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Noise & Vibration Topics

2E: In four-cylinder four-stroke engines this notation is often used to denote an engine order where the frequency is two times the engine rotational speed.
3E: Basic firing frequency of a six-cylinder four-stroke engine.
4E: Two times engine firing frequency of a four-cylinder four-stroke engine. It is the basic firing frequency of an eight-cylinder four-stroke engine.
Absolute Pitch: The exact pitch value of a musical note (for example, middle C)
Absorbent Ducts: There are many different sizes and applications of absorbent ducts.
Absorbent Material: A material which absorbs energy from an incident sound wave.
Absorption Coefficient: The fraction of sound energy that is absorbed at any surface.
Acausal: If a system is acausal it means the output begins before the input.
Accelerance: For a point excitation of a mechanical system this is the complex ratio of acceleration to applied force.
Accelerometer: An instrument for measuring acceleration, as of aircraft or guided missiles.
Acoustic Admittance: This is the ratio of volume velocity to pressure, the reciprocal of Acoustic Impedance.
Acoustic Agglomeration: High intensity sound waves are used in a fluid to group suspended particles into larger aggregates.
Acoustic Barriers: An acoustic barrier between two rooms.
Acoustic Calibrator: A device which produces a known sound pressure on the microphone of a sound level measurement system.
Acoustic Cavity: The acoustics of enclosed volumes are important when considering sound propagation within the volume.
Acoustic Coupler: An acoustic coupler is means of connecting external devices to a telephone handset avoiding direct electrical connection.
Acoustic Ducts: Ducts with an acoustic treatment or shaped to reduce or control acoustic wave propagation.
Acoustic Emission: A measure of integrity of a material determined by sound emission when a material is stressed.
Acoustic Excitation: The process of inducing vibration in a structure by exposure to sound waves.
Acoustic Feedback: Acoustic feedback is the return of acoustic energy from the output of a sound reproducing equipment.
Acoustic Filter Elements: There are a number of different types of acoustic filter elements that are used in many different applications.
Acoustic Impedance: The total reaction of a medium to the transmission of sound through it.
Acoustic Intensity: The quotient obtained when the average rate of energy flow in a specified direction and sense is divided by the area, perpendicular to that direction, through or toward which it flows.
Acoustic Material: Any material considered in terms of its acoustical properties.
Acoustic Mirrors: They are used today for sound source location in wind tunnels and for listening to conversations.
Acoustic Mobility: This is the ratio of volume velocity to pressure, the reciprocal of Acoustic Impedance.
Acoustic Ohm: The unit of acoustic impedance.
Acoustic Origin: The point in time at which the signal originates.
Acoustic Pack: Name given to all of the soft absorptive parts used to reduce noise in a vehicle.
Acoustic Source Location: There are a number of different ways by which the location of a noise source may be found.
Acoustic Sources: The simplest acoustic source is the monopole.
Acoustic Tile: A descriptive term often used to describe commercially available room acoustic treatment.
Acoustic Wedges: The acoustic treatment used on all surfaces of an anechoic chamber.
Acoustics: From the Greek akouein ( "to hear") a term sometimes used for the science of sound in general.
Action Level: An action level is basically a noise exposure level at which employers are required to take certain steps to reduce the harmful effects of noise on hearing.
Active Noise Control: This is an electronic method of reducing or removing unwanted sound by the production of a pressure wave of equal amplitude but opposite sign to the nwanted sound.
ADC: Abbreviation of Analogue to Digital converter.
Aerodynamic Noise: Sound generated by turbulent flow is just as if the field were generated by a distribution of quadrupole sources.
Aggravated Test: A test in which one or more conditions are set at a more stressful level that the test item will encounter in the field, in order to reduce test time or assure a margin of safety.
Air Intake Manifold: The air intake manifold of an internal combustion engine performs a number of functions.
Air Intake Noise: The air intake system performs four main functions, one of which is the attenuation of acoustic waves.
Airborne Sound: Sound that arrives at the point of interest, such as one side of a partition, by propagation through air.
Airflow Resistance: The quotient of the air pressure difference across a specimen divided by the volume velocity of airflow through the specimen. The pressure difference and the volume velocity may be either steady or alternating.
