Noise and Vibration
Glossary – A
The world of acoustics is overflowing with parameters. Take a simple example – Lmax is the maximum sound level but was the measurement A-weighted, C-weighted, Z-weighted and was a Fast, Slow of Impulsive Time Constant used. Already we have introduced up to 9 variables for one simple measurement, without mentioning the measurement duration.
To describe any A-weighted, Fast Time Constant, Maximum measurements, correctly, we should use the form LAFmax. However in the real world many people continue to use the ’simple’ Lmax. Similarly the terms LA dBA or dB(A) is often used to describe the same measured value.
Some instrument manufacturers use XY to indicate a range of possible descriptors where X = dBA, dBC or dBZ frequency weightings and Y = Fast(F), Slow(S) or Impulse(I) time weighting. Using this convention LXYmax describes the range of possibilities for a maximum measurement.
This list is for customers who come across a term they do not know. If the term is not strictly correct we do not go into great detail explaining where it differs from the strict convention, we just say what the term ‘means’. It follows therefore, that this table is not for the professional acousticians who will already know and use the ‘correct’ terminology.
Some common parameters need more than one or two line descriptions so we provide links on these pages and a sub-menu here.
If you find our website helpful please add a link from your website to ours, it will really help us and other people. If you would like to Copy and Paste our suggested line of HTML code to your website, please click here to go to our Links Page – thanking you in anticipation.
GLOSSARY INDEX: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
% Dose : under Dose .
1/1 Octave : see octaves
1/3-octave Filter : octave bands sub-divided into three parts, equal to 23% of the centre frequency. Used when octave analysis is not discrete enough. Divides the audio spectrum into 33 or more equal parts with constant percentage bandwidth filter – see also third-octave filter.
1/n-octave : analysis 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 Curve : see A-weighting.
A/D Converter : converts an analogue signal to a digital one.
A Filter : see A-weighting.
A Network : see A-weighting.
A-weighting : 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 audio range – 20 Hz to 20 kHz. The shape of the A-weighting curve approximates to 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 – the unit is the dBA or dB(A).
Absorption : sound waves are absorbed or soaked up by soft materials they encounter. See also room absorption. and sound absorption and sabin
Absorption Coefficient : The absorptive capabilities of various materials are rated with an absorption coefficient, which is a measure of the relative amount of sound energy absorbed by that material when a sound strikes its surface. A coefficient of 1 means 100% absorption for example an open window.
AC Coupling : The connection of a signal from one circuit to another in a manner that rejects DC components – see also DC Coupling.
Acceleration : a : is the rate of change of velocity and is a vector quantity.
The SI units are m/s2 or if using Imperial units then ‘g’ = 9.80665 m/s2 = 386.089 in/s2
v = u + at where v = velocity, u = start velocity, a = acceleration in m/s2 and t = time.
In the field of vibration acceleration a, velocity v, displacement s and angular frequency ω are related. – for example v = a/ω, s = v/ω, where ω = 2·π·f
It follows that 10 m/s2 = 0.01 m/s = 10 µm at 159 Hz
This works for all frequencies, we just chose 159 Hz to keep the numbers simple. We also have a vibration nomogram for downloading.
If 1 N = 1 Kg·m/s2, it follows that m/s2 acceleration also equals N/kg newtons per kilogram.
Acceleration Equivalent Level : Aeq : a single number to represent the equivalent acceleration energy as it varies over a working day, measured in m/s2
For example A(8) = 2.5 m/s2 indicates a equivalent level of 2.5 m/s2 measured over an 8 hour shift.
To calculate the equivalent value for other periods use the formulae A(8) = a √T/8 where T = hours
see also the Vibration at Work Regulations and Hand Arm vibration syndrome – HAVS
The human response to vibration covers a wide range from a few micrometres per second squared to tens of metres per second squared. The dB scale is a useful way to represent the immense range using a manageable set of numbers.
Acceleration Level : La = 20 log (a/ao) dB re 10 micro-m/s2
an increase or decrease in acceleration of 20 dB = a factor of 10
a 40 dB = a factor of 100
a 60 dB = a factor of 1000 … etc.,
See also the table of decibel reference levels.
Accelerometer : a sensor whose electrical output is proportional to acceleration, intended for measurement of vibrations. The two most common types are the traditional ‘charge’ type and the IEPE, Integrated Electronic PiezoElectic type with a built-in line-drive amplifier to enable the output signal to be transmitted over ‘longer cable runs’.
ACGIH : American Conference of Industrial Hygienists.
Acoustic Admittance : the reciprocal of Acoustic Impedance.
Acoustic Calibrator : an instrument providing a reference noise source used to calibrate and check the performance of sound level meters.
Acoustic Emission : the detected energy that is generated when materials are deformed or break.
Acoustic Energy : Sound Energy .
Acoustic Impedance : Z = p/vS : is the pressure p divided by the particle velocity v and the surface area S, the SI unit is N·sm5.
vS is sometimes known as the Volume Velocity.
