Sound and Vibration Dictionary (Brüel & Kjær Sound & Vibration Dictionary)

7/28/2019 Sound and Vibration Dictionary (Brel & Kjr Sound & Vibration Dictionary)

1/92

bksv.com http://www.bksv.com /Library/dictionary.aspx?key=all

Dictionary

A ref erence dictio nary o f so und and vibrat ion terms

1/3-octave Analysis

Constant Percentage Bandwidth (see CPB) spectrum analysis using 1/3-octave f ilters with bandwidth equalto 23% of the centre frequency.

1/3-octave Filter

See Third-octave Filter

1/n-oct ave Analysis

Analysis that is made o n a f ract ional part of an octave where n is t he variable. Commonly used values are1/1-, 1/3-, 1/12-, and 1/24-octave.

10/100/1000 base T

10/100/1000 Mbit Ethernet transmitted over twisted pair cable.

10 base 2

10Mbit Ethernet transmitted over coaxial cable.

A/D Converter

Converts an analogue signal to a digital one.

A-Weighted Sound Level

A measure of so und pressure level des igned to ref lect the acuity of the human ear, which does notrespond equally to all frequencies. The ear is less ef f icient at low and high f requencies than at medium orspeech-range f requencies. Therefore, to describe a sound containing a wide range of f requencies in amanner representative of the ears response, it is necessary to reduce the ef f ects o f the low and highf requencies with respect to the medium frequencies. The resultant sound level is said to be A-weighted,and the units are dBA. The A-weighted sound level is also called the noise level. Sound level meters have an

A-weight ing network f or measuring A-weighted sound level. For broadband s ounds, the A-weighted soundlevel indicates approximate relative loudness. SeeA-weight ing.

A-weighted Sound Pressure Level

The s ound pressure level of a signal which has been passed through an A weighting filter whereby both

low and high f requency components are attenuated without af f ecting the component near 1000 Hz. Theunit is the decibel, but it is usual to distinguish between this and ot her uses o f the decibel by writing theunit as dB(A). See Frequency Weighting.

A-weighting

A f requency-response adjustment of a sound level meter that makes its reading co nf orm to humanresponse. The sensitivity of the human ear is f requency dependent. At low and high f requencies, the ear isnot very sensitive, but between 500 Hz and 6 kHz t he ear is very sensitive. The A-weighting f ilter is abroadband filter t hat covers t he interval f rom 20 Hz to 20 kHz. The shape o f the A-weighting curveapproximates the f requency sensitivity of the human ear. So t he A-weighted value of a noise source is anapproximation t o how the human ear perceives the noise.

Abffusor

A proprietary panel of f ering bo th absorpt ion and dif f usion of so und.

Absorption
http://www.bksv.com/?key=Chttp://www.bksv.com/Library/Dictionary.aspx?key=Fhttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/?key=Thttp://www.bksv.com/?key=Chttp://www.bksv.com/Library/dictionary.aspx?key=all


2/92

A propert y of materials that reduces the amount of so und energy ref lected. The introduct ion of anabsorbent into the surf aces of a room will reduce the sound pressure level in that room by not ref lecting allof the so und energy striking the room's surfaces. Absorption reduces the resulting sound level produced inthe room by energy that has already entered the room

Absorption Coef ficient

A measure of the sound-absorbing ability o f a surf ace. It is def ined as the f ract ion of incident so undenergy absorbed or o therwise not ref lected by a surface. Unless o therwise specif ied, a diff use sound fieldis assumed. The values o f the absorpt ion coef f icient range f rom about 0.01 for marble slate to almos t 1.0

f or long absorbing wedges of ten used in anechoic rooms. And vary with t he f requency and angle ofincidence of the sound. Usually measured in octave bands.

AC Coupling

The connection of a signal fro m one circuit t o another in a manner that rejects DC components . See alsoDC Coupling.

Accelerance

The f requency response f unction of acceleration/fo rce. Also known as inertance.

Acceleration

A vector quant ity that specif ies rate o f change o f velocity.

Acceleration Due to Rotational Motion

G = 0.000028 42 rn2

where:

G = acceleration, in g

r= radius arm, in inches

n = revolutions per minute

G = 0.10225 rf2

where:

r= radius o f arm, in inches

f= revolutions per second

G = 4.02568 rf2

where:

r= radius of arm, in meters

f= revolutions per second

Acceleration Formulae

Multiply by to obtain

acceleration due to gravity (g) 9.80665 meters/second2

32.174 feet/second2

386.088 inches/second2

cm/second2 0.010

meters/second2

feet/second2 0.3048 meters/second2

inches/second2 0.02540 meters/second2
http://www.bksv.com/Library/Dictionary.aspx?key=D


3/92

Accelerometer

A senso r whose electrical output is proport ional to accelerat ion, these t ransducers are int ended f ormeasurement o f vibrations. A transducer whose o utput is an electrical signal directly proportional t oacceleration. The o utput is usually produced by the acceleration of a seismic mass, which applies a f orce toa piezoelectric crystal, thereby generating a current proportional to the applied f orce. This current is thenamplif ied fo r processing and analysis.

Accuracy

How close a measurement is t o t he absolute quantity.

Acoustic and Vibration Decibels

All quantities are expressed in root- mean-square (rms) values ( f or interpo lations, see Decibel Formulae).

Acceleration Velocity Sound Pressure Level in Air

dB g m/s Pa (N/m2) psi

0 1 10-6 1 10-8 2 10-5 2.90 10-9

20 1 10-5 1 10-7 2 10-4 2.90 10-8

40 1 10-4 1 10-6 2 10-3 2.90 10-7

60 1 10-3 1 10-5 0.02 2.90 10-6

80 .01 1 10-4 0.2 2.90 10-5

100 0.1 1 10-3 2.0 2.90 10-4

120 1.0 0.01 20 2.90 10-3

140 10 0.1 200 0.0290

160 100 1 .0 2 103 0.290

180 1000 10 2 104 2.90

Reference Levels

Sound Power: p0 = 1 pW = 10-12 W = 10-5 erg/s

Airborne Sound Pressure: p0 = 20 Pa = 0.0002 mbar = 0.0002 dyne/cm2

Waterborne Sound Pressure: p0 = 1 Pa = 10-5 mbar = 10-5 dyne/cm2

Accelerat ion: a0 =1 g, where g = 9.80665 m/s2 = 386.089 in/s2

Velocity: v0 = 10

-8

m/s = 10

-6

cm/s

1 psi rms corresponds to 170.8 dB re 20 mPa

1 atmosphere = 14.70 psi

Acoustic Emission

The detected energy that is generated when materials are deformed or break. For rolling-element bearinganalysis, it is the periodic energy generated by rolling over particles o r f laws and detected by the display ofthe bearing f law frequencies.

Acoustic FRF

FRF in airborne contribution measurements (sound pressure/volume velocity). Also referred to as AFRF.

Acoustic Holography

A common t erm for a set of techniques in which a sound f ield is measured at multiple po ints on a surf ace,and based on that all sound field parameters can be mapped within a volume around the measurement
http://www.bksv.com/Library/Dictionary.aspx?key=D


4/92

surf ace. Typically, measurements are taken at s ome small distance f rom a so und source and used forcalculation of pressure, particle velocity and/or sound intensity on or near the source surface. See alsoNAH and SONAH.

Acoustic indicator

In airborne contribution measurements, t he posit ion where one measures to calculate s trength at sourcepoints.

Acoustic Reflex

Bilateral contraction o f the stapedius and/or tenso r tympani muscles in response to an auditory or o thereliciting stimulus.

Acoustic Ref lex Threshold (ART)

The least sound pressure level of a sound that elicits the acoust ic ref lex.

Acoustic source

In airborne contribution measurements, o ne or several source points .

Acoustic Trauma

Damage to the hearing mechanism caused by a sudden burst o f intense noise, o r by a blast. The t ermusually implies a single traumatic event.

Acoustical Louver

A specially built louver des igned with sound-attenuat ing baf f les f or reductio n of airbo rne sound.

Acoustics

The science of the production, control, transmission, reception and eff ects of sound and of thephenomenon of hearing. The ef f ect a given environment has o n sound. The physical qualities of a room or

other enclosure (such as s ize, shape) that determine the audibility and perception o f speech and musicwithin the ro om.

Active Intensity

The propagating part of a sound f ield, producing a net f low of sound energy.

Active Noise Control

The cancellation o f sound waves by introducing a mirror image of the o riginal sound wave, 180 degrees outof phase, into the sound path.

Active side

In st ructure-borne cont ribution measurements, the s ide which exerts/sends energy (fo r example, theengine) Also referred to as Engine Side. Note: There may be sub-frames in a measurement setup, whichcould be cons idered both part o f the vehicles body and engine it is up to the user to determine what toclassify these in his SPR Model.

Active Sound Field

A so und f ield in which the part icle velocity is in phase with the sound press ure. All acous tic energy istransmitted; none is stored. A plane wave propagating in f ree f ield is an example of a purely active soundf ield and constitutes the real part o f complex sound f ield.

Acum

The unit o f measurement f or sharpness. 1 acum is t he sharpness of a 60 dB narrow-band noise, onecritical band wide with a centre frequency of 1 kHz.

