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A/D Converter
Converts an analogue signal to a digital one.<!-- SubItem DISABLED End {BCCE5486-D5A5-48AD-8461-F5CD52F5103F} --><!-- SubItem DISABLED Begin 22EEA734-EF3B-4892-9FFF-A5151B634667 -->
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. <!-- SubItem DISABLED End {22EEA734-EF3B-4892-9FFF-A5151B634667} --><!-- SubItem DISABLED Begin 80F01677-9B93-4FE8-A7DD-659B65C14E5B -->
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. <!-- SubItem DISABLED End {80F01677-9B93-4FE8-A7DD-659B65C14E5B} --><!-- SubItem DISABLED Begin 9C0E508D-B1C3-4292-A476-5995AB5ECF68 -->
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.<!-- SubItem DISABLED End {9C0E508D-B1C3-4292-A476-5995AB5ECF68} --><!-- SubItem DISABLED Begin 07114818-7BE5-4D43-98F9-387923AAE463 -->
Abffusor
A proprietary panel offering both absorption and diffusion of sound.<!-- SubItem DISABLED End {07114818-7BE5-4D43-98F9-387923AAE463} --><!-- SubItem DISABLED Begin 71FCCAB1-E5AE-4259-A6C8-812696543377 -->
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<!-- SubItem DISABLED End {71FCCAB1-E5AE-4259-A6C8-812696543377} --><!-- SubItem DISABLED Begin 443ABEA4-4BF8-4FAA-AF43-47AB76658ABC -->
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.<!-- SubItem DISABLED End {443ABEA4-4BF8-4FAA-AF43-47AB76658ABC} --><!-- SubItem DISABLED Begin 4BBAF2B4-DA26-4295-96F9-F38F712EF4FF -->
AC Coupling
The connection of a signal from one circuit to another in a manner that rejects DC components. See also DC Coupling. <!-- SubItem DISABLED End {4BBAF2B4-DA26-4295-96F9-F38F712EF4FF} --><!-- SubItem DISABLED Begin 627FA8C8-F2C3-469F-A2EA-908860E2D594 -->
Accelerance
The frequency response function of acceleration/force. Also known as inertance.<!-- SubItem DISABLED End {627FA8C8-F2C3-469F-A2EA-908860E2D594} --><!-- SubItem DISABLED Begin 12E7609C-C66E-4965-836A-A97202C158B3 -->
Acceleration
A vector quantity that specifies rate of change of velocity.<!-- SubItem DISABLED End {12E7609C-C66E-4965-836A-A97202C158B3} --><!-- SubItem DISABLED Begin 2ECFCB32-3EC1-42C8-9054-6294F0D7BD1D -->
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
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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 |
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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.<!-- SubItem DISABLED End {D17E4064-C15E-41D7-8DE9-076703F8F16F} --><!-- SubItem DISABLED Begin D8A67080-F1F5-4429-8614-3F64031FD812 -->
Accuracy
How close a measurement is to the absolute quantity.<!-- SubItem DISABLED End {D8A67080-F1F5-4429-8614-3F64031FD812} --><!-- SubItem DISABLED Begin 59ACEC20-921F-447A-B3C9-7D0A34214990 -->
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
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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.<!-- SubItem DISABLED End {A792A0A2-BC88-4A24-9FCD-F4E074A15468} --><!-- SubItem DISABLED Begin A58086F9-B739-4722-B013-8858EA3E5114 -->
Acoustic FRF
FRF in airborne contribution measurements (sound pressure/volume velocity). Also referred to as AFRF.<!-- SubItem DISABLED End {A58086F9-B739-4722-B013-8858EA3E5114} --><!-- SubItem DISABLED Begin 9DBD599F-7766-460A-BA18-357A405EA5E4 -->
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. <!-- SubItem DISABLED End {9DBD599F-7766-460A-BA18-357A405EA5E4} --><!-- SubItem DISABLED Begin 23890C4B-E008-45DE-AD76-9CD65DD3E6D9 -->
Acoustic indicator
In airborne contribution measurements, the position where one measures to calculate strength at source points.<!-- SubItem DISABLED End {23890C4B-E008-45DE-AD76-9CD65DD3E6D9} --><!-- SubItem DISABLED Begin 0EBEF5AD-7B98-4703-B242-443792E33603 -->
Acoustic Reflex
