1/3-octave Analysis

Constant Percentage Bandwidth spectrum analysis using 1/3-octave filters with bandwidth equal to 23% of the centre frequency.

1/n-octave Analysis

Analysis that is made on a fractional part of an octave where n is the variable. Commonly used values are 1/1-, 1/3-, 1/12-, and 1/24-octave.

A-Weighted Sound Level

A measure of sound pressure level designed to reflect the acuity of the human ear, which does not respond equally to all frequencies. The ear is less efficient at low and high frequencies than at medium or speech-range frequencies. Therefore, to describe a sound containing a wide range of frequencies in a manner representative of the ear’s response, it is necessary to reduce the effects of the low and high frequencies 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-weighting network for measuring A-weighted sound level. For broadband sounds, the A-weighted sound level indicates approximate relative loudness.

A-weighted Sound Pressure Level

The sound pressure level of a signal which has been passed through an “A” weighting filter whereby both low and high frequency components are attenuated without affecting the component near 1000 Hz. The unit is the decibel, but it is usual to distinguish between this and other uses of the decibel by writing the unit as dB(A).


A frequency-response adjustment of a sound level meter that makes its reading conform to human response. The sensitivity of the human ear is frequency dependent. At low and high frequencies, the ear is not very sensitive, but between 500 Hz and 6 kHz the ear is very sensitive. The A-weighting filter is a broadband filter that covers the interval from 20 Hz to 20 kHz. The shape of the A-weighting curve approximates the frequency sensitivity of the human ear. So the A-weighted value of a noise source is an approximation to how the human ear perceives the noise.


A proprietary panel offering both absorption and diffusion of sound.


A property of materials that reduces the amount of sound energy reflected. The introduction of an absorbent into the surfaces of a room will reduce the sound pressure level in that room by not reflecting all of the sound energy striking the room's surfaces. Absorption reduces the resulting sound level produced in the room by energy that has already entered the room

Absorption Coefficient

A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. Unless otherwise specified, a diffuse sound field is assumed. The values of the absorption coefficient range from about 0.01 for marble slate to almost 1.0 for long absorbing wedges often used in anechoic rooms. And vary with the frequency and angle of incidence of the sound. Usually measured in octave bands.

AC Coupling

The connection of a signal from one circuit to another in a manner that rejects DC components.


The frequency response function of acceleration/force. Also known as inertance.


A vector quantity that specifies rate of change of velocity.

Acceleration Due to Rotational Motion

G = 0.000028 42 r n2

  • G = acceleration, in g
  • r = radius arm, in inches
  • n = revolutions per minute

G = 0.10225 rf2

  • r = radius of arm, in inches
  • f = revolutions per second

G = 4.02568 rf2

  • r = radius of arm, in meters
  • f = revolutions per second
Acceleration Formulae
to obtain
acceleration due to gravity (g)

A sensor whose electrical output is proportional to acceleration, these transducers are intended for measurement of vibrations. A transducer whose output is an electrical signal directly proportional to acceleration. The output is usually produced by the acceleration of a seismic mass, which applies a force to a piezoelectric crystal, thereby generating a current proportional to the applied force. This current is then amplified for processing and analysis.


How close a measurement is to the absolute quantity.

Acoustic and Vibration Decibels

All quantities are expressed in root-mean-square (rms) values 

Sound Pressure Level in Air
Pa (N/m2)
1 ´ 10-6
1 ´ 10-8
2 ´ 10-5
2.90 ´ 10-9
1 ´ 10-5
1 ´ 10-7
2 ´ 10-4
2.90 ´ 10-8
1 ´ 10-4
1 ´ 10-6
2 ´ 10-3
2.90 ´ 10-7
1 ´ 10-3
1 ´ 10-5
2.90 ´ 10-6
1 ´ 10-4
2.90 ´ 10-5
1 ´ 10-3
2.90 ´ 10-4
2.90 ´ 10-3
1 .0
2 ´ 103
2 ´ 104

Reference Levels

  • Sound Power: p0 = 1 pW = 10-12 W = 10-5 erg/s
  • Airborne Sound Pressure: p0 = 20 µPa = 0.0002 mbar = 0.0002 dyne/cm2
  • Waterborne Sound Pressure: p0 = 1 µPa = 10-5 mbar = 10-5 dyne/cm2
  • Acceleration: a0 =1 µg, 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 bearing analysis, it is the periodic energy generated by rolling over particles or flaws and detected by the display of the bearing flaw frequencies.