Specific Acoustic Impedance : z : is the impedance at a specific point, the SI unit is N·sm3.
Characteristic Impedance : Z : is the ratio of effective sound pressure at a point to the particle velocity at that point in a free, progressive wave. This ratio is equal to the product of the density of the medium times the speed of sound in the medium.
The Characteristic Impedance of air is the density p times the speed of sound c i.e. Z = pc ≈ 415.
Acoustic Intensity : under Sound Intensity.
Acoustic Louvre : a specially built louvre designed with sound-attenuating baffles for reduction of airborne sound.
Acoustic Ohm : any one of several units measuring sound resistance. These units got their name by analogy with electric resistance, which is measured in ohms – see also acoustic impedance above.
Acoustic Power : under Sound Power .
Acoustic Pressure : under Sound Pressure.
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.
Acoustic Velocity : under Particle Velocity.
Active Noise Control : reducing unwanted sound electronically. When a sound wave of equal amplitude but opposite sign (180 degree out of phase) is added to the original sound the result is sound cancellation.
Active Intensity : the propagating part of a sound field, producing a net flow of sound energy – see also sound intensity.
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.
Aeq : Acceleration Equivalent Value .
Age Related Threshold Shift : ARTS : is the component of permanent threshold shift related to age.
see also Threshold Shift : Temporary Threshold Shift : Permanent Threshold Shift .
AI : Articulation Index
Air Density : under Density of Air
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 : A phenomenon whereby an analogue signal of frequency greater than the Nyquist frequency appears after sampling at a frequency less than the Nyquist frequency. See also anti-aliasing filter.
Ambient Noise : the total of all noise in the environment – factory noise, traffic noise, bird song, running water, etc. – including the noise from the source of interest. See also background noise, residual noise and specific noise.
Amplification Factor : Q : the 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 Hz. 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 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.
Analogue to Digital Converter : converts an analogue signal to a digital one. US spelling analog.
Anechoic : without echo – refers to the absence of audio reflections. It is almost impossible to create a truly anechoic environment, as there is no such thing as a perfect sound absorber.
Anechoic Chamber : echo free room, walls lined with sound absorbent wedges to minimize reflections and create free-field conditions, so direct sound measurements of test objects may be made.
Angular Acceleration : α : is the rate of change of angular velocity with time
Angular Displacement : Θ : is the length of arc divided by the radius = S/r in degrees
Angular Frequency : ω : the frequency expressed in radians per second. To convert a frequency in hertz to an angular frequency multiply by 2·π
Angular Momentum : L : the quantity of rotation of a body, which is the product of the moment of inertia and it’s angular velocity.
units : newton metre seconds (N·m·s)
Angular Velocity : v : the rate of change of angular position of a rotating body.
units : radians per second, rads/s
ANSI : American National Standards Institution. :
Anti-aliasing Filter : analogue low pass filters used before analogue to digital conversion to filter out the frequencies greater than half the sampling frequency and prevent aliasing.
Area : A : a quantity expressing the two-dimensional size of a defined part of a surface. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid.
The SI unit is the square metre, symbol : m2
1 m2 = 1 m by 1 m or 2 m by 0.5 m etc.,
25 mm by 25 mm = 0.025 m by 0.025 m = 0.000625 m2
Articulation Index : AI : a measure of speech intelligibility influenced by acoustical environment rated from 0.01 to 1.00. The higher the number the higher the intelligibility of words and sentences understood from 0-100%.
Artificial Ear : 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 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.
ARTS : Age Related Threshold Shift : : is the component of permanent threshold shift related to age.
ASHRAE : American Society of Heating, Refrigeration and Air-Conditioning Engineers.
Audible Range : the human ear can respond to minute pressure variations in the air if they are in the frequency range, roughly 20 Hz – 20 kHz.
Audiogram : graph showing hearing loss as a function of frequency, measured with an audiometer.
Audiometer : an instrument for testing hearing, standard equipment in ENT, Audiometry and Audiology Centres.
Auditory Masking : occurs when the perception of one sound is affected by the presence of another sound – see also critical bands .
Autocorrelation : is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal which has been buried under noise. It is frequently used for analysing time domain functions. It is the cross-correlation of a signal with itself.
Auto Scale : 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 into the available viewing area
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 constant percentage bandwidth measurements, it is the mean square of the filter output.
Averaging : when performing spectrum analysis, over a period of time, some form of time averaging must be done to accurately determine the level of the signal at each frequency. 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. 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. Exponential Averaging generates a continuous running average where the most recently sampled spectra have more influence on the average than older ones. This provides a convenient form to examine changing data with the benefit of some averaging to smooth the spectra.
Axial Mode : the room resonances associated with each pair of parallel walls in a rectangular room.
GLOSSARY INDEX: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
If you can’t find what you are looking for please try our search engine for the acronym or keyword.
E&OE – please let us know about any errors or omissions – thanks
- Gracey & Associates 01933 624212
- BSI accreditation for instrument hire and calibration
- Home
- Search
- Link to Us