Bilateral contraction of the stapedius and/or tensor tympani muscles in response to an auditory or other eliciting stimulus.<!-- SubItem DISABLED End {0EBEF5AD-7B98-4703-B242-443792E33603} --><!-- SubItem DISABLED Begin 0579122C-EB05-4FCE-9EBF-9D70753F459D -->
Acoustic Reflex Threshold (ART)
The least sound pressure level of a sound that elicits the acoustic reflex.<!-- SubItem DISABLED End {0579122C-EB05-4FCE-9EBF-9D70753F459D} --><!-- SubItem DISABLED Begin DA4309B7-696F-47A4-BD59-ADB4C25A6132 -->
Acoustic source
In airborne contribution measurements, one or several source points.<!-- SubItem DISABLED End {DA4309B7-696F-47A4-BD59-ADB4C25A6132} --><!-- SubItem DISABLED Begin 4F2BD9EB-FFF1-490E-8D40-D2A03F4BA011 -->
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.<!-- SubItem DISABLED End {4F2BD9EB-FFF1-490E-8D40-D2A03F4BA011} --><!-- SubItem DISABLED Begin 51B1722E-E1BE-4C61-967C-4BA647619681 -->
Acoustical Louver
A specially built louver designed with sound-attenuating baffles for reduction of airborne sound.<!-- SubItem DISABLED End {51B1722E-E1BE-4C61-967C-4BA647619681} --><!-- SubItem DISABLED Begin 8215B51B-F065-48D7-BA00-A23414366708 -->
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.<!-- SubItem DISABLED End {8215B51B-F065-48D7-BA00-A23414366708} --><!-- SubItem DISABLED Begin D0FFBF26-274C-4322-9682-EEC7949A5E9F -->
Active Intensity
The propagating part of a sound field, producing a net flow of sound energy.<!-- SubItem DISABLED End {D0FFBF26-274C-4322-9682-EEC7949A5E9F} --><!-- SubItem DISABLED Begin 128436CC-4BB2-4059-8CC0-D85EE96C40A1 -->
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.<!-- SubItem DISABLED End {128436CC-4BB2-4059-8CC0-D85EE96C40A1} --><!-- SubItem DISABLED Begin 11A73F0B-BA87-4912-A4EE-D2ED27EBFCE7 -->
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.<!-- SubItem DISABLED End {11A73F0B-BA87-4912-A4EE-D2ED27EBFCE7} --><!-- SubItem DISABLED Begin 75B96D81-10CB-4A42-8198-39717844EEC6 -->
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.<!-- SubItem DISABLED End {75B96D81-10CB-4A42-8198-39717844EEC6} --><!-- SubItem DISABLED Begin 48D32CDB-146A-4BC5-B2BB-0C9F1CC929E3 -->
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.<!-- SubItem DISABLED End {48D32CDB-146A-4BC5-B2BB-0C9F1CC929E3} --><!-- SubItem DISABLED Begin 3073FD0C-C384-4E36-BE5D-CB8BAB3772E7 -->
Admittance (aural)
The reciprocal of Impedance. See Immittance. <!-- SubItem DISABLED End {3073FD0C-C384-4E36-BE5D-CB8BAB3772E7} --><!-- SubItem DISABLED Begin 7F793E72-670E-49FF-9FF0-4A25621A8E72 -->
AES
Audio Engineering Society.<!-- SubItem DISABLED End {7F793E72-670E-49FF-9FF0-4A25621A8E72} --><!-- SubItem DISABLED Begin 17B4A99E-74E9-4F31-9EB7-BD9A6CCE8F4A -->
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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<!-- SubItem DISABLED End {EDB11997-B7DC-4F42-A727-05173D5B2EE6} --><!-- SubItem DISABLED Begin 59645F35-4BC5-4B97-8E99-880C2756C0EB -->
Airborne Sound
Sound that reaches the point of interest by propagation through air.<!-- SubItem DISABLED End {59645F35-4BC5-4B97-8E99-880C2756C0EB} --><!-- SubItem DISABLED Begin EBB57423-5C89-45C6-92EC-40C8E92F01AD -->
Algorithm
A specific procedure for solving mathematical problems. An FFT is an algorithm.<!-- SubItem DISABLED End {EBB57423-5C89-45C6-92EC-40C8E92F01AD} --><!-- SubItem DISABLED Begin AA5EEC2B-A99D-4BE0-BF02-886A1C114000 -->
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.<!-- SubItem DISABLED End {AA5EEC2B-A99D-4BE0-BF02-886A1C114000} --><!-- SubItem DISABLED Begin 8A072642-617A-4BC9-8796-297BCDAC9AB0 -->
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.<!-- SubItem DISABLED End {8A072642-617A-4BC9-8796-297BCDAC9AB0} --><!-- SubItem DISABLED Begin A097A515-76E8-4354-A3AA-6AA2C99CE59A -->
Alignment