Acoustic FRF

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

Acoustic Holography

A common term for a set of techniques in which a sound field is measured at multiple points on a surface, and based on that all sound field parameters can be mapped within a volume around the measurement surface. Typically, measurements are taken at some small distance from a sound source and used for calculation of pressure, particle velocity and/or sound intensity on or near the source surface.

Acoustic indicator

In airborne contribution measurements, the position where one measures to calculate strength at source points.

Acoustic Reflex

Bilateral contraction of the stapedius and/or tensor tympani muscles in response to an auditory or other eliciting stimulus.

Acoustic Reflex Threshold (ART)

The least sound pressure level of a sound that elicits the acoustic reflex.

Acoustic source

In airborne contribution measurements, one or several source points.

Acoustic Trauma

Damage to the hearing mechanism caused by a sudden burst of intense noise, or by a blast. The term usually implies a single traumatic event.

Acoustical Louver

A specially built louver designed with sound-attenuating baffles for reduction of airborne sound.


The science of the production, control, transmission, reception and effects of sound and of the phenomenon of hearing. The effect a given environment has on sound. The physical qualities of a room or other enclosure (such as size, shape) that determine the audibility and perception of speech and music within the room.

Active Intensity

The propagating part of a sound field, producing a net flow of sound energy.

Active Noise Control

The cancellation of sound waves by introducing a mirror image of the original sound wave, 180 degrees out of phase, into the sound path.

Active side

In structure-borne contribution measurements, the side which exerts/sends energy (for example, the engine) Also referred to as Engine Side. Note: There may be sub-frames in a measurement setup, which could be considered both part of the vehicle’s body and engine – it is up to the user to determine what to classify these in his SPR Model.

Active Sound Field

A sound field in which the particle velocity is in phase with the sound pressure. All acoustic energy is transmitted; none is stored. A plane wave propagating in free field is an example of a purely active sound field and constitutes the real part of complex sound field.


The unit of measurement for sharpness. 1 acum is the sharpness of a 60 dB narrow-band noise, one critical band wide with a centre frequency of 1 kHz.

Admittance (aural)

The reciprocal of Impedance.

Audio Engineering Society.
Airborne Contribution

Noise from airborne sources (radiating engine surfaces, intake/exhaust orifice, etc.) that is part of the total sound heard in a vehicle’s interior

Airborne Sound

Sound that reaches the point of interest by propagation through air.


A specific procedure for solving mathematical problems. An FFT is an algorithm.


To digitise an analog signal for processing in digital instruments such as an FFT analyzer, it first must be periodically sampled, the sampling process occurring at a specific rate called the sampling frequency. As long as the sampling frequency is more than twice as high as the highest frequency in the signal, the sampled wave will be a proper representation of the analog waveform. If, however, the sampling frequency is less than twice as high as the highest frequency to be sampled, the sampled waveform will contain extraneous components called “aliases”. The generation of aliases is called aliasing. An example of aliasing sometimes occurs in motion pictures, as for instance when the wagon wheels in a Western seem to be going backward. This is optical aliasing, caused by the fact that the frame rate of the movie camera (24 frames per second) is not fast enough to resolve the positions of the spokes. Another example of optical aliasing 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 filters in their input stages to eliminate frequency components higher than one-half the sampling frequency. These filters are automatically tuned to the proper values as the sampling frequency is changed, and this occurs when the frequency range of the analyzer is changed.

Aliasing Error

An error in digital sampling in which two frequencies cannot be distinguished. Caused by sampling at less than twice the maximum frequency in the signal.


A condition whereby the axes of machine components are coincident, parallel or perpendicular, according to design requirements, during operation.


The distinctive acoustical characteristics of a given space.

Ambient Noise

The total of all noise in the environment – factory noise, traffic noise, birdsong, running water, etc. – including the noise from the source of interest.

Ambient Noise Level

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

Ambient Sound

The combination of all near and far sounds, none of which is particularly dominant.

American National Standards Institute

Known as ANSI, this is a federation of American organisations concerned with the development of Standards. Committees of industry experts draft ANSI Standards.

Amplification Factor (Q)

The amount of mechanical gain of a structure when excited at a resonant frequency. The ratio of the amplitude of the steady state solution (amplitude at resonance) to the static deflection for the same force F at frequency 0. The amplification factor is a function of the system damping. For a damping ratio = 0 (no damping) the amplification factor is infinite, for = 1 (critically damped) there is no amplification.