Admittance (aural)
http://www.bksv.com/Library/Dictionary.aspx?key=Shttp://www.bksv.com/Library/Dictionary.aspx?key=N


5/92

The reciprocal of Impedance. See Immittance.

AES

Audio Engineering Society.

AI

SeeArticulatio n Index.

Airborne Contribution

Noise f rom airborne sources (radiating engine surfaces, intake/exhaust o rifice, etc.) that is part o f the to talsound heard in a vehicles interior

Airborne Sound

Sound that reaches the point o f interest by propagation through air.

Algorithm

A specif ic procedure f or so lving mathematical pro blems. An FFT is an algorithm.

Aliasing

To digitise an analog signal for processing in digital inst ruments such as an FFT analyzer, it f irst must beperiodically sampled, the sampling process occurring at a specific rate called the sampling frequency. Aslong as the sampling f requency is more than twice as high as the highest f requency in the signal, thesampled wave will be a proper representation of the analog waveform. If, however, the sampling frequencyis less than twice as high as t he highest f requency to be sampled, the sampled wavef orm will containextraneous components called aliases. The generation of aliases is called aliasing. An example of aliasingsometimes o ccurs in motion pictures, as f or instance when the wagon wheels in a Western seem to begoing backward. This is optical aliasing, caused by the f act that the f rame rate o f the movie camera (24f rames per second) is not f ast enough to resolve the positions of the spokes. Another example of opticalaliasing is the stroboscope, where a moving object is illuminated by a flashing light and can be made to

appear stationary, or move backward. Aliasing must be avoided in digital signal analysis to prevent errors,and FFT analyzers always contain low-pass f ilters in their input s tages t o eliminate f requency componentshigher than one-half the sampling f requency. These f ilters are automatically tuned to the proper values asthe sampling f requency is changed, and this occurs when the f requency range of the analyzer is changed.

Aliasing Error

An erro r in digit al sampling in which two f requencies cannot be dis tinguished. Caused by sampling at lessthan t wice the maximum f requency in the signal.

Alignment

A condit ion whereby the axes of machine components are co incident , parallel o r perpendicular, according t o

design requirements, during operation.

Ambience

The dist inctive acoustical characteristics o f a given space.

Ambient Noise

The total of all noise in the environment f actory noise, t raf f ic noise, birdsong, running water, etc. including the noise f rom the source of interest. See also Background Noise, Residual Noise and SpecificNoise.

Ambient Noise Level

The total noise level in the acoust ic environment, including the noise source(s) of interest.

Ambient Sound

The combination of all near and far s ounds, none of which is particularly dominant.
http://www.bksv.com/Library/Dictionary.aspx?key=Shttp://www.bksv.com/Library/Dictionary.aspx?key=Rhttp://www.bksv.com/library/dictionary.aspx/?key=B#backgroung%20noisehttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=I


6/92

American National Standards Institute

Known as ANSI, this is a f ederation o f American organisations concerned with the development o fStandards. Committees of indust ry experts draft ANSI Standards.

Amplification Factor (Q)

The amount o f mechanical gain of a st ructure when excited at a resonant f requency. The ratio of theamplitude of the steady state solution (amplitude at resonance) to the static deflection for the same force

F at f requency 0. The amplif ication f actor is a f unction of the system damping. For a damping ratio = 0 (nodamping) the amplification f actor is inf inite, f or = 1 (critically damped) there is no amplif ication.

Amplitude

The instantaneous magnitude o f an oscillating quantity such as sound pressure. The peak amplitude is t hemaximum value. In a vibrat ing object, amplitude is measured and expressed in three ways: Displacement,Velocity and Acceleration. Amplitude is also the y-axis of the vibration time waveform and spectrum; it helpsdefine the severity of the vibration.

Amplitude Demodulation

see Envelope Analysis .

Amplitude Distort ion

A dist ort ion of the wave shape o f a signal.

Amplitude Distribution

A repres entation of time-varying no ise indicat ing the percentage o f time that the no ise level is present in aseries o f amplitude intervals.

Amplitude Probability

Used to investigate the amplitude distribution o f signals.

Amplitude Scale (logarithmic)

See Logarithmic Amplitude Scale. Critical vibration components usually occur at low amplitudes compared tothe ro tational f requency vibration. These components are not revealed on a linear amplitude scale becauselow amplitudes are compressed at the bottom of the scale. But a logarithmic scale shows prominentvibration component s equally well at any amplitude. Moreo ver, percent change in amplitude may be readdirectly as dB change. Theref ore, no ise and vibration f requency analyses are usually plotted on alogarithmic amplitude scale.

Analog

Quantities in two separate physical systems having consis tently similar relationships t o each ot her arecalled analogous. One is then called the analog o f the o ther. The electrical output of a transducer is ananalog o f the vibration input of the transducer as long as the transducer is not operated in the non-linear(overloaded) range. This is in contrast to a digital representation o f the vibration signal, which is a sampledand quantisised signal consisting of a series of numbers, usually in binary notation.

Analog Signal

An electrical s ignal whose f requency and level vary cont inuously in direct relat ionship to the originalelectrical or acoustical signal.

Analog-to-Digital Conversion

The process of sampling an analog signal produces a series of numbers that is the digital representationof the same signal. The sampling f requency must be at least twice as high as the highest f requencypresent in the signal to prevent aliasing errors. SeeA/D Convert erandAliasing Erro r.

Analog-to-digital converter
http://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=Lhttp://www.bksv.com/Library/Dictionary.aspx?key=E


7/92

SeeA/D converter.

Analytical Modal Analysis

This is usually made using the f inite element method to compute a mass matrix and a st if f ness matrix,which are used in a model to represent the dynamics of a st ructure.

Anechoic

Without echo.

Anechoic Chamber

SeeAnechoic Roo m

Anechoic Room

A room des igned to suppress internal sound ref lections. Used f or acoust ical measurements . Theboundaries absorb nearly all the incident sound, thereby, ef f ectively creating essentially f ree-f ieldconditions.

Angles

Multiply by to obtain

cycle (360) 6.283 radians

degree 0.017453 radians

hertz (Hz) 6.283 radians/second

rev./minute 0.1047 radians /second

radians 57.2958 degrees

grade 0.900 degrees

Angularity

The angle between two shaf t centre lines; this angle is t he same at any point along either centreline. It isnormally specified in rise/run.

Animation

Refers t o a kind of "slow mot ion movie" that allows easy visualisation o f , f or example, a vibrating structure.

ANSI

SeeAmerican Nat ional Standards Inst itut e.

Anti-aliasing

Anti- aliasing f ilters are essential f or making a correct f requency analys is. They remove components abovethe Nyquist f requency (half the sampling f requency). If such components are present in the s ignal when it issampled, they lead to errors in the f requency domain functions, as t hey show up at lower frequencies(aliasing).

Anti-aliasing Filter

The low-pass f ilter in the input circuitry of digital signal processing equipment such as an FFT analyzer thateliminates all signal components higher in frequency than one-half the sampling frequency. See Aliasing.

Anti-resonance

A phenomenon in an elect ric, acoustic, o r o ther such syst em in which the impedance is tending to inf inity.

Apodize, Apodizat ion
http://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=A


8/92

To apodize is t o remove or smooth a sharp discontinuity in a mathematical function, an electrical signal or amechanical structure. An example would be to use a Hanning Window in an FFT analyzer to smooth thediscontinuities at the beginning and end of the sample time record. See also Hanning Window.

ART

SeeAcous tic Ref lex Threshold.

Articulation

A quant itat ive measure of the intelligibility of speech; t he percentage o f speech items correct ly perceivedand recorded.

Articulation Index (AI)

A numerically calculated measure o f the intelligibility of transmitted or pro cessed speech. It takes intoaccount the limitations of the t ransmission path and the background noise. The articulation index can rangein magnitude bet ween 0 and 1. If the AI is less t han 0.1, speech intelligibility is generally low. If it is above 0.6,speech intelligibility is generally high.

Artificial Ear

A device used to provide an aco ustic co upling bet ween an earphone and a micropho ne, thus enabling theearphone to be calibrated. The acous tic impedance of the device is made to s imulate that o f the averagehuman ear. Used to calibrate air conduction audiometers. See also Ear Simulator.

Artificial Mastoid

A device used to load a bo ne vibrat or, dynamically and st atically, enabling the bone vibrat or to be calibrated.The device includes a mechanical-elect rical transducer (usually piezoelectric). The mechanical impedance ofthe device is made to simulate that of the average human mastoid. Used to calibrate bone conductionaudiometers and to test bone conduction hearing aids.

Artif icial Reverberation

Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., Added to asignal to make it so und more lif elike.

ASA

Acous tical Society of America.

Asper

The unit o f measurement f or roughness. 1 asper is the roughness o f a 60 dB, 1 kHz s ignal with 100%modulation at 70 Hz. See also Roughness.

Asymmetrical Support

A roto r support sys tem that does not provide unif orm rest raint in all radial direct ions. This is t ypical inindust rial machinery where stif f ness in one plane may be substantially diff erent than st iff ness in theperpendicular plane. Occurs in bearings by design, or from pre-loads such as gravity or misalignment.

