# 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.
**6DOF**- Abbreviation for Six Degrees of Freedom
**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 Absorption**- Reduction of sound pressure level through sound absorption.
**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 Duct End Correction**- The correction to the physical length of a duct to take into account the impedance of the termination and the effect on the acoustic length.
**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 Generator**- A transducer which converts electric, mechanical, or other forms of energy into sound.
**Acoustic Impedance**- The total reaction of a medium to the transmission of sound through it.
**Acoustic Inertance**- The impeding effect of inertia upon the transmission of sound in a duct, equal to the mass of the vibrating medium divided by the square of the cross section.
**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 Mass**- Another name for Acoustic Inertance.
**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 Noise Source**- Any unwanted acoustic noise.
**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 Radiation Pressure**- A unidirectional, steady-state pressure exerted upon a surface exposed to a sound wave.
**Acoustic Reactance**- Acoustic impedance caused by the inertia and elasticity of the transmitting medium.
**Acoustic Refraction**- The process by which the direction of sound propagation is changed due to spatial variation in the speed of sound in the medium.
**Acoustic Resistance**- Acoustic impedance caused by the internal friction of the transmitting medium.
**Acoustic Screen**- A moveable screen, usually covered in sound absorbent material on one side and having a reflective surface on the other.
**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 Wave**- Another name for Sound Wave.
**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 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.
**Anacoustic Zone**- The region above an altitude of about 100 miles where the distance between the air molecules is greater than the wavelength of sound, and sound waves can no longer be propagated.
**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.
**Articulation Score**- A subjective measure of the intelligibility of a voice system in terms of the percentage of words correctly understood over a channel perturbed by interference.
**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 Reject Frequencies**- A tuned circuit that does not pass a specified band of frequencies.
**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.
**Characteristic Equation**- The mathematical equation whose solution defines the dynamic characteristics of the structure in terms of its natural frequencies, damping, and mode shapes.
**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 Modes**- The points on a structure have varying phase relationships between them at a natural frequency. This is unlike a real mode where the phase between points is either 0° or 180°.
**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.
**Composite Rating of Preference**- Combines low frequency sound and spectral balance i.e. modifiers for boom and high frequency.
**Compressional Wave**- Wave in an elastic medium which causes an element of the medium to change its volume without undergoing rotation.
**Computer Noise**- Noise and vibration generated by the cooling fans, disc drives and power supplies.
**Condenser Microphone**- A type of microphone in which the diaphragm is one plate of a capacitor containing an electrical charge.
**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 Damping**- The smallest value of damping required for a displaced system to return to its equilibrium position without overshooting or oscillating about that position.
**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.
**Damped Wave**- A ringing sinusoidal oscillation or oscillatory wave consisting of an initial pulse or peak followed by additional peaks of diminishing amplitude.
**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.
**Directivity Index**- The difference between sound pressure level in any given direction in the acoustic far field and the average sound pressure level in that field.
**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.
**Driving Point Measurement**- A frequency response measurement where the excitation point and direction are the same as the response point and direction.
**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 Microphone**- A device in which sound waves move a coil of fine wire that is mounted on the back of a diaphragm and located in the magnetic field of a permanent magnet.
**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.
**Early Early Sound**- Structure-borne sound may reach the microphone in a room before the airborne sound because sound travels faster through the denser materials.
**Earmuff**- Hearing protector worn over the pinna of an ear.
**Effective Mass**- The frequency response function of force/acceleration.
**Effective Perceived Noise Level**- There are a number of different noise criteria measurements used by local authorities to determine noise levels in the community.
**Eigenvalue Problem**- The mathematical formulation and solution of the characteristic equation is called the Eigenvalue problem.
**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.
**Electronic Filter**- Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
**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 Knock**- Abnormal combustion, often producing audible sound, caused by autoignition of the air/fuel mixture.
**Engine Radiated Noise**- Some empirical noise prediction models have been derived from a sample of 1m radiated engine noise measurements.
**Environmental Noise Modelling**- Computer model used to predict the potential noise impact of a proposed new development, or modifications to an existing site. This allows noise control measures to be optimised.