The instantaneous magnitude of an oscillating quantity such as sound pressure. The peak amplitude is the maximum value. In a vibrating 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 helps define the severity of the vibration.

Amplitude Distortion

A distortion of the wave shape of a signal.

Amplitude Distribution

A representation of time-varying noise indicating the percentage of time that the noise level is present in a series of amplitude intervals.

Amplitude Probability

Used to investigate the amplitude distribution of signals.

Amplitude Scale (logarithmic)

Critical vibration components usually occur at low amplitudes compared to the rotational frequency vibration. These components are not revealed on a linear amplitude scale because low amplitudes are compressed at the bottom of the scale. But a logarithmic scale shows prominent vibration components equally well at any amplitude. Moreover, percent change in amplitude may be read directly as dB change. Therefore, noise and vibration frequency analyses are usually plotted on a logarithmic amplitude scale.


Quantities in two separate physical systems having consistently similar relationships to each other are called analogous. One is then called the analog of the other. The electrical output of a transducer is an analog of 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 of the vibration signal, which is a sampled and quantisised signal consisting of a series of numbers, usually in binary notation.

Analog Signal

An electrical signal whose frequency and level vary continuously in direct relationship to the original electrical or acoustical signal.

Analog-to-Digital Conversion

The process of sampling an analog signal produces a series of numbers that is the digital representation of the same signal. The sampling frequency must be at least twice as high as the highest frequency present in the signal to prevent aliasing errors.

Analytical Modal Analysis

This is usually made using the finite element method to compute a mass matrix and a stiffness matrix, which are used in a model to represent the dynamics of a structure.

Without echo.
Anechoic Room

A room designed to suppress internal sound reflections. Used for acoustical measurements. The boundaries absorb nearly all the incident sound, thereby, effectively creating essentially free-field conditions.

to obtain
cycle (360°)
hertz (Hz)

The angle between two shaft centre lines; this angle is the same at any point along either centreline. It is normally specified in rise/run.


Refers to a kind of "slow motion movie" that allows easy visualisation of, for example, a vibrating structure.


Anti-aliasing filters are essential for making a correct frequency analysis. They remove components above the Nyquist frequency (half the sampling frequency). If such components are present in the signal when it is sampled, they lead to errors in the frequency domain functions, as they show up at lower frequencies (aliasing).

Anti-aliasing Filter

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


A phenomenon in an electric, acoustic, or other such system in which the impedance is tending to infinity.

Apodize, Apodization

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


A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.

Articulation Index (AI)

A numerically calculated measure of the intelligibility of transmitted or processed speech. It takes into account the limitations of the transmission path and the background noise. The articulation index can range in magnitude between 0 and 1. If the AI is less than 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 acoustic coupling between an earphone and a microphone, thus enabling the earphone to be calibrated. The acoustic impedance of the device is made to simulate that of the average human ear. Used to calibrate air conduction audiometers.

Artificial Mastoid

A device used to load a bone vibrator, dynamically and statically, enabling the bone vibrator to be calibrated. The device includes a mechanical-electrical transducer (usually piezoelectric). The mechanical impedance of the device is made to simulate that of the average human mastoid. Used to calibrate bone conduction audiometers and to test bone conduction hearing aids.

Artificial Reverberation

Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., Added to a signal to make it sound more lifelike.


Acoustical Society of America.


The unit of measurement for roughness. 1 asper is the roughness of a 60 dB, 1 kHz signal with 100% modulation at 70 Hz.

Asymmetrical Support

A rotor support system that does not provide uniform restraint in all radial directions. This is typical in industrial machinery where stiffness in one plane may be substantially different than stiffness in the perpendicular plane. Occurs in bearings by design, or from pre-loads such as gravity or misalignment.


Frequencies in a vibration spectrum that exceed shaft turning speed (TS), but are not integer or harmonic multiples of TS. Also commonly referred to as non-synchronous.


The beginning of a sound; the initial transient of a musical note.


To reduce the level of:

  • an electrical or acoustical signal
  • 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, diffusion, absorption, or scattering

A device, usually a variable resistance, used to control the level of an electrical signal.

Attitude Angle

The angle between the steady state pre-load through the bearing centreline, and a line drawn between the bearing centre and the shaft centreline (applies to fluid film bearings).