Asynchronous

Frequencies in a vibration spectrum that exceed shaf t turning speed (TS), but are not integer or harmonicmultiples o f TS. Also commonly referred to as non- synchronous.

Attack

The beginning of a sound; the initial transient o f a musical not e.

Attenuate

To reduce the level of :

an electrical or acoustical signal
http://www.bksv.com/Library/Dictionary.aspx?key=Rhttp://www.bksv.com/Library/Dictionary.aspx?key=Ehttp://www.bksv.com/Library/Dictionary.aspx?key=A


9/92

transmitted sound power or its electrical equivalent

sound intensity by various means (for example, air, humidity, porous materials, etc.)

sound level per unit distance by divergence, diff usion, absorption, or scattering

Attenuator

A device, usually a variable res ist ance, used to control the level o f an elect rical signal.

Attitude Angle

The angle between the steady state pre-load t hrough the bearing centreline, and a line drawn between thebearing centre and the shaf t centreline (applies to f luid f ilm bearings).

Audibility Threshold

The minimum ef f ective sound pressure level of a signal at a specified f requency that is capable of evokingan auditory sensation in a specified fraction of trials.

Audio Frequency

The f requency of oscillation of an audible sound wave, or of an acoustical or electrical signal that f allswithin the audible range of the human ear, usually taken as 20 Hz to 20 kHz.

Audio Spectrum

SeeAudio Frequency.

Audiogram

A graph s howing individual hearing acuity as a f unct ion of f requency.

Audiometer

An electrical instrument, equipped (f or air co nduct ion) with two earpho nes and a headset that provides pure

tones of known frequencies o f adjustable intensity, used to determine hearing thresho ld levels, one ear ata time. For bone conduct ion, t he audiometer is also equipped with a bo ne vibrator. A clinical audiometerincludes bot h f acilities as well as a means of generating calibrated masking noise, and usually an input f orspeech audiometry. In the industrial context, only the air conduction facility is normally required or provided.There are manual audiometers in which the tone presentations and the noting of the subjects responsesare performed manually, and self -recording audiometers in which the t one presentation and the recordingof the subjects responses are implemented automatically. In the indust rial context, a self -recordingaudiometer is set to present pulsed tones o f discrete f requencies, varied in level at a f ixed rate. In theclinical context, it may have both pulsed and cont inuous tone o utputs and continuously variable (sweep)frequency.

Audiometric Zero

see Reference zero. The terms are synonymous.

Audiometry

Measurement o f audito ry f unction. Pure-to ne audiometry means determination o f a person's hearingthresho ld levels f or pure to nes by air conduction under monaural earphone listening conditions, o r by boneconduct ion. See also Speech audiometry.

Auditory Area

The s ensory area lying between the Threshold of Hearing and the Threshold of Pain.

Auditory Cortex

The region o f the brain receiving nerve impulses f rom the ear.

Auditory System

The human hearing system made up of the external ear, the middle ear, the inner ear, the nerve pathways,
http://www.bksv.com/Library/Dictionary.aspx?key=Thttp://www.bksv.com/Library/Dictionary.aspx?key=Thttp://www.bksv.com/Library/Dictionary.aspx?key=Rhttp://www.bksv.com/Library/Dictionary.aspx?key=A


10/92

and the brain.

Aural

Having to do with the auditory mechanism.

Aures Sharpness Calculation

A correct ion applied t o the Zwicker f ormula that gives improved level independence.

Auto Correlation

Auto correlat ion is a t ime-do main funct ion that is a measure of how much a signal shape, o r wavef orm,resembles a delayed version o f itself . It is closely related to the Cepstrum. The numerical value of autocorrelation can vary between zero and one. A periodic signal such as a sine wave has an auto correlationthat is equal to one at z ero time delay, minus one at a t ime delay of one-half t he period o f the wave, andone at a time delay of one period; in other words, it is a sinuso idal waveform itself . Wideband random noisehas an auto co rrelation o f one at z ero delay, but is essentially zero at all other delays. Auto correlation issometimes used to extract periodic signals f rom noise.

Autorange

In an autorange, the measurement system detects the maximum input value on the input channels and setsthe at tenuator (dynamic range) to suit. Used befo re a calibration o r measurement.

Autoscale

In autoscaling, the axes o f the graph used to display time signal, spectra, post -processed f unctions, etc.,are automatically set by the sof tware to f it the full display (complete spectrum or signal) into the availableviewing area. Dependent o n application, it is poss ible to autos cale to the input range of a measurement,the maximum measured value or a nice round number.

Autospectrum

For FFT measurements, the Fourier Transform of a time signal is complex as it has magnitude and phase.

The autospectrum is the average of the squared magnitude. For 1/n-octave CPB measurements, it is themean square of the f ilter output.

Averaging

When performing spectrum analysis, so me f orm of time averaging must be done to accurately determinethe level of the s ignal at each f requency (unless a transient can be captured). In vibration analysis, t hemost important type o f averaging is linear spectrum averaging, where a series of individual spectra areadded together and the sum is divided by the number of spectra. Averaging is very important whenperfo rming spectrum analysis of any signal that changes with t ime, as is usually the case with vibrationsignals of machinery. Linear averaging smoothes out random noise components in a spectrum, thus makingthe discrete f requency components easier to see. Another type o f averaging that is important in machinerymonitoring is time domain averaging, or time synchronous averaging, and it requires a tachometerconnected to the trigger input o f the analyzer to synchronise each snapshot of the signal to the runningspeed of the machine. Time domain averaging is very useful in reducing the random noise components in aspectrum, or in reducing the eff ect of other interf ering signals such as components f rom a nearby machine.See also Order Analysis .

Axial

In the same direction as the shaf t centreline.

Axial Float (or End Float)

Movement of one shaf t along its centreline due to the f reedom of movement permitted by a journal bearing

or a s leeve bearing. This adjustment should be set bef ore perf orming vertical or ho rizontal moves. Thedegree of axial float can be adjusted by the pos ition of the stops, or whatever limits t he mot ion.

Axial Mode

The ro om resonances associated with each pair of parallel walls in a rectangular room.
http://www.bksv.com/Library/Dictionary.aspx?key=O


11/92

Background Noise

The to tal of all noise in a system or situation, independent of the presence of the desired signal. Inacoustical measurements, strictly speaking, the term background noise means electrical noise in themeasurement system. However, in popular usage the term background noise is of ten used to mean thenoise in the environment (airborne, structure borne, and inst rument noise), o ther than the noise f rom thesource of interest, otherwise known as Residual Noise.

Background Noise Level

The no ise level in the acous tic environment, normally excluding the no ise so urce(s) of interest.Alternatively, the value of a no ise parameter, such as LA90 (the level exceeded f or 90% of the measurement

time).

Backlash

A condit ion where a ro tor can ro tate f reely f or a cert ain angular distance bef ore enco untering any res ist ingforce. It may be measured in degrees. This term normally applies to couplings and gears.

Backup

To make a copy of data fo r sto rage or in case of the risk of data being lost.

Baffle

A movable barrier used in the reco rding s tudio to achieve separat ion of signals f rom dif f erent so urces. Thesurf ace or board upon which a loudspeaker is mounted.

Band

Any segment o f the f requency spectrum.

Band Pass Filter

A f ilter that has a single t ransmission band ext ending f rom a lower cut-of f f requency greater than zero to a

f inite upper cut-of f f requency. Outside the f ilter bandwidth, the signal is attenuated: the further outside, thegreater the attenuation.

Bandwidth

A f requency interval with an upper and lower limit and including all f requencies within t his range. A specif iedband in which (ideally) all components within this band pass completely at f ull power and all components atother f requencies are attenuated completely. The f requency range passed by a given device or st ructure.

Bandwidth (-3 dB)

The s pacing between the f requencies at which a f ilter attenuates by 3 dB. Normally expressed as f requency

difference for constant bandwidth filters and as percent of centre frequency for constant percentagebandwidth f ilters.

Bandwidth (Eff ect ive Noise)

The bandwidth of an ideal f ilter that would pass the same amount o f power fro m a white noise so urce asthe f ilter described. Used to def ine bandwidth of third-octave and octave f ilters.

Bark

The unit f or t he critical band f requency scale (1 24 Bark covers the f requency range f rom 0 15500 Hz).

Baseband

The band in which signals are measured when no zooming is applied, that is, f rom 0 Hz up to X Hz, where Xis a value greater than zero and can be up to the maximum frequency value of the input module.

Basilar Membrane
http://www.bksv.com/Library/Dictionary.aspx?key=R


12/92

A membrane inside the cochlea that vibrates in response to so und, excit ing the hair cells.

Bass

The lower range of audible f requencies.

Bass Boost

The increase in level of the lower range o f f requencies, usually achieved by electrical circuits .

Baud Rate

The t ransmiss ion rate, in data bits per second.

Bearing

There are primarily two types, rolling element and sleeve or plain bearing. Rolling element bearings consistof f our parts : an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation o fthe rolling elements. A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. A lubricatingf ilm prevents contact between the shaf t and the s leeve.

Bearing Frequencies

Faults in any o f the bearing components will generate s pecif ic f requencies dependent upon t he bearinggeometry and rotat ing speed.