**Equivalent Continuous Sound Level****Equivalent Rectangular Bandwidth**- This is the effective bandwidth of an auditory filter.
**Equivalent Spherical Diameter**- The diameter of a sphere of the same volume as the object being described.
**Equivalent Spherical Radius**- The radius of a sphere of the same volume as the object being described.
**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 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.
**Exponential Window**- A special windowing function for minimizing leakage in lightly damped structures that is used in impact testing.
**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 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.
**Fluctuation Strength**- This is similar in principle to roughness except it quantifies subjective perception of slower, up to 20Hz, amplitude modulation of a sound.
**Flutter Echo**- A repetitive echo set up by parallel reflecting surfaces.
**Force Window**- A special windowing function for minimizing noise in impact testing.
**Forced Response Analysis**- Mathematically calculating the system response to an arbitrary forcing function using modal analysis data as the system model.
**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 Field**- Field in a homogeneous, isotropic medium free from boundaries.
**Free Progressive Wave**- Wave in a medium free from boundary effects.
**Free Sound Field**- A sound field in which the effects of obstacles or boundaries on sound propagated in that field are negligible.
**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.
**Frequency Response Function**- The output to input relationship of a structure. Mathematically, it is the Fourier transform of the output divided by the Fourier transform of the input. It is also the transfer function measured along the j axis in the s-plane.
**Frequency Response Matrix**- For an N degree of freedom system, it is an N x N symmetrical matrix whose elements are the frequency response functions between the various points on the structure.
**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 is composed of a number of components of frequencies which are all integer multiples of one 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 Level**- A measured threshold of hearing at a specified frequency, expressed in decibels relative to a specified standard of normal hearing.
**Hearing Loss**- An impairment of auditory acuity.
**Hearing Protector**- Personal device worn to reduce harmful auditory or annoying subjective effects of sound.
**Hearing Sensitivity**- The human ear is less sensitive at low frequencies than in the midrange.
**Hearing Threshold Level**- The amount in decibels by which an individual′s threshold of audibility differs from a standard audiometric threshold.
**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.
**High Pass Filter**- A filter that passes signal frequencies above a specific, or cut off, frequency is called a high pass filter.
**Holes in Acoustic Barriers**- A hole in an acoustic barrier drammatically reduces the acoustic performance.
**Human Vibration**- The effect of mechanical vibration in the environment on the human body.
**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.
**Impact Sound**- The sound produced by the collision of two solid objects.
**Impact Testing**- A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system′s response.
**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.
**Impulse Response**- The response of a system to a unit impulse or Dirac′s delta function. The Fourier transform of the impulse response is the frequency response function.
**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.
**Integration**- The inverse of differentiation. Mathematical process used in calculus.
**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.
**Interaural Time Difference**- This is the difference in arrival time of an acoustic signal at each ear.
**Interleaving**- The placing of a sheet of paper between two adjacent layers of metal to facilitate handling and shearing of rectangular sheets, or to prevent sticking or scratching.
**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}**Laser Vibrometer**- A sensing system which uses laser beams to measure the motion of a vibrating surface.
**Law of the First Wavefront**- The first wavefront falling on the ear determines the perceived direction of the sound.
**L**_{dn}**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}**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}**L**_{NP}**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.
**Loudness Level**- Measured in phons it is numerically equal to the median sound pressure level (dB) of a free progressive 1000 Hz wave presented to listeners facing the source, which in a number of trials is judged by the listeners to be equally loud.
**Loudspeaker**- An electroacoustical transducer that changes electrical energy to acoustical energy.
**Low Frequency Noise**- There is no international agreement on how exactly to define low frequency noise.
**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.
**Macrosonics**- The technology of sound at signal amplitudes so large that linear approximations are not valid.
**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.
**Mass Law**- A rule for estimating the transmission loss of a barrier in its mass controlled region.
**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.
**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.
**Near Field**- Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source.
**NEF****Neper**- A unit expressing the ratio of two numbers as a natural logarithm.
**Newtonian Speed of Sound**- Approximation to the speed of sound a in a perfect gas.
**Nibble**- A group of four bits or half a byte (8-bits).
**Node**- A point or line on a vibrating structure that remains stationary.
**Noise**- Noise is any undesired signal.
**Noise Climate**- This is sometimes calculated as L
_{A10}- L_{A90}and represents the fluctuations in noise. **Noise Control**- The technology of obtaining an acceptable noise environment.