A condition whereby the axes of machine components are coincident, parallel or perpendicular, according to design requirements, during operation.<!-- SubItem DISABLED End {A097A515-76E8-4354-A3AA-6AA2C99CE59A} --><!-- SubItem DISABLED Begin AECA79C4-2AD6-49E6-8CDF-796393BE3936 -->
Ambience
The distinctive acoustical characteristics of a given space.<!-- SubItem DISABLED End {AECA79C4-2AD6-49E6-8CDF-796393BE3936} --><!-- SubItem DISABLED Begin 9E5EE69E-A8A7-40B7-BF33-F3625EB70633 -->
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. <!-- SubItem DISABLED End {9E5EE69E-A8A7-40B7-BF33-F3625EB70633} --><!-- SubItem DISABLED Begin 8C0C017C-5096-45D4-8910-A0074A0541A8 -->
Ambient Noise Level
The total noise level in the acoustic environment, including the noise source(s) of interest.<!-- SubItem DISABLED End {8C0C017C-5096-45D4-8910-A0074A0541A8} --><!-- SubItem DISABLED Begin D1496595-547C-4A58-9559-0223D0E53FD5 -->
Ambient Sound
The combination of all near and far sounds, none of which is particularly dominant.<!-- SubItem DISABLED End {D1496595-547C-4A58-9559-0223D0E53FD5} --><!-- SubItem DISABLED Begin FFFC01D0-8C0A-42BC-873E-BE43FAD2AFF6 -->
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.<!-- SubItem DISABLED End {FFFC01D0-8C0A-42BC-873E-BE43FAD2AFF6} --><!-- SubItem DISABLED Begin DA2ED3A1-8878-49F7-9503-DC1DA14F13C8 -->
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.<!-- SubItem DISABLED End {DA2ED3A1-8878-49F7-9503-DC1DA14F13C8} --><!-- SubItem DISABLED Begin C849CE89-F17B-4663-854E-731BBD2F2E5D -->
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.<!-- SubItem DISABLED End {C849CE89-F17B-4663-854E-731BBD2F2E5D} --><!-- SubItem DISABLED Begin 04A80329-67BB-42BE-B381-1046E839FAE8 -->
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Amplitude Distortion
A distortion of the wave shape of a signal.<!-- SubItem DISABLED End {444AD3F6-DF17-4C48-87A9-4BAFE01381F3} --><!-- SubItem DISABLED Begin 78F975E3-D805-4FA7-A960-42F5B84219F6 -->
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.<!-- SubItem DISABLED End {78F975E3-D805-4FA7-A960-42F5B84219F6} --><!-- SubItem DISABLED Begin B40748FD-E515-45FB-B9B3-275CA25F1038 -->
Amplitude Probability
Used to investigate the amplitude distribution of signals.<!-- SubItem DISABLED End {B40748FD-E515-45FB-B9B3-275CA25F1038} --><!-- SubItem DISABLED Begin F09EB8F2-8070-4169-8792-4E31F14DBA8A -->
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. <!-- SubItem DISABLED End {F09EB8F2-8070-4169-8792-4E31F14DBA8A} --><!-- SubItem DISABLED Begin 4C13F489-0914-46EB-B2AD-852DDCD29B39 -->
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.<!-- SubItem DISABLED End {4C13F489-0914-46EB-B2AD-852DDCD29B39} --><!-- SubItem DISABLED Begin 1C5553F3-2A75-4712-868D-0A74B4DDAEDA -->
Analog Signal
An electrical signal whose frequency and level vary continuously in direct relationship to the original electrical or acoustical signal.<!-- SubItem DISABLED End {1C5553F3-2A75-4712-868D-0A74B4DDAEDA} --><!-- SubItem DISABLED Begin 790B5A65-FFBA-49E1-BDD3-669E98CC0845 -->
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. <!-- SubItem DISABLED End {790B5A65-FFBA-49E1-BDD3-669E98CC0845} --><!-- SubItem DISABLED Begin 208B9386-E489-466C-8213-A35ECA529999 -->
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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.<!-- SubItem DISABLED End {0C98E8CE-4F07-4CD9-8738-384C61F28947} --><!-- SubItem DISABLED Begin 6C161FFF-2C1C-4344-9A25-89F3FBDCAE06 -->
Anechoic
Without echo.<!-- SubItem DISABLED End {6C161FFF-2C1C-4344-9A25-89F3FBDCAE06} --><!-- SubItem DISABLED Begin B0AEB2E1-CAB2-4298-B0F9-E5BF3EFFFD79 -->
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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.<!-- SubItem DISABLED End {3C855BCE-90F3-458F-8792-F11577E1249A} --><!-- SubItem DISABLED Begin 9F749D91-A7A8-4FDE-9A0A-D7B67E91460B -->