Bearing Misalignment

A misalignment that results when the bearings support ing a shaf t are no t aligned with each o ther. Thebearings may not be mounted in parallel planes, cocked relative to the shaf t, o r distorted due to f oundationsett ling or t hermal growth.

Bearing Nomenclature

Each bearing manufacturer has s pecif ic codes applied as pref ixes and suf f ixes to their bearings. These

codes inform the user of the const ruction, materials, clearances, and other f actors used in theconst ruction of the bearing. Consult the individual manufacturer s handbook f or specif ic code meaning.

Beat Frequency

If two vibration components are quite close to gether in frequency and if t hey are present at t he same timeat t he same place, they will combine in such a way that their sum will vary in level up and down at a rat eequal to t he diff erence in f requency between the two components. This phenomenon is known as beating,and its f requency is the beat f requency. There is conf usion in some areas between beating and amplitudemodulation, which also can produce an undulating vibration level. Amplitude modulation is dif f erent f rombeating, and is caused by a high-frequency component being multiplied by a lower-frequency componentand is thus a non- linear eff ect, whereas beating is s imply a linear addition o f two components whosef requencies are close to one another.

Beats

Periodic fluctuations t hat are heard when sounds of slightly diff erent f requencies are superimposed.

Bi-directional

Data can be transferred in both directions , that is , both transmitted and received.

Binaural Loudness

In a real sound f ield, the signals at the two ears of a listener of ten diff er f rom one another, the st imulationof the auditory system being dichotic. Dichot ic loudness perception has been investigated, but t ypicallyusing artificial stimuli over headphone playback without spatial information. In a real sound field, wheresound signals at the ears o f a listener are scattered depending on the sound incidence angle and theindividual characteristics o f the listener, dichotic loudness has no t been fully investigated. In s tudiesperformed at the Sound Quality Research Unit at Aalborg in 2005, a binaural loudness model wasdeveloped that enables loudness prediction f or any so und f ield where diotic and dichot ic at-ear signals areused as in uts and sound incidence an le does not la a role. The Binaural loudness model makes no


13/92

assumptions about the sound f ield f or t he user, everything is t aken care of by the at-ear signals. In thismodel f or any inputs t o the lef t and right ears, a corresponding frontal sound pressure is determined fo reach f requency band producing equal loudness.

Binaural Recording

Recording sound using two microphones t o preserve the directional characteristics o f the so und, binauralrecording is not the same as stereo recording. A typical binaural recording unit has two microphonesmounted in a dummy head, inset in ear-shaped moulds to capture all he adjustments (known as Head

Related Transf er Function (HRTF) in psychoacoust ic research) that happen naturally as so und wrapsaround the head and is shaped by the form of the outer and inner ear.

Bins

In an FFT spectrum, the individual f requencies at which the amplitudes are calculated, commonly calledlines. The binwidth equals the frequency span divided by the number of lines. Effective binwidth equals thebinwidth times the window noise f actor.

Bit

Short for binary digit. A number expressed in binary notation utilises the digits 1 and 0, and these are calledbits. Any number can be expressed with combinations of them.

Bode Plot

The f requency response f unction is actually a three-dimensional quantity, consist ing of amplitude vs.phase vs. f requency. Theref ore a true plot of it requires three dimensions, and this is diff icult t o representon paper. One way to do it is the so-called Bode plot, which consis ts of two curves, one of amplitude vs.f requency and one o f phase vs. f requency. Another way to look at the f requency response f unction is toresolve the phase port ion into t wo orthogonal components, one in-phase part (called the real part), andone part 90 degrees o ut o f phase (called the quadrature or imaginary part). Sometimes these two phaseparts are plott ed against each other, and the result is the so -called Nyquist Plot.

Bone Vibrator

An electro-mechanical transducer applied, usually, to the mastoid process .

Boomy

Colloquial expression f or excessive bass response in a recording, playback, or so und-reinfo rcing syst em.

Bow

A shaf t condit ion such that the geo metric centreline o f the shaf t is not s traight .

Bow-t ie Correct ion

Multispectra measurements f or co rrelation f unctions are performed using spectrum averaging. Half of thenormal time record is set to zero. Accordingly, correlation f unctions f or continuous s ignals will decreasewith the delay. The bow-tie correction compensates f or this.

Broadband Noise

Noise with components over a wide range of f requencies.

Buffer

A memory locat ion in a computer or digital instrument t hat is s et aside f or temporary storage o f digitalinfo rmation while it is waiting to be processed.

Bump Testing

A single-channel approximation to a two- channel impact test . This method works because the impactingf orce approximates an impulse and imparts broadband excitat ion over a limited f requency range. Since theFourier Transform of the impulse response function is the frequency response function, it provides a goodmethod of estimating the natural frequencies of a structure.
http://www.bksv.com/Library/Dictionary.aspx?key=N


14/92

Byte

A term used in digital syst ems. Equal to 8 bits o f data. See Bit.

Calculated Peak

Term used to describe the spectral overall RMS level multiplied by 2. Sometimes referred to asderived peak or pseudo peak.

Calibration

The process o f measuring to determine the accuracy of your measurement chain. This result can then beused to of f set measured values and take account o f this inaccuracy.

Calibrator

A device that produces a known so und press ure on a microphone in a sound level measurement sys tem, ora known vibration (acceleration, velocity, or displacement) on an accelerometer in a vibration measurementsystem. It is used to adjust the system to standard specifications.

Campbell Diagram

A graph which s hows f requency on axis and machine RPM on t he horizontal axis. The spect ral amplitude isindicated by the diameter o f a circle (or t he side length of a square) at each point in the diagram.

Capacitor

An electrical component t hat passes alternat ing currents but blocks direct current s. Also called acondenser, it is capable of storing electrical energy.

CCLD

Constant current line drive, as used by DeltaTron accelerometers and microphone preamplifiers.

Centre Frequency

The arithmetic centre of a constant bandwidth f ilter, or t he geometric centre (midpoint on a logarithmicscale) of a const ant percentage bandwidth f ilter. When specifying an FFT zo om f requency span, the centref requency is the middle point of the f requency span that you want to analyse. For example, if measuring a 2kHz zoom f requency span and the centre f requency is 6 kHz, the measured range is f rom 5 kHz to 7 kHz.

Cepstrum

The cepstrum is the fo rward Fourier transf orm of the logarithm of a spectrum. It is thus the spectrum of aspectrum, and has certain properties that make it usef ul in many types of signal analysis. One of its morepowerful attributes is the f act that any periodicities, or repeated patterns, in a spectrum will be sensed asone or two specific components in the cepstrum. If a spectrum contains several sets o f sidebands or

harmonic series, they can be confusing because of overlap. But in the cepstrum, they will be separated in away similar to the way the spectrum separates repetitive t ime patterns in the wavef orm. Gearboxes androlling element bearing vibrations lend themselves especially well to cepstrum analysis. The cepstrum isclosely related to the auto correlation function.

Characteristic Equation

The mathematical equation whose so lution def ines the dynamic characteristics o f the st ructure in terms ofits natural f requencies, damping, and mode s hapes. The mathematical formulation of the characteristicequation is called the Eigenvalue problem. The characterist ic equation is obtained from the equations o fmotion f or the st ructure.

Charge Amplifier

An amplif ier with low input impedance whose output volt age is proport ional to the output charge f rom apiezoelectric transducer. Has the advantage that vo ltage output is not af f ected by length of connectingcable f rom the transducer.

CIC
http://www.bksv.com/Library/Dictionary.aspx?key=B


15/92

Charge Injection Calibration (CIC) is a t echnique patented by Brel & Kjr f or on- line verification of theintegrity of the ent ire measurement chain, f or example, microphone, preamplif ier and cabling. Evenmicrophones remote f rom the input s tage/conditioning amplif ier can be verified. The basic philosophybehind CIC is that if we have a known condition (f or example, a properly calibrated microphone) andestablish a ref erence measurement, then as long as the reference value does not change, nothing haschanged, f or example, the microphone calibrat ion will st ill be valid. Additionally CIC verifies the cable andpreamplifier.

Circle Fit

A single degree of f reedom curve- f itt ing routine that tr ies t o f it a mode t o a circle (Nyquis t plot of a singledegree of f reedom system). The modal coef f icient is determined by the diameter of the circle and thephase by its location relative to the imaginary axis. For a real mode, it should be either completely above orcompletely below the imaginary axis.

Circular Correlation Effect

When using FFT to calculate auto- o r cross-correlation f rom auto- or cross-spectra, rectangular weightingis used so t hat the signal is not disto rted. This introduces leakage, which af f ects both the spectra and thecorrelation f unctions. Due to the discrete Fourier transf orm, the analyzer treats the signal as periodic.When correlation f unctions are calculated f rom such signals, there is a contribution f rom the end of oneperiod multiplied by the beginning of the next. This is called the circular correlation ef f ect:

To avoid this eff ect, zero pad is used. This also introduces an error into the calculations, but as the erroris the same for all signals it can be compensated f or using the bow-tie correction:


16/92

Further info rmation can be found in Brel & Kjr Technical Review, No. 2, 1984, p.29.

Classical Damping (in structural dynamics)

It is assumed that the damping matrix may be expressed as a linear combination of the mass matrix and thestif f ness matrix.