**Noise Criteria**- Standard spectrum curves by which a given measured noise may be described by a single NC number.
**Noise Exposure Forecast****Noise Exposure Levels**- There are a number of different calculations for exposure levels for different times of the day, environment and types of noise source.
**Noise Exposure Limits**- The amount of hearing loss is dependent on the noise level and duration.
**Noise Floor**- Normally the lowest threshold of useful signal level.
**Noise Isolation Class**- A single-number rating calculated in accordance with Classification E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.
**Noise Level**- For airborne sound , unless specified to the contrary, it is the A-weighted sound level.
**Noise Path Analysis**- An analysis technique that uses noise transfer functions, mount stiffness and input force calculations to determine the dominant noise paths for any particular frequency range or operating condition.
**Noise Pollution**- Unwanted noise, including traffic noise, machinery noise or music.
**Noise Pollution Level****Noise Rating Curves**- Developed by the International Standards Organisation in 1971 to rate noisiness with the 1000 Hz octave band as reference point.
**Noise Reduction**- The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure.
**Noise Reduction Coefficient**- Arithmetic mean of sound absorption coefficients.
**Noise Reduction Rating**- Indicates a hearing protectors noise reduction capability.
**Nondeterministic Signal**- For a nondeterministic signal the physical phenomenon cannot be represented mathematically.
**Non-Impulsive Noise**- All noise not included in the definition of impulsive noise.
**Nonlinear Damping**- Damping due to a damping force that is not proportional to velocity.
**Nonstationary**- A characteristic of a time series for which the distribution changes over time.
**Normal Mode of Vibration**- A mode of free vibration of an undamped system. In general, any composite motion of a vibrating system ban be analyzed into a summation of its normal modes.
**Normalized Noise Isolation Class**- A single-number rating calculated in accordance with Classification E413 using measured values of normalized noise reduction.
**Normalized Noise Reduction**- Between two rooms, in a specified frequency band, the value that the noise reduction in a given field test would have if the reverberation time in the receiving room were 0.5s.
**Norris Eyring Formula**- A formula for calculating the reverberation time of a room based on the absorption coefficient of the walls, surface area and volume.
**Notch Filter**- An electronic network which passes signals having frequencies above and below the band-elimination frequency span.
**NVH**- Commonly used abbreviation for Noise, Vibration and Harshness
**Nyquist Frequency**- Half the sampling frequency. Any analog frequency component above the Nyquist frequency will, after sampling, be converted (aliased) to a frequency below the Nyquist frequency.
**Nyquist Plot**- A plot of real versus imaginary spectral components that is often used in servo analysis. Should not be confused with a polar plot of amplitude and phase.
**Octave**- Two frequencies are an octave apart if the ratio of the higher frequency to the lower frequency is two.
**Octave Band**- A segment of the frequency spectrum separated by an octave.
**Octave Band Level**- The integrated sound pressure level of only those components in a specified octave band.
**Office Noise Index**- This is based on the same principle of LNP, with the difference that background noise replaces equivalent sound pressure level and a factor of 2.4 affects the ?uctuation level in order to maximize the correlation with the percentage of dissatis?ed subjects.
**Oil Whirl/Whip**- An unstable free vibration whereby a fluid-film bearing has insufficient unit loading.
**Order**- Harmonic of the rotational speed of rotating machinery. As an example, in the case of the shaft rotating at 6,000 rpm, the first order component occurs at a frequency of 100 Hz (6,000/60), whereas the third order component would occur at a frequency of 300 Hz.
**Order Analysis**- Order analysis is simply frequency analysis where the frequency axis of the spectrum is expressed in orders of rpm rather than in Hz or rpm.
**Order Tracking**- Tracking the evolution of a specific order as rpm changes.
**Otologist**- A physician who specializes in the ear and its diseases.
**Out of Phase**- The offset in time of two related signals.
**Overtone**- A harmonic frequency related to the fundamental.
**Pa**- The SI derived unit of pressure is the pascal.
**Particle Velocity**- A fluctuating velocity superimposed by the presence of sound on the other velocities with the particles of the medium may have.