Audibility Threshold

The minimum effective sound pressure level of a signal at a specified frequency that is capable of evoking an auditory sensation in a specified fraction of trials.

Audio Frequency

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


A graph showing individual hearing acuity as a function of frequency.


An electrical instrument, equipped (for air conduction) with two earphones and a headset that provides pure tones of known frequencies of adjustable intensity, used to determine hearing threshold levels, one ear at a time. For bone conduction, the audiometer is also equipped with a bone vibrator. A clinical audiometer includes both facilities as well as a means of generating calibrated masking noise, and usually an input for speech 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 subject’s responses are performed manually, and self-recording audiometers in which the tone presentation and the recording of the subject’s responses are implemented automatically. In the industrial context, a self-recording audiometer is set to present pulsed tones of discrete frequencies, varied in level at a fixed rate. In the clinical context, it may have both pulsed and continuous tone outputs and continuously variable (sweep) frequency.


Measurement of auditory function. Pure-tone audiometry means determination of a person's hearing threshold levels for pure tones by air conduction under monaural earphone listening conditions, or by bone conduction. See also Speech audiometry.

Auditory Cortex

The region of the brain receiving nerve impulses from the ear.

Auditory System

The human hearing system made up of the external ear, the middle ear, the inner ear, the nerve pathways, and the brain.


Having to do with the auditory mechanism.

Aures Sharpness Calculation

A correction applied to the Zwicker formula that gives improved level independence.

Auto Correlation

Auto correlation is a time-domain function that is a measure of how much a signal shape, or waveform, resembles a delayed version of itself. It is closely related to the Cepstrum. The numerical value of auto correlation can vary between zero and one. A periodic signal such as a sine wave has an auto correlation that is equal to one at zero time delay, minus one at a time delay of one-half the period of the wave, and one at a time delay of one period; in other words, it is a sinusoidal waveform itself. Wideband random noise has an auto correlation of one at zero delay, but is essentially zero at all other delays. Auto correlation is sometimes used to extract periodic signals from noise.


In an autorange, the measurement system detects the maximum input value on the input channels and sets the attenuator (dynamic range) to suit. Used before a calibration or measurement.


In autoscaling, the axes of the graph used to display time signal, spectra, post-processed functions, etc., are automatically set by the software to fit the full display (complete spectrum or signal) into the available viewing area. Dependent on application, it is possible to autoscale to the input range of a measurement, the maximum measured value or a “nice” round number.


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 the mean square of the filter output.


When performing spectrum analysis, some form of time averaging must be done to accurately determine the level of the signal at each frequency (unless a transient can be captured). In vibration analysis, the most important type of averaging is linear spectrum averaging, where a series of individual spectra are added together and the sum is divided by the number of spectra. Averaging is very important when performing spectrum analysis of any signal that changes with time, as is usually the case with vibration signals of machinery. Linear averaging smoothes out random noise components in a spectrum, thus making the discrete frequency components easier to see. Another type of averaging that is important in machinery monitoring is time domain averaging, or time synchronous averaging, and it requires a tachometer connected to the trigger input of the analyzer to synchronise each “snapshot” of the signal to the running speed of the machine. Time domain averaging is very useful in reducing the random noise components in a spectrum, or in reducing the effect of other interfering signals such as components from a nearby machine.


In the same direction as the shaft centreline.

Axial Float (or End Float)

Movement of one shaft along its centreline due to the freedom of movement permitted by a journal bearing or a sleeve bearing. This adjustment should be set before performing vertical or horizontal moves. The degree of axial float can be adjusted by the position of the stops, or whatever limits the motion.

Axial Mode

The room resonances associated with each pair of parallel walls in a rectangular room.

Background Noise

The total of all noise in a system or situation, independent of the presence of the desired signal. In acoustical measurements, strictly speaking, the term “background noise” means electrical noise in the measurement system. However, in popular usage the term “background noise” is often used to mean the noise in the environment (airborne, structure borne, and instrument noise), other than the noise from the source of interest, otherwise known as Residual Noise.

Background Noise Level

The noise level in the acoustic environment, normally excluding the noise source(s) of interest. Alternatively, the value of a noise parameter, such as LA90 (the level exceeded for 90% of the measurement time).


A condition where a rotor can rotate freely for a certain angular distance before encountering any resisting force. It may be measured in degrees. This term normally applies to couplings and gears.