Clipping

An electrical s ignal is clipped if the signal level exceeds the capabilit ies of the amplif ier. It is a disto rt ion ofthe s ignal.

Coast-down

The s lowing down of a rot ating or reciprocating machine where no braking is applied.

Cochlea

A spirally coiled organ lo cated within the inner ear t hat contains the recepto r organs essential to hearing. Itshair cells respond to pressure f luctuations caused by mechanical vibrations of the cochlea f luid and

generate nerve impulses that are interpreted by the brain as s ound. It is the f requency-analysing port ion o fthe auditory syst em.

Coef ficient of Determination

An indicator of how well the equat ion resulting f rom regressio n analys is explains the relat ionship amongthe variables. It is o btained by comparing est imated and actual pref erence. It ranges in value f rom 0 to 1,where 1, represents perf ect correlation in the sample (that is, there is no dif f erence between theestimated y-value and the actual y-value) and 0 represents no correlation (t hat is, t he regress ionequation is not helpful in predicting a y-value). Used in Brel & Kjrs Psychoacoust ic Test Bench program,BZ 5301

Coherence

Coherence is a number between one and zero, and is a measure of the degree of linearity between tworelated signals, such as the excitation f orce (input) of a structure related to the vibration response to thatf orce (output). Coherence is thus a t wo-channel measurement, and does no t apply to single-channelmeasurements of vibration signatures. In a f requency response measurement o f a mechanical st ructure, if
http://www.bksv.com/doc/bv0014.pdf


17/92

the s tructure is linear, the coherence will be one (100% coherent), but if there is some non- linearity in thest ructure, if t here is noise in a measurement channel, or if the impulse response is truncated by theanalysis (if the f requency resolut ion is inadequate) the coherence will be less than one. A dual-channel FFTanalyzer is able to measure the coherence between the two channels, and it is a usef ul tool in determininggood f rom noisy or meaningless data.

Coherent Power

If the signal-to -no ise ratio is high, it is equal to the product of the coherence and the measured outputpower spectrum. Usef ul for determining the contributions of sources in a measured signal that is made up

of a number of s ources.

Cold Alignment

Machine condition in which alignment procedures are normally performed. Changes in off-line to on-linerunning conditions should be allowed for during this procedure so that the machine can "grow" intoalignment during operation. Also known as static alignment or primary alignment.

Coloration

The distort ion of a signal detectable by the ear.

Comb Filter

A dist ort ion produced by combining an elect rical or acoustical signal with a delayed replica o f its elf . Theresult is constructive and destructive interference that results in peaks and nulls being introduced into thef requency response. When plotted to a linear f requency scale, the response resembles a comb, hence thename.

Combination Met ric

A new object ive metric created as a weighted sum of a number o f single metrics . Can be created us ing Brel& Kjr's Psychoacoustic Test Bench too l BZ5301.

Common-mode Rejection

A measure of a measurement s yst ems capability o f reject ing common-mode s ignals.

Common-mode Signals

Signals o f identical magnitude and phase that appear simultaneously at two inputs of an amplif ier.

Common-mode Voltage

The vo ltage arising f rom common-mode signals.

Complex Acoustic Admitt ance

The ratio of the sound particle velocity through the surface to the surface sound pressure.

Complex Acoustic Impedance

The ratio of the surface sound pressure to the sound particle velocity through the surface.

Complex Intensity

Complex intensity is the combined real intensity and imaginary intensity.

Complex Modes

The po ints o n a st ructure have varying phase relationships between them at a natural f requency. This is

unlike a real mode where the phase between points is either 0 or 180.

Complex Reflection Coeff icient

The complex ratio of the pressure of the ref lected wave to the pressure o f the incident wave.


18/92

Complex Spectrum

In a Complex or Fourier spectrum, the lines in the spectrum resulting from FFT analysis are equidistant, sothe time signal is analysed in constant bandwidths. The analyzer analyses the time signal in blocks andeach block is recorded in memory and a Fast Fourier Transform (FFT) is performed on each block.

Complex Time

This is the analytical time signal where the real part is the time signal itself. The imaginary part is the Hilberttransf orm of the real part. The magnitude of the complex time signal is the envelope of the real signal.

Compliance

Frequency response f unction of displacement/f orce the ease with which a system may be displaced orcompressed f or a given force. The reciprocal of Stiff ness. Also known as Dynamic Compliance. See alsoImmittance.

Compression

Reducing the dynamic range of a signal by electrical circuits that reduce the level of loud passages.

Condenser

A Capacitor.

Conductive Hearing Loss

Hearing loss caused by blockage of the outer ear or by derangement o f the middle ear.

Constant Bandwidth Analysis

Analysis using Constant Bandwidth Filters . This gives a unif orm f requency resolutio n on a linear f requencyscale.

Constant Bandwidth Filter

A f ilter with f ixed f requency bandwidth, express ed in hertz (Hz), regardless o f cent re f requency.

Constant Percentage Bandwidth

see CPB.

Constant Percentage Filter

A f ilter whos e bandwidth is a f ixed percentage o f centre f requency. The width of the individual f ilters isdef ined relative to t heir posit ion in the range of interest. The higher the centre f requency of the f ilter, thehigher the bandwidth. The bandwidth is def ined in octaves or as a f ixed percentage of the centre f requencyof the filter.

Continuous Sound

Sound having a st eady nature that is not impulsive.

Contour Graph

A graph t hat plots all point values and links together all po ints that belong within a band or level. Each ofthese levels is t hen shown in a unique colour. It divides all points into groups o f closed curves. Every curvelinks to gether all points in the group that possess t he same data value.

Contribution

Part o f the to tal sound that comes to a particular receiver in a vehicles interior f rom one or severalsources

Control System

A sys tem in which deliberate guidance or manipulat ion is used to achieve a prescribed value of a variable.
http://www.bksv.com/Library/Dictionary.aspx?key=Chttp://www.bksv.com/Library/Dictionary.aspx?key=I


19/92

Correlogram

A graph s howing the correlat ion of one signal with another.

Cortex

seeAudito ry Cortex.

Cosine Taper

Attenuat ion by a numerical f actor that varies with t ime as a co sine f unct ion.

Coulomb Damping

Non-linear damping that is a result of rubbing, loo seness, etc.

Coupling

Mechanical f ixture for jo ining two shaf ts . See also Signal Ground Coupling.

CPB

Constant Percentage Bandwidth (analysis); the width of the f ilters is def ined relative to their pos ition in thef requency range of interest. The higher the centre f requency of the f ilter, the higher the bandwidth. Thebandwidth is def ined in octaves, o r as a f ixed percentage of the centre f requency of the f ilter. A typical CPBanalysis uses 1/3-o ctave filters, which correspond to 23% of the centre f requency.

Crest Factor

The term used to represent the ratio o f the peak (crest) value to t he rms value of a waveform. Forexample, a sine wave has a crest factor of 1.4 (or 3 dB), since the peak value equals 1.414 times the rmsvalue. Music has a wide crest f actor range of 4 10 (o r 12 20 dB). This means that music peaks o ccur 12

20 dB higher than the rms value, which is why headro om is so important in audio design.

Critical Band

In human hearing, only those frequency components within a narrow band, called the critical band, will maska given tone. Critical bandwidth varies with frequency but is usually between 1/6- and 1/3-octave. The earsact like a set of parallel filters, each with its own bandwidth. 24 critical bands make up the frequency rangefrom 20 Hz to 16 kHz. Critical bands are approximated by 1/3-octave bands above 500 Hz and by 100 Hzwide bands at lower f requencies.

Critical Damping

The minimum viscous damping that will allow a displaced system to return to its original pos ition withoutoscillation.

Cross Correlation

Cross correlation is a measure of the s imilarity of two t ime domain signals. If the signals are identical, thecross correlation will be one, and if they are completely dissimilar, the cross correlation will be zero.

Cross-spectrum

The cross-spectrum is the forward Fourier transform of the cross-correlation function. Generally, thecross -spectrum is a complex f unction.

Cross-talk

The s ignal of one channel, track, or circuit interfering with another.

Crossover Frequency

In a loudspeaker with multiple radiators, t he crossover f requency is the 3 dB point of the network dividingthe s ignal energy.
http://www.bksv.com/Library/Dictionary.aspx?key=Shttp://www.bksv.com/Library/Dictionary.aspx?key=A


20/92

Cumulative Distribution

A method of repres ent ing t ime-varying no ise by indicat ing the percentage o f time that the no ise level ispresent above (or below) a series of amplitude levels.

Cursor

A thin hairline t hat can be dis played and posit ioned on spect rum or t ime signal graphs to obtain a readout.Various types of cursor are available. For example, single cursor a line, delta cursor selects a band,harmonic cursor marks a set o f harmonics.

Cursor Handles

Points on a movable cursor t hat can be selected by the mouse and then dragged to a chosen position.

Cursor Readings

Information that can be read out f rom displayed functions when interrogated using curso rs.

Curve Fit

The process of f itting a curve to a set of polynomials.

Cut-off Frequency

The f requencies that mark the ends o f a band, or the points at which the characteristics of a f ilter changef rom pass t o no-pass.

Cycle

The complete sequence of values of a periodic quantity that occurs during one period.