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 |
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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.<!-- SubItem DISABLED End {EEED52D1-5CEC-4E28-B997-754B98AE9016} --><!-- SubItem DISABLED Begin 548EBE14-1890-4F20-AE5E-A66551D7CC99 -->
Animation
Refers to a kind of "slow motion movie" that allows easy visualisation of, for example, a vibrating structure.<!-- SubItem DISABLED End {548EBE14-1890-4F20-AE5E-A66551D7CC99} --><!-- SubItem DISABLED Begin 73A32751-66F6-4CE8-88E8-91B33CF2CFD2 -->
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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).<!-- SubItem DISABLED End {19997E54-901C-4D48-A077-FE40831B47D7} --><!-- SubItem DISABLED Begin BF91A4A2-2A24-40CD-A5FA-62019619A8FE -->
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.<!-- SubItem DISABLED End {BF91A4A2-2A24-40CD-A5FA-62019619A8FE} --><!-- SubItem DISABLED Begin 1EE5E191-C2F6-467A-BB49-B9D7CED158DA -->
Anti-resonance
A phenomenon in an electric, acoustic, or other such system in which the impedance is tending to infinity.<!-- SubItem DISABLED End {1EE5E191-C2F6-467A-BB49-B9D7CED158DA} --><!-- SubItem DISABLED Begin 0F1E91AE-71FD-458B-BF98-0F56A6A92E18 -->
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.<!-- SubItem DISABLED End {0F1E91AE-71FD-458B-BF98-0F56A6A92E18} --><!-- SubItem DISABLED Begin 9161869D-7412-4139-BC24-542F1DC8B3B4 -->
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Articulation
A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.<!-- SubItem DISABLED End {9D428945-EC5B-4C05-A3B2-B26EEFFF8C68} --><!-- SubItem DISABLED Begin EA2FCD0B-DCFA-4070-905B-8B4E6C627BAA -->
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.<!-- SubItem DISABLED End {EA2FCD0B-DCFA-4070-905B-8B4E6C627BAA} --><!-- SubItem DISABLED Begin 334AC816-09F6-4061-8B20-CFDB08AAD572 -->
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. <!-- SubItem DISABLED End {334AC816-09F6-4061-8B20-CFDB08AAD572} --><!-- SubItem DISABLED Begin C38F61B2-B9E9-481A-A52E-1A2F620D4CBC -->
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.<!-- SubItem DISABLED End {C38F61B2-B9E9-481A-A52E-1A2F620D4CBC} --><!-- SubItem DISABLED Begin 207409EC-4B39-476A-B2E7-75AA695B8475 -->
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.<!-- SubItem DISABLED End {207409EC-4B39-476A-B2E7-75AA695B8475} --><!-- SubItem DISABLED Begin A947A726-084C-4C0E-9335-2848D1DC44DB -->
ASA
Acoustical Society of America.<!-- SubItem DISABLED End {A947A726-084C-4C0E-9335-2848D1DC44DB} --><!-- SubItem DISABLED Begin 01C56DF9-8CF1-4DC0-8895-7E606278B206 -->
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. <!-- SubItem DISABLED End {01C56DF9-8CF1-4DC0-8895-7E606278B206} --><!-- SubItem DISABLED Begin 20639604-7A90-444E-95F0-3B78FDC7E954 -->
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.<!-- SubItem DISABLED End {20639604-7A90-444E-95F0-3B78FDC7E954} --><!-- SubItem DISABLED Begin D244C648-6669-4363-B087-ACD8F1CB1910 -->
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.<!-- SubItem DISABLED End {D244C648-6669-4363-B087-ACD8F1CB1910} --><!-- SubItem DISABLED Begin 7FF653D7-38EA-4321-AF6B-E02F37D81842 -->
Attack
The beginning of a sound; the initial transient of a musical note.<!-- SubItem DISABLED End {7FF653D7-38EA-4321-AF6B-E02F37D81842} --><!-- SubItem DISABLED Begin 6204D899-A987-4886-BF06-E882B3635655 -->
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
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Attenuator
A device, usually a variable resistance, used to control the level of an electrical signal.<!-- SubItem DISABLED End {3F0365D9-27BD-4D09-904D-46A9A9CED0D3} --><!-- SubItem DISABLED Begin C7218B31-0496-437C-BB74-18B03A1B6D3A -->