**Partition**- A division between two volumes.
**Pascal**- The SI derived unit of pressure.
**Pass-by Noise**- Another name for Vehicle Exterior Noise.
**Passenger Compartment Acoustic Absorption**- The noise within a passenger compartment are controlled by a number of parameters, one of those is the acoustic absorption within the compartment.
**Passive Absorber**- A sound absorber that dissipates sound energy as heat.
**Peak Hold**- The peak signal level for each frequency component is held for display.
**Peak Pick**- A parameter estimation technique where the peak value of the imaginary part of the frequency response function is used to estimate the mode shape value at that point.
**Peak Sound Pressure Level**- ten times the common logarithm of the square of the ratio of the largest absolute value of the instantaneous sound pressure in a stated frequency band during a specified time interval to the reference sound pressure of 20 micro pascals.
**Peak to Peak Value**- The over-all amplitude of a signal measured from its lowest peak to its most highest peak.
**Peak Value**- The instantaneous maximum value of a waveform.
**Perceived Noisiness**- The metric for Perceived Noisiness turned out to be far less robust than the metric for Loudness. It was an early indication that the acceptability of a sound was product specific to a certain degree.
**Percentage Articulation Loss of Consonants**- Estimate of speech intelligibility.
**Period**- The time taken for a full cycle of motion in a system executing harmonic motion.
**Periodic**- A term used to refer to a pattern of motion which repeats exactly after a given interval or period.
**Periodic Quantity**- Oscillating quantity whose values recur for certain increments of the independent variable.
**Permanent Threshold Shift**- A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level.
**Phase**- Phase identifies the position at any instant which a periodic wave occupies in its cycle. A portion of a material system whose properties and composition are homogeneous and which is physically distinct from other parts of the system.
**Phase Shift**- The time or angular difference between two signals.
**Phase Velocity**- The velocity with which planes of equal phase, crests or troughs, progress through the medium.
**Phon**- The unit of measurement for loudness level.
**Phoneme**- The basic classes of sounds used to form the words of a language. Examples in English are "k", "oo", and "th". They are often represented by single written letters.
**Phonon**- A single quantum of vibrational or elastic energy.
**Piezoelectric**- Any material which provides a conversion between mechanical and electrical energy.
**Pink Noise**- A random-like signal in which power is proportional to the inverse of frequency.
**Pitch**- Pitch is the distance between the teeth on a gear.
**Place Effect**- The theory that pitch perception is related to the pattern of excitation on the basilar membrane of the cochlea.
**Point Mobility**- The velocity at a point per unit force at the same point and same direction.
**Pop**- An undesirable short duration microphone output, often caused by explosive exhalation of air during the formation of certain vocal sounds.
**Power Spectral Density**- A method of scaling the amplitude axis of spectra of random rather than deterministic signals.
**Pressure**- Defined as the force exerted per unit area.
**Pressure Zone**- As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.
**Principal Component Analysis**- Constructing new features which are the principal components of a data set.
**Privacy Index**- A measure for rating the speech privacy performance of an architectural space.
**PSD**- A method of scaling the amplitude axis of spectra of random rather than deterministic signals.
**Psychoacoustics**- The study of the interaction of the auditory system and acoustics.
**Pulse Range**- Difference in decibels between the peak level of an impulsive signal and the root-mean-square level of a continuous noise.
**Pure Tone**- A tone with a sinusoidal waveform.
**Q-Factor**- Abbreviation of Quality Factor. The quality factor defines the sharpness of resonance.
**Quality Factor**- The quality factor defines the sharpness of resonance.
**Quantization Error**- When a continuous time signal is digitized, because there isn′t an infinite number of discrete digital levels, the difference between the actual analog value and the digital representation of that value is defined as the quantization error.
**Quarter Wave Tube**- A resonator used most commonly on automotive air intake systems to reduce resonance.
**Radial Vibration**- Shaft dynamic motion or casing vibration which is in a direction perpendicular to the shaft centerline.