Cycles Per Second

A measure of f requency numerically equivalent t o hertz (Hz).

Cylindrical Wave

A wave in which the surf aces of constant phase are co axial cylinders . A line of closely s paced soundsources radiating into an open space produces a f ree sound f ield of cylindrical waves.

Daily Acoust ic Immission (DAI)

See Sound Exposure Level.

Damped Natural Frequency

The damped natural f requency is t he f requency at which a damped system will oscillate in a f ree vibration

situation.

Damping

Any means o f dissipating vibrat ion energy within a vibrating syst em.

The diss ipation of energy with time or distance. The t erm is generally applied to the att enuation ofsound in a structure owing to the internal sound-dissipative properties of the structure or to theaddition of sound-dissipative materials.

The action o f f rictional or dissipative forces on a dynamic system causing the system to lose energyand reduce the amplitude of movement.

Removal of echoes and reverberation by the use of sound absorbing materials. See also

Reverberation Time.

Damping Ratio

Ratio o f actual damping to Critical Damping, at a resonant f requency. Also, the rat io o f the decrease insignal amplitude as a f unction of time.
http://www.bksv.com/Library/Dictionary.aspx?key=Rhttp://www.bksv.com/Library/Dictionary.aspx?key=S


21/92

DAT Recorder

Digital audio tape recorder.

Data Source

The identity and location of the data that are used in an analysis.

Day-Night Average Sound Level (Ldn)

A-weight ed equivalent continuous so und expo sure level f or a 24- hour period with a 10 dB adjus tmentadded to the sound levels occurring during nighttime hours (10 p.m. to 7 a.m.).

dB

decibel. A bel (aft er Alexander Graham Bell) is def ined as the logarithm to base ten o f the ratio of twoacoust ical powers, o r intensities. One tenth o f a bel, the decibel, is the generally used unit. See Decibel.

dBA, dB(A)

A so und- level meter reading with an A-weighting network simulat ing the human-ear res ponse at a loudnesslevel of 40 phons . The weighting is specif ied in ANSI Specif ications f or Sound Level Meter, S1.4-1983

DC Coupling

The connection o f a signal from one circuit t o another in a manner that passes both AC and DCcomponents. See alsoAC Coupling.

DC Offset

The change in input voltage required to produce zero o utput vo ltage when no s ignal is applied to anamplifier.

Decade

Ten times any quantity or f requency range. The range of the human ear is about 3 decades.

Decay Rate

A measure of the decay o f acoust ical signals, expres sed as a slope in dB/second. The rate at which asignal drops off.

Decibel (dB)

The primary unit o f sound measurement; used to quantify both so und pressure level and sound powerlevel. Used for measuring the relative magnitude based on a logarithmic scale. See dB and Sound PressureLevel.

Decibel Formulae

Quantity Symbol Formula Ref. Level*

Sound Pressure Level Lp 20 log(p/p0) dB 20 Pa (in air )1 Pa (in water)

Accelerat ion Level La 20 log(a/a0) dB 1 m/s2

Velocity Level Lv 20 log(v/v0) dB 1 nm/s

Force Level LF 20 log(F/F0) dB 1 N

Power Level LW 10 log(P/P0) dB 1 pW

Intensity Level LI 10 log(I/I0) dB 1 pW/m2

Energy Density Level Lw 10 log(w/w0) dB 1 pJ/m3
http://www.bksv.com/Library/Dictionary.aspx?key=Shttp://www.bksv.com/Library/Dictionary.aspx?key=Dhttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=D


22/92

Energy Level LE 10 log(E/E0) dB 1 pJ

* p0, a0, etc.

Reference: ISO 1683-2008 - SI Units

Decibel Scale

A linear numbering scale used to def ine a logarithmic amplitude scale, thereby compres sing a wide range ofamplitude values t o a small set o f numbers.

Degree of Freedom

In a mechanical system, equals the minimum number of independent co-ordinates required to completelydefine the position of all parts of the system at any instant of time. Refers to a point and a direction inwhich a structure vibrates, or in which the vibration is measured.

Delay

An entered t ime dif f erence that has to elapse af ter (o r bef ore) t riggering of a measurement and samplingof the time history data begins. There are situations where entering a trigger delay is usef ul.

Delay Compensation

Compensation f or t he delay on the f inite detector response time on a swept f requency analysis comparedwith t he true result, which would be achieved with an inf initely slow rate of sweep.

Delay Line

A digital, analog, or mechanical device employed t o delay one audio signal with respect to another.

Detector

An electronic circuit that determines the amplitude level o f a signal in acco rdance with certain rules . The

simplest type of detector consists of a resisto r and a capacitor, and it measures the rectif ied averagevalue of a f luctuating DC signal. A more complex, but much more useful type of detector is an RMS detectorwhose out put is proportional to the power or energy present in the s ignal or a vibration.

Deterministic

A type of signal whose spectrum consis ts of a collectio n of discrete components, as opposed to a rando msignal, whose spectrum is spread out or smeared in frequency. Some deterministic signals are periodic,and their spectra cons ist of harmonic series. Vibration signatures o f machines are in general deterministic,containing one o r more harmonic series, but they always have non-deterministic components, such asbackground noise. See also Stationary Signal. Transients can also be deterministic.

DFCF

Diffuse field correction filter.

DHCP

Dynamic Host Conf iguration Proto col.

Diaphragm

Any surf ace that vibrates in response to so und o r is vibrated to emit sound, such as in microphones andloudspeakers. Also applied to wall and floo r surf aces vibrating in response to sound or in transmittingsound.

Dielectric

An insulating material. The material bet ween the plates of a capacitor.

Differentiation
http://www.bksv.com/Library/Dictionary.aspx?key=S


23/92

In vibration analysis, diff erentiation is a mathematical operation that converts a displacement s ignature to avelocity s ignature, or a velocity s ignature to an acceleration s ignature. It is perf ormed electronically on ananalog signal or can be performed digitally on a spectrum. Dif f erentiation is an inherently noisy operation, ifperfo rmed on an analog signal, adding a signif icant amount o f high-f requency noise to the signal, and isgenerally not used very much in machinery vibration analysis. It is no t inherently noisy if it is done digitallyon t he FFT spectrum. See also Integration, which is the inverse of diff erentiation.

Diffraction

The distortion of a wave front caused by the presence of an obstacle in the sound field. The scattering ofradiation at an object smaller than one wavelength and the subsequent interf erence of the scattered wavefronts.

Diffuse Field

A so und f ield in which the sound press ure level is the same everywhere and the f low o f energy is equallyprobable in all directions.

Diffuse Sound

Sound that is completely random in phase; sound that appears t o have no s ingle source.

Diffusion

The act o f sound waves spreading out over a wide area, reflecting of f a convex or o ther uneven surf ace.

Diffusor

(Diffuser) A proprietary device for the diffusion of sound through reflection-phase-grating means.

Digital

Digital instrumentation consist s o f devices that convert analog signals into a series of numbers through asampling process and an analog to digital converter. They then perf orm operations on the numbers toachieve such eff ects as equalisation, data s to rage, data compression, f requency analysis, etc. This

process in general is called digital signal processing. It is characterised by several advantages anddisadvantages. One advantage is that the converted signals can be manipulated, transf ormed and copiedwithout introducing any added noise or disto rtion. The disadvantage is t hat the signal representation maynot be truly representative of the o riginal signal. Digital s ignal process ing requires analog t o digital (A/D)conversion of the input signal. The f irst s tep in A/D conversion is sampling of the instantaneous amplitudesof the signal at specif ic times determined by the sampling rate. If the s ignal contains any info rmation atfrequencies above one-half the sampling frequency, the signal will not be sampled correctly, and thesampled version o f the s ignal will contain spurious components. This is called aliasing. The t heoreticalmaximum f requency that can be correct ly sampled is equal to one-half the sampling rate, and is called theNyquist frequency. In all digital signal processing systems, including FFT analyzers, the sampling rate ismade to be significantly greater t han twice the highest f requency present in the signal in order to be certainthe aliasing will not occur.

Digital Filter

A digital processo r that receives a sequence of input data values, executes an operat ion on them, andoutputs a corresponding sequence of values that have been f iltered with respect to the input.

Digital Filter Analyzer

Uses constant percentage (or relative) bandwidth resolution. This is often preferred for acousticmeasurements because it best simulates the way in which the human ear perceives sound.

Digital Signal Processing (DSP)

DSP is the analysis of digital signal data. The original analog signal is sampled at regular time intervals, andan Anolog to Digital converter converts the sampled amplitudes into a number series.

Dimensionless Frequency Domain Functions

see Coherence.
http://www.bksv.com/Library/Dictionary.aspx?key=C


24/92

Dimensionless Time Domain Functions

seeAuto Correlation.

Direct Field

The region in which the so und measured can be att ributed to the source alone without t he eff ects o fobstructions, walls, etc.

Directivity Factor

The ratio o f the mean-square pressure (or intensity) on the axis of a transducer at a certain distance tothe mean-square pressure (or intensity) that a spherical source radiating the same power would produce atthat point.

Directivity Index

In a given direction f rom a so und source, the diff erence in decibels between (a) the so und pressure levelproduced by the so urce in that direction, and (b) the space-average sound pressure level of that s ource,measured at the same distance.

