accepted national or international standard. It consists of a microphone, amplifier, square-law rectifier, averaging circuits and indicating instrument, having a specified performance in respect of directivity, frequency response, rectification characteristic, and time-weighted averaging. The instrument is normally equipped with F, S and possibly (time-weightings as an aid to measuring fluctuating sounds. With suitable circuitry it can also perform frequency analysis, typically either with octave or one-third octave bands. See also Integrating-averaging sound level meter, Time weighting.Sound Level Meter Weighting Networks
Sound Level Prediction in Semi-reverberant Fields
Sound Power
The total sound energy radiated by a sound source per unit time. The unit of measurement is the watt.Sound Power Level (LW or PWL)
The level, in dB, at which a source produces sound energy per unit of time, usually given in octave bands. A power expressed in dB above the standard reference level of 1 picowatt. Sound Power Level = 10 log10 (W/W0), where W is the emitted power and W0 is the reference power (10-12 W).Sound Pressure
The instantaneous difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space. A dynamic variation in atmospheric air pressure. It is a scalar entity describing the level of the sound pressure. At a point in a medium, the difference between the pressure existing at the instant considered and the static pressure. Sound pressure is measured in pascals (Pa), 1 Pa = 1 newton per square metre (N/m2).Sound Pressure Level (SPL)
Sound Pressure Level = 20 log10 (p/p0) dB: The sound pressure level of a sound in decibels, is equal to 20 times the logarithm to base 10 of the ratio of the RMS sound pressure to the reference sound pressure 20 mPa (2 × 10-5 Pa).Sound Quality Metrics
Objective algorithms such as loudness, sharpness, etc., that are used to characterise sound quality.Sound Spectrograph
An instrument that displays the time, level, and frequency of a signal.Sound Transmission Class (STC)
A single-number rating system used to compare the sound-isolating characteristics of partitions used to separate occupied spaces. The preferred single figure rating system designed to give an estimate of the sound insulation properties of a structure or a rank ordering of a series of structures. Used to define sound transmission loss of a wall or partition. Expressed in decibels, it is 10 times the logarithm to base 10 of the reciprocal of the sound transmission coefficient of the configuration.Sound Transmission Loss
Ratio of the sound energy emitted by an acoustical material or structure to the energy incident upon the opposite side.Source Group
A group of related source points.Source Point
A particular position (and in some cases, direction) on the structure where an excitation point is applied. The excitation is either a force (structure-borne contribution) or volume velocity (airborne contribution). Example: “Front Engine Mount Vertical 2Z-”.Source Substitution method
A method of analysing airborne contributions, wherein a FRF matrix is used to estimate operating point source strengths. Also referred to as indicator method.Source-to-Indicator Matrix
A matrix of transfer functions (Hxv) used to estimate operating source strengths from indicator operating dataSpacers
A generic term for any coupling that has 2 flex planes separated by a connecting shaft without bearings or other supports (between the flex points). Sometimes called an insert or spider.SPC
Source Path ContributionSpecific Noise
Noise from the source under investigation. Specific noise is a component of Ambient Noise and can be identified and associated with the specific source. See also Residual Noise.Spectral Lines
The number of constant bandwidth lines used in the measurement of spectra.Spectrum
The spectrum is the result of transforming a time domain signal to the frequency domain. It is the decomposition of a time signal into a collection of sine waves. The plural of spectrum is spectra. Spectrum analysis is the procedure of doing the transformation, and it is most commonly done with an FFT analyzer.Spectrum Analyzer
An instrument for measuring, and usually recording, the spectrum of a signal. A spectrum analyzer converts a signal from the time domain into the frequency domain, and the FFT analyzer is the most common type today, but there are many other types.Spectrum Comparison
The display and examination of two or more spectrum for similarities between them.Specularity
A term devised to express the efficiency of diffraction-grating types of diffusers.Speech Audiometry
The presentation of speech material (usually word lists) to determine the percentage of material correctly received. In the simplest forms, listening is monaural by earphone in quiet to recorded material. Variations include live-voice presentation, free-field binaural listening, added noise, etc. In all forms, the speech level or speech/noise ratio is varied to plot a speech audiogram from which various measures are derived, principally speech reception threshold (level at which a defined percentage correct score is obtained, usually 50%), and optimum discrimination score (the maximum percentage correctly heard).Speech Interference Level (SIL)
A calculated quantity providing a guide to the interference of a noise with the reception of speech. The speech-interference level is the arithmetic average of the octave band levels of the interfering noise in the most important part of the speech frequency range. The levels in octave bands centred at 500, 1000, and 2000 Hz are commonly averaged to determine the speech-interference level.