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).<!-- SubItem DISABLED End {C7218B31-0496-437C-BB74-18B03A1B6D3A} --><!-- SubItem DISABLED Begin 78573DB3-9446-4DF5-939D-7E80155076B0 -->
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.<!-- SubItem DISABLED End {78573DB3-9446-4DF5-939D-7E80155076B0} --><!-- SubItem DISABLED Begin 6EDEBBB2-75E2-47C7-B92A-6CE36D955A3F -->
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.<!-- SubItem DISABLED End {6EDEBBB2-75E2-47C7-B92A-6CE36D955A3F} --><!-- SubItem DISABLED Begin 05288EE6-2452-4FFE-A529-41F6E5335344 -->
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Audiogram
A graph showing individual hearing acuity as a function of frequency.<!-- SubItem DISABLED End {0E1713AF-D2D5-4C39-8A69-0E725ABC95EE} --><!-- SubItem DISABLED Begin C3475EB0-10DA-408F-9C34-4C988FECA37D -->
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.<!-- SubItem DISABLED End {C3475EB0-10DA-408F-9C34-4C988FECA37D} --><!-- SubItem DISABLED Begin 3D02CF72-119E-4E26-8E85-AA69A0CCF31F -->
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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.<!-- SubItem DISABLED End {AAA8F6D7-5BC5-4D71-BC1C-AC432D13957A} --><!-- SubItem DISABLED Begin 9BD5AFA4-907A-4FA2-9AB9-F656359B79CF -->
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Auditory Cortex
The region of the brain receiving nerve impulses from the ear.<!-- SubItem DISABLED End {2DA722A2-8A14-47F3-AEA8-FDA074ABDE95} --><!-- SubItem DISABLED Begin EFDFCE8A-0A7B-41A0-AD8C-25D95F796C46 -->
Auditory System
The human hearing system made up of the external ear, the middle ear, the inner ear, the nerve pathways, and the brain.<!-- SubItem DISABLED End {EFDFCE8A-0A7B-41A0-AD8C-25D95F796C46} --><!-- SubItem DISABLED Begin D931F93B-BBD9-478D-B02A-C1C88EDE66FA -->
Aural
Having to do with the auditory mechanism.<!-- SubItem DISABLED End {D931F93B-BBD9-478D-B02A-C1C88EDE66FA} --><!-- SubItem DISABLED Begin 9D43E41E-2484-4531-AB99-3F0C55D19790 -->
Aures Sharpness Calculation
A correction applied to the Zwicker formula that gives improved level independence.<!-- SubItem DISABLED End {9D43E41E-2484-4531-AB99-3F0C55D19790} --><!-- SubItem DISABLED Begin 33427BA4-7AA2-41F0-AFA6-5A2B949F7F8C -->
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.<!-- SubItem DISABLED End {33427BA4-7AA2-41F0-AFA6-5A2B949F7F8C} --><!-- SubItem DISABLED Begin 65A57021-0066-4BAD-8E00-DD82718CEF27 -->
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.<!-- SubItem DISABLED End {65A57021-0066-4BAD-8E00-DD82718CEF27} --><!-- SubItem DISABLED Begin 783C9E72-62AA-45C3-81F0-EF233B3D2DAF -->
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.<!-- SubItem DISABLED End {783C9E72-62AA-45C3-81F0-EF233B3D2DAF} --><!-- SubItem DISABLED Begin E079992C-2FDE-401D-8F69-FC7F9D1A3F60 -->
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.<!-- SubItem DISABLED End {E079992C-2FDE-401D-8F69-FC7F9D1A3F60} --><!-- SubItem DISABLED Begin BDE304B9-3FEB-477F-9419-49472C821186 -->
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. <!-- SubItem DISABLED End {BDE304B9-3FEB-477F-9419-49472C821186} --><!-- SubItem DISABLED Begin 9A8A0FD3-6492-4FC5-ADAA-40D57D021718 -->
Axial
In the same direction as the shaft centreline.<!-- SubItem DISABLED End {9A8A0FD3-6492-4FC5-ADAA-40D57D021718} --><!-- SubItem DISABLED Begin EB13A938-1C84-4D30-A650-8EAF8258C0EC -->
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.<!-- SubItem DISABLED End {EB13A938-1C84-4D30-A650-8EAF8258C0EC} --><!-- SubItem DISABLED Begin 32D62A00-AE21-4BE0-89C4-90048622473F -->
Axial Mode
The room resonances associated with each pair of parallel walls in a rectangular room.