Directivity Pattern

A graphical descript ion, usually in po lar co-ordinates, of the response of the t ransducer as a f unct ion ofthe direction of the t ransmitted o r incident so und waves in a specified plane and at a specif ied frequency.

Discrete

With ref erence to a spectrum, discrete means consist ing of separate distinct points , rather thancontinuous. An example of a discrete spectrum is a harmonic series. An FFT spectrum, which consists ofinfo rmation o nly at specific f requencies (the FFT lines), is actually discrete regardless of the input signal.For instance, the true spectrum of a transient is cont inuous, and the FFT of a transient appearscontinuous on the screen, but st ill only contains information at t he f requencies of the FFT lines. The inputsignal to an FFT analyzer is continuous, but the sampling process necessary to implement t he FFTalgorithm converts it into a discrete form, with information only at t he specific sampled times.

Discrete Fourier Transform

The mathematical calculation that converts o r transf orms a sampled and digitised waveform into asampled spectrum. The f ast Fourier transf orm, or FFT, is an algorithm that allows a computer to calculatethe discrete Fourier transform very quickly. See also Fast Fourier Transform.

Discrimination Score

See Speech Audiometry.

Displacement

A vector quant ity that specif ies the change of posit ion of a body. Usually measured f rom the rest posit ion.

Displacement, Velocity, Acceleration Relat ionships

(for sinusoidal motion only)

do= peak displacement

D = pk-pk displacement

f= f requency in Hz

t= 1/ fseconds = period in seconds

g= 9.80665 m/s2

= 386.09 in/s2

= 32.174 f t/s2

G = acceleration in g units

Displacement:

Velocity:
http://www.bksv.com/Library/Dictionary.aspx?key=Shttp://www.bksv.com/Library/Dictionary.aspx?key=A


25/92

Accelerat ion:

G = a/g

vo = 6.28 f do = 3.14 fD

vo = 61.42 in/s pk = 1.560 m/s pk

do = 9.780 inches pk; orG = 0.0511 f2D (forD in inches pk-pk)

=0.2484 metres pk; orG = 2.013 f2D (forD in metres pk-pk)

Distance Double Law

In pure spherical divergence of sound f rom a point source in f ree space, the sound pressure leveldecreases 6 dB fo r each doubling of the distance. This condition is rarely encountered in practice,but it is a handy rule to remember in estimating sound changes with distance.

Distortion

Any change in t he wavef orm or harmonic content of an original s ignal as it passes t hrough a device.The result of non- linearity within the device.

Distortion, Harmonic

Changing the harmonic content of a signal by passing it through a non- linear device.

Dodd Bars

A seco ndary alignment method. Consist s o f two bars that are s imilar in conf igurat ion to revers e dialindicator bars. However, these bars are not mounted on t he shaf t, t hey are mounted t o the machine. Eachbar is f itted with a proximity probe and it corresponds to a block on the other bar. As the machines move totheir on-line condition the gap between the proximity probes and the metal blocks changes, which changesthe voltages. The analyzer converts t he voltages to dist ances f rom which the alignment corrections can becalculated.

Domain

A domain is a s et of co-ordinates in which a mathematical f unct ion resides. A wavef orm, for inst ance, hasdimensions of amplitude and time, and it is said to exist in the time domain, while a spectrum hasdimensions of amplitude and f requency, and is said to exist in the f requency domain.

Doppler Effect (Doppler Shift)

The apparent upward shift in frequency of a sound as a noise so urce approaches the listener or theapparent downward shif t when the no ise so urce recedes. The classic example is the decrease in pitch of arailroad whistle as a locomotive passes by.

Dosimeter

A device worn by a wo rker f or determining the worker's accumulated noise exposure with regard to level andtime according to a pre-determined integration f ormula.

Driving Point Measurement

A f requency response measurement where the excitat ion point and directio n are t he same as t he responsepoint and direction.

Dynamic Capability (for Sound Intensity)

The dynamic capability of an Intens ity measurement system is determined by adding normally 7 or 10 dB(f or a measuring error less t han 1 or 0.5 dB, respectively) to the pressure-Residual Intensity Index.

Dynamic Compliance

see Compliance.
http://www.bksv.com/Library/Dictionary.aspx?key=C


26/92

Dynamic Mass

Ratio o f applied force to resulting acceleration during simple harmonic motion.

Dynamic Modulus

Ratio o f stress to strain under vibratory conditions.

Dynamic Range

All audio sys tems are limited by inherent noise at low levels and by overload distort ion at high levels . Theusable region between these two extremes is the dynamic range of the system. Expressed in dB.

Dynamic St iff ness

The frequency response function of force/displacement.

Dyne

The f orce that will accelerate a 1 gram mass at the rate of 1 cm/s. The old standard reference level fo rsound pressure was 0.0002 dyne/cm2. The same level today is expressed as 20 micro pascals, or 20 Pa.

Ear Canal

The external auditory meatus; the canal between the pinna and the eardrum.

Ear Protector

see Hearing Protector.

Ear Simulator

Synonymous withArtif icial Ear.

Eardrum

The tympanic membrane located at t he end of the ear canal that is at tached to t he oss icles o f the middleear.

Earmuff

An ear pro tecto r consist ing o f two earcups , each covering t he pinna and having a sof t ring to be pressedagainst the head around the pinna.

Earphone

An electroacoustic t ransducer o perat ing f rom an electrical syst em to an acoustical sys tem and designed tobe applied to the ear, usually without leakage. See Headphones.

Earplug

An ear pro tecto r that is ins erted into the ear canal.

Eccentricity, Mechanical

The variation of the outer diameter of a shaf t surf ace when ref erenced to the true geometric centreline ofthe shaf t. Out-of - roundness.

Eccentricity Ratio

The vecto r dif f erence between the bearing centreline and the average steady-st ate journal centreline.

Applies t o sleeve bearings not ant i-f rict ion bearings .

Echo

A delayed return o f so und that is perceived by the ear as a discrete sound image. Some signals may cont ainan echo due to ref lection. The time delay asso ciated with the echo may be of interest. For t he detection of
http://www.bksv.com/Library/Dictionary.aspx?key=Hhttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=H


27/92

echoes and measurement o f the ass ociated time delay, power cepstra are used.

Echograms

A record o f the very early reverberatory decay o f so und in a roo m.

Eddy Current Probe

A non-contact electrical device t hat measures the displacement of one surf ace relat ive to the t ip of theprobe. Construction consist s o f an electrical coil of various lengths and diameters. This coil located in the

tip of the probe is energised producing an electrical field around the tip o f the probe. When a conductivesurf ace is placed in the f ield and the distance f rom the probe is no ted, variations in this gap can bedetermined by the variations in the voltage f low to the probe tip.

EES

Early, early sound. Structure-bo rne sound may reach the microphone in a room before the airborne soundbecause so und travels f aster through the denser materials.

EFC

Energy-frequency curve.

Effective Mass

The frequency response function of force/acceleration.

Effect ive Noise Bandwidth

The width of an ideal f ilter with t he same transmission level, and which transmits the same power f rom awhite no ise source.

EFTC

Energy-frequency-time curve.

Eigenvalue

The roots o f t he Characteristic Equation.

Eigenvalue Problem

The mathematical f ormulation and solution of the Characteristic Equation is called the Eigenvalue problem.

Eigenvectors

The mode shape vectors .

End Float

seeAxial Float .

Engineering Format

The engineering format (Eng.) is used to set up the s pectrum values with a metric pref ix (..., m, k, M...).

Engineering Units, EU

The units in which a measurement is made; for instance velocity may be expressed in millimetres persecond, feet per second, miles per hour, etc., depending on the use to which the data will be put. Moderninstrumentation allows you t o specif y what the engineering units are and to apply conversion f actors if

needed.

Enhanced Spectrum

The Fourier transf orm of the enhanced time signal. See Enhanced Time.
http://www.bksv.com/Library/Dictionary.aspx?key=Ehttp://www.bksv.com/Library/Dictionary.aspx?key=Ahttp://www.bksv.com/Library/Dictionary.aspx?key=C


28/92

Enhanced Time

This is an average of the time signal.

Envelope Analysis

Frequency analysis of the envelope of the original time signal. If the original signal is amplitude modulated,the envelope analysis extracts the modulating signal (amplitude demodulation). Envelope analysis is usefulf or detecting cracks in ro ller bearings.

Equal Loudness Contour

A contour represent ing a constant loudness f or all audible f requencies. The contour having a soundpressure level of 40 dB at 1000 Hz is arbitrarily def ined as t he 40-phon contour.

Equalisation

The process of adjusting the frequency response of a device or system to achieve a flat or other desiredresponse.

Equaliser

A device f or adjusting the f requency response of a device o r syst em.

Equivalent Continuous A-Weighted Sound Level (LAeq)

The constant sound level that, in a given time period, would convey the same sound energy as the actualtime-varying A-weighted sound level.

Equivalent Continuous Sound Pressure Level (Leq)

The no tional so und pressure level that would cause the same sound energy to be received as that due tothe actual (f luctuating) sound over the same to tal duration. Sample equivalent continuous sound pressurelevel (Leq(s)): the value o f equivalent continuous sound pressure level (as def ined above) measured over asampling period shorter than the whole duration of the sound.