|
m/s |
| Air, 21°C |
344 |
| Alcohol |
1213 |
| Lead |
1220 |
| Hydrogen, 0ºC |
1269 |
| Water, fresh |
1480 |
| Water, salt, 21ºC |
1520 at 3.5% salinity |
| Human body |
1558 |
| Plexiglas |
1800 |
| Wood, soft |
3350 |
| Concrete |
3400 |
| Mild steel |
5050 |
| Aluminium |
5150 |
| Glass |
5200 |
Components in a spectrum that are dependent upon the speed of the object under test.
Speed (Velocity) of Sound in Air
344 m/s at 21°C (1128 ft/s at 70°F) in air at sea level.Spherical Divergence
The condition of propagation of spherical waves that relates to the regular decrease in intensity of a spherical sound wave at progressively greater distances from the source. Under this condition the sound pressure level decreases 6 decibels with each doubling of distance from the source. Sound diverges spherically from a point source in free space.Spherical Wave
A sound wave in which the surfaces of constant phase are concentric spheres. A small (point) source radiating into an open space produces a free sound field of spherical waves.Splaying
Walls are splayed when they are constructed somewhat “off square”, that is, a few degrees from the normal rectilinear form.SPR model
An SPC “template” representing the test vehicle (or part of vehicle) decomposed into source, receiver and operating condition points/nodes. During the course of testing and analysis, the model will be filled with dataSpring Constant of Materials (in compression)
where:
- E = elastic modulus
- A = area of material
- t = thickness of material
Standard Deviation
If the instantaneous distances from an equilibrium position of a vibrating body are squared and averaged, the result is called the variance of the vibration. The square root of the variance is the standard deviation. It is also equal to the rms (root mean square) value.Standing Wave
A periodic wave having a fixed distribution in space that is the result of interference of progressive waves of the same frequency and kind. Characterised by the existence of maxima and minima amplitudes that are fixed in space.Static Pressure vs. Altitude
Stationary Operating Condition
An Operation Deflection Shape is not unique in the same sense that a Mode shape is. It is dependent on the operating condition and the choice of frequency and is thus only valid for one particular stationary condition.Stationary Signal
A stationary signal is a signal whose average statistical properties over a time interval of interest are constant, and it may be deterministic or not. In general, the vibration signatures of rotating machines are stationary. Stationary signals are either deterministic or random.Steady-State Sounds
Sounds whose average characteristics remain relatively constant in time. A practical example of a steady-state sound source is an air conditioning unit.Stereo
A stereophonic system with two channels. See Binaural.Uses constant bandwidth analysis. This is preferable for vibration analysis applications.
Stiffness
Ratio of the change in force to the corresponding change in displacement of an elastic element.Stiffness Matrix
In modal testing, this represents the coefficients of the restoring forces.Stopband Frequencies
Frequencies that are outside those of the selected region.Strain
Strain is defined as extension divided by original length.Stress
Stress is defined as force divided by area.Structural Integrity
This is achieved using modal parameter monitoring. The modal parameters of a structure are like a fingerprint. If a change in the structure occurs, for example, due to structural damage, it is instantaneously reflected in the modal parameters in terms of a change in the modal frequencies. Advanced techniques combining the mass or stiffness matrix data obtained from a finite element method analysis may be used to localise and quantify the damage.Structural Modification
Mathematically determining the effect of changing the mass, stiffness, or damping of a structure and determining its new modal parameters. A modal analysis provides, in essence, a mathematical model of the structure. This model can be manipulated to determine the effect of modifications to the structure. The modal model can be generated either experimentally or using a finite element program.Structure-borne Contribution
Vibrational noise from structure-borne sources (engine, gearbox, exhaust line, etc.) that is part of the total sound heard in a vehicle’s interiorStructure-borne Sound
Sound that radiates from a construction assembly after travelling through a building's structure in the form of vibration.STSF uses scanned cross-spectrum measurements to create a graphical representation of a sound field. The main application areas are the automotive industry (engine tests, whole vehicles on dynamometers and tyre noise, domestic and industrial goods industries.