Airflow Resistivity: For a homogeneous material it is the quotient of its specific airflow resistance divided by its thickness.
ALCONS: The measured percentage of Articulation Loss of Consonants by a listener.
Aliasing: The erroneous interpretation of high-frequency signals as lower-frequency signals.
Ambient Noise: The composite of airborne sound from many sources near and far associated with a given environment.
Amplitude Distortion: A distortion of the wave shape of a signal.
Amplitude Envelope: The function describing how the maximum amplitude of a waveform evolves over time.
ANC: This is an electronic method of reducing or removing unwanted sound by the production of a pressure wave of equal amplitude but opposite sign to the nwanted sound.
Anechoic: Without echo.
Anechoic Chamber: A room whose boundaries effectively absorb all incident sound over the frequency range of interest, thereby creating essentially free field conditions.
Angular Frequency: The frequency of a steady recurring phenomenum in radians per second.
Angular Rate Sensor: A sensor that measures rotational velocity around its sensitive axis.
Anharmonic: Of or pertaining to an oscillating system that is not undergoing simple harmonic motion.
Anisochrony: A sequence of events where the time seperating each pair is not equal.
Anti Resonance: In multi degree of freedom systems or continuous systems there will exist frequencies at which a dynamic force applied at a point will not cause any motion of the system at that point.
Anti-Aliasing Filter: A device that attenuates signal content outside the desired bandwidth.
Antinode: Point of maximum displacement.
Aperiodic: Vibrations or oscillations with no apparent period.
Apical Turn: The final turn in the spiral of the cochlea furthest from the oval and round windows.
Apodize: To apodize is to remove or smooth a sharp discontinuity in a mathematical function, an electrical signal or a mechanical structure.
Apparent Mass: Force per unit acceleration.
Articulation Class: A measure for rating the speech privacy performance of a ceiling in an open plan environment.
Articulation Index: A quantitative measure of the intelligibility of speech; the percentage of speech items correctly perceived and recorded.
Articulation Loss of Consonants: The measured percentage of Articulation Loss of Consonants by a listener.
Artificial Reverberation: Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc.
Attack: The beginning of a sound; the initial transient of a musical note.
Attenuation of Sound in Air: The attenuation of sound in air at 20�C due to viscous, thermal and rotational loss mechanisms is 1.6E-10f�dB/m.
Audio Frequency: Generally in the range 20Hz to 20KHz.
Auditory Area: The sensory area lying between the threshold of hearing and the threshold of feeling or pain.
Autocorrelation: The autocorrelation function gives a measure of the extent to which a signal correlates with a displaced version of itself, as a function of the displacement.
Autocovariance: This is the degree to which a function is correlated with itself as a function of time.
Autospectrum: A spectrum with the coefficients of the components expressed as the square of the magnitudes
Average Room Absorption Coefficient: Total room absorption in sabins or metric sabins, divided by total room surface area in consistent units of square feet or square metres.
A Weighting: This is the most generally used filter when making overall noise measurements.
Axial Mode: The room resonances associated with each pair of parallel walls.
B: An abbreviation of bel, the logarithm of the ratio of two powers. The decibel is one tenth of a bel.
Background Noise: Noise from all sources unrelated to a particular sound that is the object of interest.
Baffle: A movable barrier used to achieve separation of signals from different sources.
Balancing: A procedure for adjusting the mass distribution of a rotor so that vibration of journals, or the forces on the bearings at once-per-revolution, are reduced or controlled.
Band Stop Filter: An electronic network which passes signals having frequencies above and below the band-elimination frequency span.
Bandpass Filter: An electronic network which transfers through it a specific band of signal frequencies.
Bandwidth: The bandwidth of a filter is the separation between the lower and upper frequencies at which the amplitude of a sinusoidal signal is attenuated by a factor of 2.
Bandwidth Time Product: Used in calculating the confidence limit of a measurement.
Bark: The Bark is the standard unit corresponding to one critical band width of human hearing.
Bass Ratio: In concert hall acoustics, the ratio of the average reverberation times at 125 and 250 Hz to the average of the RT′s at 500 and 1000 Hz. It is determined only for a hall when fully occupied.
Beats: Periodic fluctuations that are heard when sounds of slightly different frequencies are superimposed.