ESD (Energy Spectral Density)

The energy of a transient as a f unction o f f requency. Energy Spectral Density (ESD) is a scaling unit.

Essinger Bars

A seco ndary alignment method used to measure the dif f erence between on- line and o f f - line runningconditions. The method measures the change in distance and a change in angle between two tooling balls.One ball is fixed to the bearin and the other ball is fixed to a fixed reference oint usuall the floor . The


29/92

balls are connected by means o f an inside micrometer with a resolution of at least 0.001". This s hould beset up for bo th s ides of the bearing, so the readings can be taken simultaneously. And readings should betaken at every bearing. As the machine grows the distances between the balls and the angle between theinside micrometer and a f ixed location (also usually the f loor) will change. These changes can be used todetermine the changes in alignment.

EU

See Engineering Units.

Eustachian Tube

The tube running from the middle ear into the pharynx that equalises middle ear and atmospheric pressure.

Excitation

An external f orce or mot ion applied t o a syst em that causes the syst em to respond in some way.

Experimental Modal Analysis

This is used with a physical structure, typically a proto type of a scale model. See Modal Analysis and ModalTesting.

Exponential

A decaying exponent ial weight ing f unct ion, specif ied by a s tart ing po int (shif t) and a constant t ime (length).Use exponential weighting f or exponentially decaying transients longer than one time record, to avoidleakage caused by truncation.

Exponential (Response) Window

A special windowing f unct ion used in impact test ing f or minimising leakage in light ly damped structures. In alightly damped structure, oscillations may not die out within the sampled time data block, T, which results inleakage error. An exponential window adds damping to the time signal to force it to die out within the time T,thus minimising leakage. The added damping is then removed mathematically after the signal is processed.

Exponential Taper

Attenuat ion by a numerical f actor that varies exponent ially with t ime.

External Meatus

The ear canal terminated by the eardrum.

External Sampling

Where time record capture is triggered by an external signal.

Fall-off Rate

The rate at which a signals amplitude diminishes.

Far Field

Describes a region in f ree space at a much greater distance f rom a sound source than the lineardimensions o f the so urce itself where the sound pressure level obeys the inverse-square law (the soundpressure level decreases 6 dB with each doubling of distance f rom the source). Also , in this region t hesound particle velocity is in phase with the so und pressure. Closer to the so urce, where these twoconditions do not hold, constitutes the near field region. In the f ar f ield, the sound waves can becons idered planar. See also : Diffraction.

Fast Fourier Transform (FFT)

The FFT is an algorithm, or digital calculation routine, that ef f iciently calculates the Discrete FourierTransf orm f rom the sampled time waveform. In other words it converts, or transf orms, a signal f rom thetime domain into the frequency domain.
http://www.bksv.com/Library/Dictionary.aspx?key=Dhttp://www.bksv.com/Library/Dictionary.aspx?key=Mhttp://www.bksv.com/Library/Dictionary.aspx?key=Mhttp://www.bksv.com/Library/Dictionary.aspx?key=E


30/92

Feedback, Acoustic

Unwanted interaction between the output and input o f an acoustical system, f or example, between theloudspeaker and the microphone of a system.

Feedback Control System

A control sys tem that achieves prescribed relationships between selected syst em variables by comparingf unctions o f these variables and using the dif f erences to eff ect control.

FFT

Fast Fourier Transf orm. Gives the mathematical connection between time and f requency and vice versaand, given a time signal, allows the calculation of its spectrum. An iterative program that computes theFourier Transf orm in a shorter time.

FFT Analyzer

An FFT analyzer is a device that uses the FFT algo rithm to calculate a spectrum f rom a time domain signal,and is the mos t common t ype of spectrum analyzer available to day. The FFT analyzer is a very usefuldevice, and is available in a great variety of models of varying complexity. It is the heart of any machinerypredictive maintenance program. An FFT analyzer uses constant (o r abso lute) bandwidth reso lution. This is

of ten pref erred in vibration analysis because it gives a better f requency resolution. See also Fast FourierTransform.

Fidelity

As applied to so und qualit y, the f aithf ulness t o the original.

Field Impact Insulation Class (FIIC)

A single-number rat ing f or impact s ound insulat ion, calculated f rom measured values of normalised impactsound pressure levels.

Field STC (FSTC)

Sound Transmission Class based on field measurements of sound transmission loss of a partition.

Filter

A device f or separat ing components of a signal on the bas is o f their f requency. It allows co mponents inone or more f requency bands to pass relatively unattenuated, and it att enuates components in ot herf requency bands. Modif ies the f requency spectrum of a signal usually while it is in electrical f orm.

Filter, Bandpass

A f ilter that passes all f requencies between a low-f requency cut-of f point and a high- f requency cut-of fpoint.

Filter, High-pass

A f ilter that passes all f requencies above a cut -of f f requency.

Filter, Low-pass

A f ilter that passes all f requencies below a cut-of f f requency.

Finite Element Analysis or Modelling

A computer-aided design technique f or mathematically modelling a s tructure. Finite element modelling is

used f or s tructural analysis, heat transf er analysis, and modal analysis.

Fixed Format

Fixed format numbering is where numbers be entered as their absolute values. The whole number isentered f ollowed by a decimal point, if required, and the required number of significant f igures.
http://www.bksv.com/Library/Dictionary.aspx?key=F


31/92

Flanging

The term applied to the use of comb f ilters to obtain special sound ef f ects.

Flanking Pat h

An indirect so und t ransmission path, such as the structure-borne pat h between two adjacent rooms.

Flat Top Window

The f lat top window is a special time window with low ripple that is used in some FFT analyzers in additionto the more common Hanning Window and Rectangular Time Window. The flat top window does not allowas fine a frequency resolution as the Hanning window, but it will accurately measure the amplitude level of asignal at any f requency, even if the f requency is between the lines of the FFT analysis. The maximumpicket-fence-error is 0.008 dB. It is used in transducer calibration systems to increase amplitude accuracy.

Fluctuation Strength

A measure of the low-f requency (below 20 Hz) amplitude and f requency modulat ion of so und. It isperceived as changes in t he f requency and volume of the sound with t ime. Fluctuating signals o f this t ypesound louder, and more annoying, than a s teady signal of the same RMS magnitude. The ef f ect is mostnot iceable when the modulation f requency is around 4 Hz.

Flutter

A repet itive echo set up by parallel ref lecting surf aces.

Flyover Noise

Measurement of aircraf t no ise as aircraf t f ly over.

Force Window

A special windowing f unct ion f or minimising no ise in impact t est ing. Since the duration of the actual impactis usually very short relative to t he overall digitised time sample, the f requency response f unction of the

f orce signal can have a low signal-to- noise ratio. The force window does not alter the actual f orce pulsebut minimises the noise in the rest of the data block giving a much improved signal to no ise ratio.

Forced Def lection Shape

When a st ructure is f orced to vibrate by applying one or more excitation f orces, the structure will exhibitwhat is known as a Forced Def lection Shape.

Forced Response Analysis

Calculating the syst em response to an arbitrary f orcing function using modal analysis data as t he systemmodel.

Forced Vibration

The os cillation of a system under the action of a f orcing f unction.

Foundation

A sur f ace to which a machine baseplate is mount ed.

Fourier Analysis

Applicatio n of the Four ier t ransf orm to a signal to determine its s pectrum. Fourier analysis is another termf or spectrum analysis, although it generally refers to analysis us ing an FFT analyzer.

Fourier, Jean Bapt iste

The famous, many-talented French engineer, mathematician, and one time president o f Egypt, who devisedthe Fourier series and Fourier Transform for the conversion of time functions into frequency functions andvice versa.
http://www.bksv.com/Library/Dictionary.aspx?key=Rhttp://www.bksv.com/Library/Dictionary.aspx?key=H


32/92

Fourier Spectrum

The line spect rum result ing f rom an FFT analysis is equally spaced, so t he time signal is analysed inconstant bandwidths. The analyzer analyses the time signal in blocks and each block is recorded in memoryand a Fast Fourier Transf orm (FFT) is perf ormed on each block (the o ld instantaneous spectrum). See alsoFFT Analyzer.

Fourier Transform

A mathemat ical operat ion f or decomposing a t ime funct ion into its f requency components (amplitude and

phase). The process is reversible, and the signal can be reconst ructed f rom its Fourier components. Seealso Fast Fourier Transform.

Free Field

An environment in which there are no ref lective surf aces within the f requency region of interest and thesound is isotropic and homogenous.

Free-field Voltage Sensitivity

The ratio of the output o pen-circuit voltage of a microphone to the f ree-f ield sound pressure in anundisturbed plane progress ive wave. Frequency and angle of incidence must be specif ied.

Free Run

When using free run triggering, the analyzer runs continuously capturing time records and calculating thecorresponding spectra.

Free Sound Field

A so und f ield in which the ef f ects of obstacles or boundaries on sound pro pagated in that f ield are

Date post:	03-Apr-2018
Category:	Documents
Upload:	marlin-goose
View:	258 times
Download:	2 times

Sound and Vibration Dictionary (Brüel & Kjær Sound & Vibration Dictionary)

Documents