Dictionary

A reference dictionary of sound and vibration terms




A/D Converter

Converts an analogue signal to a digital one.

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. See A-weighting.

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). See Frequency Weighting.

A-weighting

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.

Abffusor

A proprietary panel offering both absorption and diffusion of sound.

Absorption

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. See also DC Coupling.

Accelerance

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

Acceleration

A vector quantity that specifies rate of change of velocity.

Acceleration Due to Rotational Motion

G = 0.000028 42 r n2

where:

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

G = 0.10225 rf2

where:

  • r = radius of 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

Accelerometer

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.

Accuracy

How close a measurement is to the absolute quantity.

Acoustic and Vibration Decibels

All quantities are expressed in root-mean-square (rms) values (for interpolations, 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 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. See also NAH and SONAH.

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.

Acoustics

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.

Acum

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. See Immittance.

AES

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.

Algorithm

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

Aliasing

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.

Alignment

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

Ambience

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. See also Background Noise, Residual Noise and Specific Noise.

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.

Amplitude

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 Demodulation

see Envelope Analysis.

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)

See Logarithmic Amplitude Scale. 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.

Analog

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. See A/D Converter and Aliasing Error.

Analog-to-digital converter

See A/D converter.
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.

Anechoic

Without echo.

Anechoic Chamber

See Anechoic Room

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.

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

Animation

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

Anti-aliasing

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.

Anti-resonance

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.

Articulation

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. See also Ear Simulator.

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.

ASA

Acoustical Society of America.

Asper

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

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.

Asynchronous

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.

Attack

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

Attenuate

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

Attenuator

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.

Audio Spectrum

See Audio Frequency.

Audiogram

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

Audiometer

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.

Audiometric Zero

see Reference zero. The terms are synonymous.

Audiometry

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 Area

The sensory area lying between the Threshold of Hearing and the Threshold of Pain.

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.

Aural

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.

Autorange

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.

Autoscale

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.

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 the mean square of the filter output.

Averaging

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. See also Order Analysis.

Axial

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.




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