Behavioural Envelope: Envelope of response versus frequency.
bel: The bel is the logarithm of the ratio of two powers, and the decibel is one tenth of a bel.
Belt Noise: There are many different mechanisms by which drive belts generate noise.
Bending Mode: Mode of vibration in which cross sections of a beam, shaft or structure undergo translation and rotation. Type of translational mode usually found in slender structures with evenly distributed mass and stiffness.
Bimodal: In the home entertainment context, pertaining to presentations involving the visual and auditory sensory modalities.
Binaural: A situation involving listening with two ears.
Blackman-Harris Window: A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis. The Blackman-Harris window has much the same performance as the Kaiser-Bessel window, except that it suppreses the sidelobes more than 92dB at a cost of an 11% wider noise bandwidth.
Blade Passing Frequency: A potential vibration frequency on any bladed machine (turbine, axial compressor, fan, etc.). It is represented by the number of blades times shaft-rotating frequency.
Block Companding: Normalising of the digital representation of an audio signal within a certain time period.
Bode Plot: A plot of the frequency response function that includes log magnitude versus frequency plus phase versus frequency.
Bohman Window: A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis.
Boomy: Listening term, refers to an excessive bass response that has a peak(s) in it.
Bounce: A term which is used to describe the oscillations of a rigid body, for example, the sprung and unsprung masses of a vehicle, that consists primarily of vertical displacement.
Boundary Element Method: A mathematical formulation used to predict acoustic parameters such as pressure and power. This technique may be applied to interior and exterior acoustic problems.
Brake Graunch: The name given to the noise made by the brakes when they are just slipping when starting (or stopping) from rest. The brake graunch is mainly exhibited on the vehicles fitted with automatic transmission, but can also occur on manual vehicles e.g., on a steep hill. It is caused by brake pad stick-slip when static and dynamic friction is very similar. The term 'creep/groan' is also used to describe the brake graunch.
Brake Judder: Brake judder is the phenomenon where with medium to heavy braking from high speed, severe vibration is felt throughout the whole vehicle. It is often attributed to a mode of vibration in which the front suspension executes a predominantly fore and aft vibration with the two road wheels in-phase with each other. Excitation is often provided by the oscillatory forcing generated when applying the brakes to a brake disk that has disk thickness variation.
Brake Squeal: High frequency continuous tone when brakes are applied, this may be a single or multiple tones. The sound is generated by vibration on the surface of the disc itself.
Bright: In acoustic terms this refers to too much high frequency energy.
Broad Band Signal: A random signal containing a wide range of frequency components. Some types of electrical noise in a cable and acoustic noise from turbulent flow tend to be broad band.
Broadband Noise: Noise with components over a wide range of frequencies.
Building Acoustics: Acoustics associated with the operation and use of a building.
Bulk Modulus: The bulk modulus of a gas is a measure of its compressibility (elastic property).
B Weighting: The B-weigthing curve approximately follows the 70 phon curve.
Campbell Diagram: A mathematically constructed diagram used to check for coincidence of vibration sources with natural resonances.
Capacitor Microphone: Microphone whose operation depends on variations in capacitance caused by varying air pressure on the movable plate of a capacitor.
Carbon Microphone: Microphone whose operation depends on pressure variation in carbon granules causing a change in resistance.
Cauchy Window: A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis.
Cavity Acoustics: The acoustics associated with enclosed volumes.
Ceiling Attenuation Class: This is a rating value for the efficiency of the ceiling as a barrier to airborne sound transmission.
Cent: In musical research, a unit of pitch change equal to 0.01 semitones.
Centre Frequency: Frequency to which an amplifier is tuned. The frequency half way between the cut-off frequencies of a tuned circuit.
Cepstrum: A cepstrum is a time history defined as the power spectrum of the logarithm of the Power spectrum.
Circle Fitting: This is a common method of extracting mode shape vectors from a set of frequency response functions.
Circular Frequency: The frequency of a steady recurring phenomenum in radians per second.
Clipping: Refers to a type of distortion that occurs when an amplifier is driven into an overload condition.
CNEL: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Cochlea: A snail shaped mechanism in the inner ear that contain hair cells of basilar membrane that vibrate to aid in frequency recognition.