**Radiated Noise**- The dissemination of sound energy as acoustic waves from a source (e.g. a vibrating panel, running engine etc.).
**Random Noise**- A noise signal, commonly used in measurements, which has constantly shifting amplitude, phase, and a uniform spectral distribution of energy.
**Random Vibration**- A vibration whose instantaneous magnitude is not specified for any given instant of time.
**Rasp**- A sort of blowing noise emitted often from the exhaust pipes of internal combustion engines, usually, it is dominated by a mixture of 4E, 6E and 8E engine orders on 4 cylinder engines.
**Rattle**- A sound such as that made when shaking a tin full of coins.
**Ray**- At higher audio frequencies, sound may be considered to travel in straight lines, in a direction normal to the wave front.
**Ray Tracing**- The calculation of the propagation, reflection and refraction of rays from source to receiver. This technique is used extensively within the optics and acoustics industry.
**Rayl**- The Rayl is the SI derived unit of specific acoustic impedance.
**Rayleigh′s Method**- A method used for calculating approximate natural frequencies for a vibrating system assuming a deflected shape and balancing kinetic and strain energies.
**Reactive Absorber**- A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as well as resistance.
**Reactive Silencer**- A silencer that uses reflection effects for its action.
**Reactive Sound Field**- A sound field in which the particle velocity is 90° out of phase with the pressure.
**Reactivity Index**- The reactivity index in a given direction at a point is defined as the difference between the sound intensity level and the sound pressure level measured in the given direction at that point.
**Real Modes**- In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass through equilibrium at the same time.
**Receptance**- Displacement per unit force.
**Reciprocity**- If in a space filled with air which is partly bounded by finitely extended fixed bodies and is partly unbounded, sound waves being excited at any point A, the resulting velocity-potential at a second point B is the same both in magnitude and phase, as it would have been at A, had B been the source of the sound.
**Rectangular Window**- An amplitude weighting used to truncate continuous time signals to fit within the length of the DFT window.
**Reference Sound Source**- A calibrated source of sound power with well defined operating characteristics and a predictable performance.
**Reflected Sound**- That portion of a sound wave which bounces off a surface and reverses direction.
**Reflection Phase Grating**- A diffuser of sound energy using the principle of the diffraction grating.
**Residual Terms**- Terms added to a curve fit algorithm to take into account the effects of modes outside the range being fitted.
**Resilient Mounting**- A suspension system or cushioned mounting designed to reduce the transmission of vibration between a device and the mounting surface.
**Resonant Frequency**- Any system has a resonance at some particular frequency.
**Reverberation**- The persistence of sound in an enclosed space, as a result of multiple reflections, after the sound source has stopped.
**Reverberation Room**- Room having a long reverberation time, specially designed to make the sound field therein as diffuse as possible.
**Reverberation Time**- The time in seconds required for sound pressure at a specific frequency to decay 60 dB after a sound source is stopped.
**Ride Quality**- A measure of the isolation from road inputs the suspension transfers to the vehicle without compromising vehicle control.
**Rigid Body Modes**- These modes exist for all objects and are the modes of vibration where the object itself remains rigid and are the rotation or translational modes of the object on its mounting system.
**Ringing**- Continued oscillation after an external force or excitation is removed, as after a guitar string is plucked.
**Rise Time**- The time required for the output of a transducer to rise from 10% to 90% of its final value, as it responds to a step change in the measurand.
**Room Absorption**- Sum of Sabine absorption due to objects and surfaces in a room, and due to dissipation of energy in the medium within the room.
**Room Acoustics**- The general acoustic requirements of a room depend on the use.
**Room Constant**- The ratio of room sound absorption to the quantity 1 minus the average room absorption coefficient.
**Room Modes**- Frequencies at which sound waves in a room resonate (in the form of standing waves), based on the room dimensions.
**RT60**- Abbreviation of Reverberation Time, the time taken for the sound pressure level to drop 60dB.
**SDOF**- Abbreviation of Single Degree of Freedom.
**SEA**- Abbreviation of Statistical Energy Analysis.
**SEA Coupling Loss Factors**- Statistical energy analysis coupling between different types of structure.
**SEL****Sensor**- A device that converts a physical stimulus (such as force, sound, pressure, motion) into a corresponding electrical signal.
