专业音频术语

audicable

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.       
       
Scientific Format       
       
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.       
       
Short Time Fourier Transform       

audicable

        
see STFT.        
        
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.        
        
SISO (Single-input/Single-output)        
        
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 Assessment        
        
Consists of subjective and objective tests, and analysis. See Psychoacoustic Test Bench.        
        
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 data        
        
Spacers        
        
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.        
        
Spatial Transformation of Sound Fields        

audicable

see STSF.       
       
SPC       
       
Source Path Contribution       
       
Specific 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.       
       
Speed of Sound in Various Media       
       
        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
       
Speed-related Component       
       
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.       
       
SPL       
       
See Sound Pressure Level.       
       
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 data       
       
Spring 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.       
       
STFT (Short Time Fourier Transform)       
       
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 interior       
       
Structure-borne Sound       
       
Sound that radiates from a construction assembly after travelling through a building's structure in the form of vibration.       
       
STSF (Spatial Transformation of Sound Fields)       
       
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.       
       
Subjective Test       
       
A sound quality test (either Semantic Differential or Paired Comparison) carried out by a jury who indicate their preferences for particular sounds.       
       
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)
a(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
m(t)        input noise signal
N        transform size
NA        averaging number
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
p0        sound pressure reference
pA        sound pressure of the channel A signal
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
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
W(f)        weighting spectrum
x(t)        displacement
xi(t)        input time signal (multi-channel analysis)
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
e1        GMM/GUU input error
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.       

CHUNYU

哈哈！也顶一下！！:victory:

warmer

:victory: 写的不错，顶！

audicable

嗬嗬嗬嗬  
谢谢各位捧场啊 :P