Definitions, Terms, Units and Parameters
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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/D Converter: converts a analogue signal to a digital one.
Absorption : A: sound waves are absorbed by 'acoustically soft' materials they encounter, as opposed to being reflected by 'acoustically hard' materials. An open window is an example of 100% absorption and a Reverberant Room or Chamber has 'hard' - very low absorption surfaces.
See also room absorption. and sound absorption and sabin and reverberation time measurements
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 = 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.
velocity = a/ω and displacement = v/ω, where ω = 2·π·f
So at 159 Hz an acceleration of 10 m/s2 = 0.01 m/s and = 10 μm
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.
aw: is time-averaged, frequency-weighted, single-axis vibration acceleration.
See also Particle Acceleration, used in acoustic wave theory, see also Angular Acceleration.
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 dB: La = 20 lg (a/ao) dB re 1 µm/s2
Acceleration Reference Level : ao = 10-6 m/s2 (defined in ISO 1683) *
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 other Standard 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 PiezoElectric 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 energy that is generated when materials are under stress or break.
Acoustic Impedance : Za: is the opposition to the flow of sound through a material. Unit: pascal second per cubic metre Pa·s/m3, see also rayl
Za = p/v : is the sound pressure divided by the particle velocity of an acoustic wave.
Za = I/v2 : is the sound intensity divided by the particle velocity.
Za = p2/I : the sound pressure squared divided by the sound intensity.
Acoustic Intensity: is the sound power per unit area. More details under Sound Intensity.
Acoustic Louvre: a specially built louvre designed with sound-attenuating baffles for reduction of airborne sound.
Acoustic Ohm: a measurement of sound resistance. These units got their name by analogy with electric resistance, which is measured in ohms - see acoustic impedance above.
Acoustic Power: also known as sound power is the sound energy measured over a stated period of time.
Acoustic Pressure : p: more commonly known as sound pressure is the difference between the pressure produced by an acoustic wave and the atmospheric pressure at a given point in space, and is measured in pascal - symbol Pa, units newton per square metre.
1 Pa = 1 N/m2 = 1 J/m3 = 10-5 bar = 1 kg/(m·s2)
more details 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 Speed of Sound.
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.
Other Sound Fields.
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 .
Airborne Sound: sound that reaches the point of interest by propagation through air.
Airborne Sound Insulation Index : Ia'': former name for - Weighted Apparent Sound Reduction Index : R'w
Air Condenser Microphones: are widely used in noise measurements because they offer the best linearity, frequency range and high stability.
Because of their importance we have prepared more details Measurement Microphones
Algorithm: a specific procedure for solving mathematical problems. An FFT is an algorithm.
Aliasing: digital sampling requires an analogue signal is sampled at twice the frequency of interest otherwise aliasing occurs. If the signal is not filtered to eliminate the high frequencies, they appear as 'false' lower frequency signals. Once 'introduced' these aliased signals cannot be distinguished from valid sampled data.
See also Anti-aliasing filter and the Nyquist Frequency.
Ambient Noise: the total of all noise in the environment - factory noise, traffic noise, bird song, running water, etc., near and far. See also background noise, residual noise and specific noise.
Ambient Pressure: the ambient pressure is the pressure of the surrounding medium. The SI unit is the Pascal - Pa which is very small relative to the atmospheric pressure.
Ampere : A: the basic SI unit of electric current; the constant current that, when maintained in two parallel conductors of infinite length and negligible cross section placed 1 metre apart in free space, produces a force of 2 x 10-7 newton per metre between them.
1 ampere is equivalent to 1 coulomb per second
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. Also 'known' as probability amplitude,
Analogue to Digital Converter: converts an analogue signal to a digital one. US spelling analog.
Anechoic: without echo - refers to the absence of sound 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.
Angles: angles are usually expressed in degrees or radians. Less often as Grads or Gradians (except on some electronic calculators)
Angular Acceleration : α: is the rate of change of angular velocity with time
units : radians per second squared, rads/s2
Angular Displacement : θ: is measured in radians rather than degrees. This is because it provides a very simple relationship between distance travelled around the circle and the distance r from the centre.
θ = s/r = length of arc divided by the radius in radians
Angular Frequency : ω: the frequency expressed in radians per second (rad/s). To convert a frequency in hertz to an angular frequency multiply by 2·π For an oscillation with period T, the angular frequency ω = 2·π / T
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 - see also Nyquist Frequency.
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 to 1. The higher the number the higher the intelligibility of words and sentences understood from 0-100%. An Articulation Index of less than 0.3, generally suggests unintelligible speech and one over 0.7 indicates excellent intelligibility.
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.
Asymmetric: a waveform not identical on both sides of the mean or zero line, lacks symmetry.
Atmospheric Pressure : atm: is the force per unit area exerted on a surface by the weight of air above that surface.
Standard Atmospheric Pressure, at sea level, is equal to 101.325 kPa the preferred SI units or 8760 mmHg and 1013.25 millibars.
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: Exponential Linear RMS Spatial Spectrum Time Domain
aw: is the time-averaged frequency-weighted, single-axis vibration acceleration.
Axial Mode: the room resonances associated with each pair of parallel walls in a rectangular room.
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