Cocktail Party Effect: This is the ability of the human hearing system to effectively reject spurious noises and allow the hearing to "home in" on the sound that is arriving from a particular direction.
Coherence: This gives a measure of the linear dependence between signal A and B.
Coherent: A property of two or more sources of waves that have the same wavelength and maintain constant phase differences.
Coincidence: The sound radiation efficiency of a plate is dependant upon the coupling of sound waves in the air and flexural waves (vibration) in the plate. Optimum efficiency (maximum energy transfer from vibration to sound or visa versa) is achieved when the plate is vibrated such that the wavelength of flexural waves in the plate is equal to the wavelength of acoustic waves in the air.
Colouration: A listening term. A visual analog. A "coloured" sound characteristic adds something not in the original sound. The colouration may be euphonically pleasant, but it is not as accurate as the original signal.
Colourmap: This is a method of plotting noise or vibration versus engine speed and frequency simultaneously.
Community Noise Equivalent Level: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Complex Tone: Sound wave containing simple harmonic components of different frequencies.
Compliance: The displacement caused by a unit force applied to a spring or structural component. It is the reciprocal of stiffness (Units m/N).
Composite Acoustic Barriers: Acoustic barriers are generally made from panels of different acoustic performance.
Compressional Wave: Wave in an elastic medium which causes an element of the medium to change its volume without undergoing rotation.
Condition Monitoring: The measurement, recording and analysis of machinery parameters (such as acceleration) to determine machinery health. Today′s condition is compared with earlier condition, as when a machine was new.
Conductive Hearing Loss: Hearing loss due to the impairment of sound transmission before it reaches the inner ear.
Consonance: Two or more sounds that, when heard together, sound pleasant.
Constant Percentage Bandwidth Filter: A band-pass filter whose bandwidth is a constant percentage of centre frequency. 1/3 octave filters are constant percentage bandwidth.
Constrained-Layer Damper: A layer of damping material between the structure′s surface and an additional elastic layer.
Convolution: Convolution meaning ..intertwined, coiled�
Correlation: Generally speaking, a similarity between data; the extent to which data are related.
Coupled Modes: Coupled modes are modes of vibration that are not independent but which influence one another because of energy transfer from one mode to the other.
CPS: Abbreviation of cycles per second.
Crank Rumble: This is an amplitude modulation of engine noise perceived inside a car with a modulation frequency of 0.5 order.
Critical Band: In human hearing, only those frequency components within a narrow band, called the critical band, will mask a given tone.
Critical Distance: The distance from a sound source at which direct sound and reverberant sound are at the same level.
Critical Frequency: A particular resonant frequency at which damage or degradation in performance is likely.
Critical Speeds: Any rotating shaft will have lateral (or flexural) and torsional natural frequencies. If the shaft is subjected to a force at any of these frequencies, the amplitude of vibration will be particularly large.
Cross Correlation: The cross-correlation function gives a measure of the extent to which two signals correlate with each other as a function of the time displacement between them.
Cross Spectrum: The cross spectrum F_xy(f) of f_x(t) and f_y(t) is the forward Fourier Transform of the cross correlation function R_xy(T). The cross spectrum is in general complex.
Crossover Frequency: In a loudspeaker with multiple radiators, the crossover frequency is the 3dB point of the network dividing the signal energy.
Cutoff Frequency: The cutoff frequency of an anechoic wedge or set of wedges is the lowest frequency above which the normal incidence sound absorption coefficient is at least .990
C Weighting: The C-weighting curve approximately follows the 100 phon curve.
Cycle: One complete repetition of a periodic motion.
Cycle Counting: Cycle counting is used to summarize lengthy, irregular load-versus-time histories by providing the number of times cycles of various amplitudes occur.
Cycles per Second: The rate of repetition of periodic motion measured in hertz (cycles per second).
Cylindrical Wave: A wave in which the surfaces of constant phase are coaxial cylinders.
Damped Natural Frequency: The frequency at which a damped system will oscillate in a free vibration situation.
Damping: Dissipation of energy in a system, either through time or distance.
Damping Factor: The ratio of actual damping in a system to its critical damping.
Damping Pad: Material applied to add damping to another material to reduce structural vibrations. This layer may be constrained or unconstrained.