Sub Harmonic
Sub harmonics are synchronous components in a spectrum that are multiples of 1/2, 1/3, or 1/4 of the frequency of the primary fundamental. They are sometimes called “sub-synchronous” components. In the vibration spectrum of a rotating machine, there will normally be a component at the turning speed along with several harmonics of turning speed. If there is sufficient looseness in the machine so that some parts are rattling, the spectrum will usually contain sub harmonics. Harmonics of one-half turning speed are called “one-half order sub harmonics”, etc.Sub-synchronous
Frequencies in a vibration spectrum that are lower than the fundamental frequency.Superposition
Many sound waves may traverse the same point in space, the air molecules responding to the vector sum of the demands of the different waves.Sweep Random Vibration Test
A test in which a specimen is excited at a constant acceleration spectral density level in a narrow frequency band, the centre frequency of which is swept (varied) up and down.Symbols (commonly used)
|
a(t) |
(input) time signal |
|
ã(t) |
Hilbert transform of a(t) |
|
â(t) |
enhanced (input) time signal |
|
ap(t) |
(input) time signal made periodic by FFT |
|
A(f) |
frequency spectrum of a(t) |
|
Ã(f) |
Hilbert transform of A(f) |
|
Â(f) |
spectrum of a(t) |
|
Ap(f) |
frequency spectrum corresponding to periodic input |
|
A |
amplitude |
|
a |
acceleration |
|
A0 - A9 |
1k FFT spectra in 10k FFT zoom |
|
B |
bandwidth |
|
b(t) |
output time signal |
|
B(f) |
output Fourier spectrum |
|
B3dB |
3dB bandwidth |
|
B60dB |
60dB bandwidth |
|
BNoise |
noise bandwidth |
|
c |
external sampling frequency/internal sampling frequency |
|
c |
damping coefficient |
|
C80 |
f80dB/fc |
|
c |
capacitance |
|
CAA(t) |
(input) cepstrum |
|
CBB(t) |
output cepstrum |
|
CAB(f) |
coincident part of the cross-spectrum |
|
D |
decimation factor |
|
f |
frequency |
|
Df |
line spacing |
|
fs |
sampling frequency |
|
f1 |
pitch frequency |
|
ffund |
fundamental frequency |
|
F |
multiplication factor |
|
fspan |
frequency span of analyzer |
|
fN |
Nyquist frequency |
|
f(t) |
force signal |
|
F(f) |
force spectrum |
|
f0 |
centre frequency |
|
fu |
upper limiting frequency |
|
fl |
lower limiting frequency |
|
fc |
Cut-off frequency |
|
fcar |
carrier frequency |
|
fk, fl |
discrete frequency variable |
|
fstart |
sweep start frequency |
|
fstop |
sweep stop frequency |
|
freal-time |
real-time bandwidth |
|
G(f) |
glottis spectrum |
|
GAA(f) |
(input) autospectrum |
|
GBB(f) |
output autospectrum |
|
GAB(f) |
cross-spectrum |
|
GAAL(f) |
liftered (input) spectrum |
|
GBBL(f) |
liftered output spectrum |
|
GBC(f) |
complex spectrum |
|
GVV(f) |
coherent power |
|
GNN(f) |
noncoherent power |
|
GXiXj |
input cross-spectrum |
|
GXiXi |
input autospectrum |
|
GXiYj |
input/output cross-spectrum |
|
GYiYj |
output cross-spectrum |
|
GYiYi |
output autospectrum |
|
H(f) |
frequency response function |
|
H1(f), H2(f), H3(f) |
different estimators of H(f) |
|
h(t) |
impulse response function |
|
h1(t), h2(t), h3(t) |
different estimators of h(t) |
|
I |
sound intensity |
|
Ir |
sound intensity in the r direction |
|
I0 |
sound intensity reference |
|
Ia |
absorbed sound intensity |
|
Ii |
incident sound intensity |
|
Ir |
reflected intensity |
|
k |
stiffness |
|
K |
pressure intensity index |
|
k |
wave number |
|
Lp |
sound pressure level |
|
Ll |
sound intensity level |
|
LW |
sound power level |
|
m |
mass |
|
M |
zoom factor |
|
M(f) |
input noise spectrum |
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m(t) |
input noise signal |
|
N |
transform size |
|
NA |
averaging number |
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n1 |
line number for fundamental frequency |
|
Nspan |
number of lines in fspan |
|
n(t) |
(output) noise signal |
|
N(f) |
(output) noise spectrum |
|
Nx |
number of input signals |
|
Ny |
number of output signals |
|
p |
sound pressure |
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p0 |