To make a copy of data for storage or in case of the risk of data being lost.


A movable barrier used in the recording studio to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted.


Any segment of the frequency spectrum.

Band Pass Filter

A filter that has a single transmission band extending from a lower cut-off frequency greater than zero to a finite upper cut-off frequency. Outside the filter bandwidth, the signal is attenuated: the further outside, the greater the attenuation.


A frequency interval with an upper and lower limit and including all frequencies within this range. A specified band in which (ideally) all components within this band pass completely at full power and all components at other frequencies are attenuated completely. The frequency range passed by a given device or structure.

Bandwidth (-3 dB)

The spacing between the frequencies at which a filter attenuates by 3 dB. Normally expressed as frequency difference for constant bandwidth filters and as percent of centre frequency for constant percentage bandwidth filters.

Bandwidth (Effective Noise)

The bandwidth of an ideal filter that would pass the same amount of power from a white noise source as the filter described. Used to define bandwidth of third-octave and octave filters.


The unit for the critical band frequency scale (1 – 24 Bark covers the frequency range from 0 – 15500 Hz).


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

Basilar Membrane

A membrane inside the cochlea that vibrates in response to sound, exciting the hair cells.


The lower range of audible frequencies.

Bass Boost

The increase in level of the lower range of frequencies, usually achieved by electrical circuits.

Baud Rate

The transmission rate, in data bits per second.


There are primarily two types, rolling element and sleeve or plain bearing. Rolling element bearings consist of four parts: an inner race, an outer race, balls or rollers, and a cage to maintain the proper separation of the rolling elements. A sleeve bearing is a cylinder of alloy metal surrounding the rotating shaft. A lubricating film prevents contact between the shaft and the sleeve.

Bearing Frequencies

Faults in any of the bearing components will generate specific frequencies dependent upon the bearing geometry and rotating speed.

Bearing Misalignment

A misalignment that results when the bearings supporting a shaft are not aligned with each other. The bearings may not be mounted in parallel planes, cocked relative to the shaft, or distorted due to foundation settling or thermal growth.

Bearing Nomenclature

Each bearing manufacturer has specific codes applied as prefixes and suffixes to their bearings. These codes inform the user of the construction, materials, clearances, and other factors used in the construction of the bearing. Consult the individual manufacturer’s handbook for specific code meaning.

Beat Frequency

If two vibration components are quite close together in frequency and if they are present at the same time at the same place, they will combine in such a way that their sum will vary in level up and down at a rate equal to the difference in frequency between the two components. This phenomenon is known as beating, and its frequency is the beat frequency. There is confusion in some areas between beating and amplitude modulation, which also can produce an undulating vibration level. Amplitude modulation is different from beating, and is caused by a high-frequency component being multiplied by a lower-frequency component and is thus a non-linear effect, whereas beating is simply a linear addition of two components whose frequencies are close to one another.


Periodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.


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

Binaural Loudness

In a real sound field, the signals at the two ears of a listener often differ from one another, the stimulation of the auditory system being dichotic. Dichotic loudness perception has been investigated, but typically using artificial stimuli over headphone playback without spatial information. In a real sound field, where sound signals at the ears of a listener are scattered depending on the sound incidence angle and the individual characteristics of the listener, dichotic loudness has not been fully investigated. In studies performed at the Sound Quality Research Unit at Aalborg in 2005, a binaural loudness model was developed that enables loudness prediction for any sound field where diotic and dichotic at-ear signals are used as inputs and sound incidence angle does not play a role. The Binaural loudness model makes no assumptions about the sound field for the user, everything is taken care of by the at-ear signals. In this model for any inputs to the left and right ears, a corresponding frontal sound pressure is determined for each frequency band producing equal loudness.

Binaural Recording

Recording sound using two microphones to preserve the directional characteristics of the sound, binaural recording is not the same as stereo recording. A typical binaural recording unit has two microphones mounted in a dummy head, inset in ear-shaped moulds to capture all he adjustments (known as Head Related Transfer Function (HRTF) in psychoacoustic research) that happen naturally as sound wraps around the head and is “shaped” by the form of the outer and inner ear.


In an FFT spectrum, the individual frequencies at which the amplitudes are calculated, commonly called “lines”. The binwidth equals the frequency span divided by the number of lines. Effective binwidth equals the binwidth times the window noise factor.


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

Bode Plot