Damping Ratio: Also known as Damping Factor, the ratio of actual damping in a system to its critical damping.
dB: Short for decibel.
Decibel: The human ear responds logarithmically and it is convenient to deal in logarithmic units in audio systems.
Decimation: .. to lose one tenth of one's military forces in battle or as punishment... Decimation is a terminology used in signal processing for the discarding of data in oder to compact the data or to match the sampling rate to another data set.
Decruitment: The rapid decline in the loudness of a tone that decreases continuously in level.
De-Emphasis: Filtering applied to an audio signal after storage or transmission to undo a linear distortion due to emphasis.
Demodulation: The process of recovering intelligence from a signal, some parameter of which was modified to carry the intelligence.
Deterministic Signal: For a deterministic signal the physical phenomenon can be represented by a mathematical relationship to a reasonable degree of accuracy.
DFT: The Discrete Fourier Transform, the digital version of the fourier transform.
Dichotic: Listening condition in which different signals are applied to the two ears.
Diffuse Sound Field: Sound field in which the time average of the mean-square sound pressure is everywhere the same and the flow of acoustic energy in all directions is equally probable.
Diffusers: A combination of sound reflection and absorption.
Diffusion: The mixing of two substances caused by random molecular motions.
Digital Signal Processing: The term used for calculations performed on digital signals.
Diotically: The same signal at both ears.
Dirac Delta Function: The limiting case of a pulse with unit area that is infinitely short and at the same time infinitely high.
Direct Sound: Sound reaching the listening location without reflections.
Directivity Factor: The ratio of the sound pressure squared, radiated directly ahead of a sound source, to the sound pressure squared radiated in all directions.
Discrete Fourier Transform: The digital version of the fourier transform.
Discrete Tone: A prominent single frequency component within a broadband noise signal.
Displacement: Displacement is a vector quantity that specifies the change of position of a body or particle and is usually measured from the mean position or position of rest.
Distortion: Any change in the waveform or harmonic content of an original signal as it passes through a device.
Dither: The noise added to a signal prior to quantization which reduces the distortion and noise modulation resulting from the quantization process.
Dolby Prologic: This is a four-channel sound reproduction system consisting of left, center, right and rear channel, (the single rear channel is usually played through two speakers).
Door: Opening to a room, compartment or system.
Doppler Sonar: An acoustic instrument that measures the change in the acoustic frequency of the scattered sound or echo from that of the transmitted pulse. The magnitude and direction of the shift in frequency is related to the relative motion of the sensor and the scatterer.
Dosimeter: A device worn by a worker for determining the worker's accumulated noise exposure with regard to level and time according to a pre-determined integration formula.
Double Acoustic Barrier: An acoustic barrier consisting of two panels with a gap between.
Double Glazing: Two layers of glazing used to improve thermal and acoustic radiation.
Downsampling: Downsampling is used to reduce the sampling rate of data.
Drumming Noise: Often used to describe noise in the frequency range 30 to 60Hz.
DSP: Abbreviation of Digital Signal Processing, the term used for calculations performed on digital signals.
Duct Acoustics: Ducts with acoustic waves propagating through them exist in many forms.
Duplex Theory of Localization: This is the combination of the two methods that describe human binaural sound localization first proposed by Lord Rayleigh around 1900.
Dynamic Headroom: The ability of an audio device to respond to musical peaks.
Dynamic Mass: Ratio of applied force to resulting acceleration during simple harmonic motion.
Dynamic Modulus: Ratio of stress to strain under vibratory conditions.
Dynamic Range: The power range over which a component or system functions properly.
Dynamic Sealing: Vehicle door seals must function when the aerodynamic load on the door pulls the door away from the seals.
Dynamic Stiffness: Ratio of force to unit displacement during simple harmonic motion.
Dynamic Unbalance: The condition which exists when unbalanced masses on a shaft or its attachments create a couple on the shaft when it rotates.
Dynamic Vibration Absorber: A dynamic vibration absorber is an auxiliary mass-spring system which tends to neutralise vibration of a structure to which it is attached.
Ear Canal: The external auditory meatus; the canal between the pinna and the eardrum.
Earmuff: Hearing protector worn over the pinna of an ear.