sound pressure reference |
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pA |
sound pressure of the channel A signal |
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pB |
sound pressure of the channel B signal |
|
P |
sound power |
|
P0 |
sound power reference |
|
p(a) |
amplitude probability density of a(t) |
|
q |
number of bits |
|
QAB(f) |
quadrature part of the cross-spectrum |
|
Q |
quality factor |
|
Dr |
distance? |
|
R |
residue |
|
R |
resistance |
|
Raa(t) |
autocorrelation function of a(t) |
|
Rab(t) |
crosscorrelation function between a(t) and b(t) |
|
r |
radius |
|
Sij |
power spectrum (instantaneous autospectrum) |
|
S |
surface area |
|
SAA(f) |
two-sided (input) autospectrum |
|
SBB(f) |
two-sided output autospectrum |
|
SAB(f) |
two-sided cross-spectrum |
|
S(f) |
speech spectrum |
|
S/N |
signal-to-noise ratio |
|
T |
record length |
|
TA |
averaging time |
|
TR |
filter response time |
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Tco |
cut-off time |
|
Dt |
sampling interval |
|
T1 |
pitch time |
|
Ti |
initial reverberation time |
|
Tp |
time period |
|
t |
time variable |
|
t |
temperature |
|
T0 |
1/f0 |
|
ti, tm, tn |
discrete time variables |
|
TAN |
analysis time |
|
Td |
pulse duration |
|
u |
instantaneous particle velocity |
|
ur |
instantaneous particle velocity in the r-direction |
|
V |
volume |
|
v |
velocity |
|
v0 |
velocity of hammer before contact |
|
w(t) |
weighting function |
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W(f) |
weighting spectrum |
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x(t) |
displacement |
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xi(t) |
input time signal (multi-channel analysis) |
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X(f) |
displacement spectrum |
|
Xi(f) |
input Fourier spectrum (multi-channel analysis) |
|
yi(t) |
output time signal (multi-channel analysis) |
|
Yi(f) |
output Fourier spectrum (multi-channel analysis) |
|
a |
Ia/Ii |
|
D1(t), D2(t) |
time signals consisting of d functions |
|
D1(f), D2(f) |
frequency spectra consisting of d functions |
|
g2 |
coherence |
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e1 |
GMM/GUU input error |
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e0 |
GNN/GVV output error |
|
er |
normalised standard deviation |
|
f |
phase |
|
F |
phase or angle |
|
F(fk) |
phase compensation |
|
rxy2 |
correlation coefficient |
|
r |
mass density |
|
z |
damping ratio |
|
s |
decay rate |
|
sx2, sy2 |
variance |
|
sxy2 |
covariance |
|
t |
time constant |
|
t |
delay variable |
|
h |
loss factor |
|
q |
angle |
|
w |
angular frequency |
|
w0 |
undamped natural frequency |
|
wd |
damped natural frequency |
|
[ ]2 |
squaring |
|
û |
estimate |
|
|
Fourier transform |
|
|
Hilbert transform |
|
|
Laplace transform |
Synchronised
Where two signals have the same period or one has a period equal to a multiple of the others.Synchronous
Synchronous literally means “at the same time”, but in spectrum analysis, synchronous components are defined as spectral components that are integral multiples, or harmonics, of a fundamental frequency. They may in some cases exist as multiples of an integral fraction of the fundamental frequency, in which case they are called sub harmonics.Synchronous Averaging
A type of signal averaging where successive records of the time waveform are averaged together. This is also known as time domain averaging. The important criterion is that the start of each time record must be triggered from a repetitive event in the signal, such as 1 rpm. The triggering assures that the phase of the waveform components that are synchronised with the trigger are the same in each record. Then in the averaging process, these in-phase components will add together while the rest of the signal components will gradually average out because of their random relative phases. The technique is excellent for extracting signals from noisy environments.Brüel & Kjær Sound & Vibration Dictionary End User Agreement