Effective Perceived Noise Level: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Electric Motor Noise: There are numerous applications of electric motors and the type of noise produced by each may be very specific to the installation and type of motor.
ELF: That portion of the radio frequency spectrum from 30 to 3000 hertz.
Emphasis: Filtering applied to an audio signal before storage or transmission to improve the signal-to-noise ratio at high frequencies.
Engine Excitation Mechanisms: The physics behind the vibrations generated by the internal mechanisms of an internal combustion engine.
Engine Radiated Noise: Some empirical noise prediction models have been derived from a sample of 1m radiated engine noise measurements.
Equivalent Continuous Sound Level: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Ergodic Process: A stationary random process.
Eustachian Tube: The tube running from the middle ear into the pharynx that equalizes middle-ear and atmospheric pressure.
Exhaust Catalyst: A catalytic converter is known to have two distinct acoustic effects: Reactive Effect and Resistive Effect.
Exhaust Noise: The exhaust system of an internal combustion engine has a number of functions, one of which is to reduce the noise of the waste gases as they are expelled from the engine to the atmosphere.
Exhaust Silencer: Acoustic filter elements used to reduce sound pressure levels in an exhaust system.
Expansion Chamber: The expansion chamber can be considered as a simple low pass filter.
Exponential Smoothing: A statistical technique commonly used to forecast time series data or to smooth the values on a control chart.
Exterior Noise: Noise produced by a vehicle and radiating to its surroundings.
Extremely Low Frequency: That portion of the radio frequency spectrum from 30 to 3000 hertz.
Fan Noise: For small fans such as those used on electric motors the noise generated by their centrifugal cooling fans can be reduced by reducing the fan rotational speed; noise has been shown to increase at a rate of 53 to 64 dB/decade increase of speed.
Far Field: The distribution of sound energy at a very much greater distance from a sources than the linear dimensions of the source and in which the sound waves can be considered to be plane waves.
Fast Fourier Transform: An algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform.
FFT: An abbreviation of Fast Fourier Transform, an algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform.
Field Transmission Loss: Sound transmission loss measured in accordance with Annex A1 of Test Method E 336.
Filter: Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
Finite Impulse Response Filter: A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
Fir: A light coloured and soft wood. Needs to be dried very well before use.
FIR Filter: A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
Flanking Sound Transmission: The transmission of sound from the sound source room to the receiving room by paths other than through the partition under test.
Flattop Window: An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Fletcher-Munson Curves: Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.
Flexure Mode: Flexure modes are associated with thin slender beams.
Flow Noise: A term generally used to describe aerodynamic noise produced when a gas flows within a duct or when the gas exits the duct.
Flutter Echo: A repetitive echo set up by parallel reflecting surfaces.
Forced Vibration: The oscillation of a system under the action of a forcing function.
Forcing Frequency: In sinusoidal vibration testing or resonance searching, the frequency at which a shaker vibrates.
Fourier Transform: The mathematically rigorous operation which transforms from the time domain to the frequency domain and vice versa.
Free Progressive Wave: Wave in a medium free from boundary effects.
Free Vibration: Free vibration occurs without forcing, the vibration of a string after it has been plucked.
Free-Layer Damper: A treatment to control the vibration of a structural by bonding a layer of damping material to the structure′s surface.
Frequency Leakage: In a Fourier Transform the signal is assumed to be periodic. If a pure sine wave does not repeat exactly within the time window, it is truncated. this truncation will lead to the frequency domain resultant being smeared (leakage) and not a single frequency.
Frequency Range: The bandwidth of a filter or signal.
Frequency Response: An important parameter in specifying the performance of measuring or recording devices.
Fricative: A speech sound produced by frication, that is, by forcing air through a constriction in the vocal tract. Examples are "s" and "f".
Full Scale Deflection: The maximum value on the scale of an instrument.
Fundamental: In periodic forced vibration, the term fundamental refers to the lowest frequency component present in a harmonic train.
Fusion Zone: All reflections arriving at the observer's ear within 20 to 40 msec of the direct sound are integrated, or fused together, with a resulting apparent increase in level and a pleasant change of character. This is the Haas effect.
Gear Mesh Frequency: A potential vibration frequency on any machine that contains gears; equal to the number of teeth multiplied by the rotational frequency of the gear.
Gear Noise: Gear noise such as whine are generated by meshing gears due to the vibration caused by failure of the rolling action between the mating teeth.
Geometric Scattering: Acoustic scattering in which the wavelength of the sound used is much smaller than the size of object causing the scattering.
Grommets: The design of grommets is critical to the acoustic performance of any partition such as engine bay bulkhead.
G Weighted Overall Level: This purportedly reflects human response to infrasound.
Haas Effect: Delayed sounds are integrated by the auditory system if they arrive at the ear within 20 to 40 msec of the direct sound.
Hamming Window: An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Hanning Window: An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
Hard Room: A room in which the surfaces have very low values of sound absorption and are therefore highly reflective.
Harmonic: If a signal (representing acceleration, displacement, sound pressure etc.) is composed of a number of components of frequencies which are all integer multiples of one (fundamental) frequency, these components are said to form a harmonic train.
Harmonic Distortion: Distortion of a signal by adding content that is harmonically related to the original signal. Clipping overload of an amplifier adds odd-order harmonics to the signal.
Harshness: A typical transient interdisciplinary phenomenon of noise and vibration.
Head Related Transfer Function: The frequency response between the point in space where a sound source is located, and the ear, due to anatomical features of the head, upper torso and pinnae.
Headroom: A term related to dynamic range, used to express in dB, the level between the typical operating level and the maximum operating level above which the signal will be clipped.
Hearing: The subjective human response to sound.
Hearing Damage: A person exposed to high noise levels can suffer hearing damage. The damage may be gradual or traumatic.
Hearing Protector: Personal device worn to reduce harmful auditory or annoying subjective effects of sound.
Helmholtz Hermann: German physicist, anatomist and physiologist.
Helmholtz Number: The cut-off frequency of a duct is generally described in terms of the Helmholtz Number.
Helmholtz Resonator: An acoustic filter element.
Hertz: The SI unit of frequency indicating the number of cycles per second (symbol Hz).
Heterodyne: A Heterodyne is generated when two frequencies which are close together (and of similar level) interact to give a "beating" effect i.e. a noise which gets quieter and louder alternatively.
Holes in Acoustic Barriers: A hole in an acoustic barrier drammatically reduces the acoustic performance.
Hz: An abbreviation of hertz. The SI unit of frequency.
Imbalance: Unequal radial weight distribution on a rotor system.
Impact Harshness: The interior sound and vibration resulting from tyre interactions with discrete road disturbances.
Impact Isolation Class: A measure or specification of isolation effectiveness of building structures from impact noises such as slammed doors, dropped objects, footfalls, shuffled furniture, etc. The higher the IIC rating, the better such isolation.
Impedance: Mechanical impedance is the ratio of a force-like quantity to a velocity like quantity when the arguments of the real (or imaginary) parts of the quantities increase linearly with time.
Impulse: The product of the force and the time during which it acts.
Impulsive Noise: A noise level that fluctuates over a range greater than 10 dB during observation is classified as impulsive.
In Phase: Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."
Infinite Impulse Response Filter: A commonly used type of digital filter. This recursive structure accepts as inputs digitized samples of the audio signal, and then each output point is computed on the basis of a weighted sum of past output (feedback) terms, as well as past input values.
Infrasound: Acoustic waves below 20Hz are normally inaudible.
Initial Time-Delay Gap: The time gap between the arrival of the direct sound and the first sound reflected form the surfaces of the room.
Inner Ear: The inner ear is a labyrinth of twisting fluid-filled passages associated with hearing and balance.
Insertion Loss: This may be applied to a silencer or other sound-reducing element, in a specified frequency band, the decrease in sound power level, measured at the location of the receiver, when a sound insulator or a sound attenuator is inserted in the transmission path between the source and the receiver.
Interaural Cross-Correlation Coefficient: The measure of the difference in the sounds arriving at the two ears of a listener.
Interaural Fluctuation Strength: Phase differences between ears lead to an inferred source location. If this location changes as a function of time this may be inferred as a signal instability.
Interaural Level Difference: In binaural hearing the interaural level difference has an impact on lateralization throughout the frequency spectrum.
Interaural Phase Difference: Coincident with the interaural time delay, varies systematically with source azimuth and wavelength due to distance from source and refraction around the head.
Isochrony: A sequence of events is called isochronous if the time separating each pair of successive events is strictly equal.
Jury Evaluation: A name given to a field where the customer is given the opportunity to compare products and rank one versus the other.
Knudson′s Law: The difference in frequency between pure tones of equal strength and of which the difference in pitch is just perceptible is defined by Knudson′s law.
Kurtosis: A tendency for a distribution to form a sharp narrow peak.
L_AE: Abbreviation for sound exposure level.
L_AeqT: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Laser Vibrometer: A sensing system which uses laser beams to measure the motion of a vibrating surface.
L_dn: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Leakage: In a Fourier Transform the signal is assumed to be periodic. If a pure sine wave does not repeat exactly within the time window, it is truncated. this truncation will lead to the frequency domain resultant being smeared (leakage) and not a single frequency.
L_EPN: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Level: The condition where an item is perpendicular to the force of gravity. The level of a signal is its amplitude.
Line Source: A continuous line acoustic source such as a vibrating string.
Lissajou′s Figures: The pattern traced out when the displacements of two simple harmonic motions are traced in the x and y direction.
Live Room: Room characterized by a relatively small amount of sound absorption.
L_N: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
L_NP: There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
Localization Accuracy: Our ability to detect changes in localization or lateralization is not absolute.
Lombard Effect: As noise levels increase, talkers increase their voice levels to compensate and to maintain adequate conditions for verbal communication.
Longitudinal Wave: A wave in which the vibrations of the medium are parallel to the direction the wave is moving.
Loudness: A subjective term for the sensation of the magnitude of sound.
Loudspeaker: An electroacoustical transducer that changes electrical energy to acoustical energy.
Low Frequency Sound: Acoustic waves below 20Hz are normally inaudible, but may be experienced as being in a region of high pressure.
Machinery Vibration: The vibration of rotating machinery may infer certain defects.
Magnetostriction: The change of size and/or shape of a ferromagnetic material due to the application of a magnetic field.
Mains Noise: Unwanted noise in electrical signals related to mains signals.
Masking: A property of the human auditory system by which an audio signal cannot be perceived in the presence of another audio signal.
Masking Sound: Electronically generated sound used to mask intrusive sound and enhance speech privacy.
Masking Threshold: A function in frequency and time below which an audio signal cannot be perceived by the human auditory system.
Mechanical Impedance: The mechanical properties of a machine system (mass, stiffness, damping) that determine the response to periodic forcing functions.
Mel: A proportional scale, in which equal intervals correspond to equal perceived interval sizes.
Mercalli Scale: A measure of the intensity of an earthquake. This is a subjective value that varies depending on where the earthquake is observed.
Metric Sabin: This is the product of surface area and absorption coefficient. Therefore, the absorption within a room may be specified by a single number. One metric sabin is the equivalent to one square metre of a perfectly absorptive surface.
MHz: Abbreviation of Megahertz, 1000000Hz.
Microphone: An acoustical-electrical transducer by which sound waves in air may be converted to electrical signals.
Middle Ear: The cavity between the eardrum and the cochlea housing the ossicles connecting the eardrum to the oval window of the cochlea.
Millington-Sette Formula: A formula for calculating the reverberation time of a room based on the absorption coefficient of the walls, surface area and volume.
Missing Fundamental: The phenomenon by which an observer, presented with a harmonic tone in which the fundamental is absent, hears the same pitch as would be heard if the fundamental had been present.
Mobility: The ratio of the carrier velocity to the applied electric field.
Modal Analysis: The process of determining a set of generalized coordinates for a system such that the equations of motion are both inertially and elastically uncoupled.
Mode Shape: The relative position of all points on a structure at a given natural frequency.
Motion Sickness: Defined specifically to describe nausea, vomiting and colour changes. It does not include discomfort.
M/S Stereo: A method of removing imaging artefacts as well as exploiting stereo irrelevance or redundancy in stereophonic audio programmes based on coding the sum and difference signal instead of the left and right channels.
Natural Frequency: The frequency of free vibration of a system.
NC: Standard spectrum curves by which a given measured noise may be described by a single Noise Criteria (NC) number.