**Sensory Pleasantness**- An acoustic metric derived by Zwicker this is a combination of loudness, roughness, sharpness and tonality these are all expressed as relative values making sensory pleasantness a relative quantity.
**Shake Table Test**- A laboratory test for vibration tolerance, in which the device to be tested is placed on a vibrator.
**Shaker**- An electromagnetic device capable of imparting known vibratory acceleration to a given object.
**Sharpness**- Sharpness (in acum) is defined as the ratio of high frequency level to overall level.
**Shear Wave**- The axis of vibration is perpendicular to the direction of propagation of the wave.
**Shell Radiated Noise**- Noise radiated from the shell of an object that is vibrating, eg an exhaust silencer can.
**Side Branch Orifice**- The side branch orifice is an acoustic filter element and behaves as a high pass filter.
**Silencer**- An acoustic filter element used to reduce sound pressure levels inside a duct.
**Simple Harmonic Quantity**- Periodic quantity that is a sinusoidal function of the independent variable.
**Single Acoustic Barrier**- A single layer acoustic barrier.
**Single Degree of Freedom**- A system whose position in space can be completely described by one coordinate.
**Single Event Level**- Also known as Sound Exposure Level, used to describe the amount of noise from an event such as an individual aircraft flyover.
**Single Event Noise Exposure Level**- The dB(A) level which, if it lasted for one second, would produce the same A-weighted sound energy as the actual event.
**Six Degrees of Freedom**- An unconstrained mass has six degrees of freedom, 3 translations and 3 rotations.
**Skew**- An expression of the so-called "third moment".
**Sound Absorption**- Reduction of sound pressure level through sound absorption.
**Sound Absorption Coefficient**- The practical unit between 0 and 1 expressing the absorbing efficiency of a material. It is determined experimentally.
**Sound Definition**- This compares ‘useful’ sound with total sound.
**Sound Energy**- Energy added to an elastic medium by the presence of sound, consisting of potential energy in the form of deviations from static pressure and of kinetic energy in the form of particle velocity.
**Sound Energy Density**- The quotient obtained when the sound energy in a region is divided by the volume of the region.
**Sound Energy Flux**- The average rate of flow of sound energy for one period through any specified area.
**Sound Energy Flux Density**- This is Sound Intensity.
**Sound Exposure Level**- Used to describe the amount of noise from an event such as an individual aircraft flyover.
**Sound Field**- A region containing sound waves. Normally broken down into Near Field and Far Field.
**Sound Focusing**- Focusing of sound can be caused by reflection from large concave surfaces.
**Sound Level Meter**- Device used to measure sound pressure level.
**Sound Masking**- The process of using electronic masking sound to cover unwanted or intrusive sound such as speech or equipment noise, typically used to enhance speech privacy and productivity.
**Sound Paths**- A trace of the path taken by specific sound waves as they move outward from the source of the sound to the receiver location.
**Sound Power Level**- Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.
**Sound Pressure**- A fluctuating pressure superimposed on the static pressure by the presence of sound.
**Sound Pressure Level**- Ten times the logarithm of the ratio of the time-mean-square pressure of a sound to the square of the reference sound pressure.
**Sound Propagation**- At distances large compared to the size of the source, sound intensity diminishes according to the inverse square law.
**Sound Quality**- Pertaining to the quality or other aspects of the machine that might be inferred from the sound it makes.
**Sound Source**- A calibrated source of sound that is used for measurements of the acoustic performance of materials, partitions and complete rooms and buildings.
**Sound Transmission Class**- In acoustics, a single number rating for describing sound transmission loss of a wall or partition.
**Sound Waves**- Sound waves can be thought of like the waves in water.
**Source**- The object that produces the wave or disturbance.
**Specific Airflow Resistance**- The product of the airflow resistance of a specimen and its area.
**Spectral Centroid**- This is the centre of gravity of a frequency spectrum.
**Spectral 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.
**Spectrogram**- A three-dimensional intensity plot displaying the amplitude of spectral components as a function of both time and frequency.
**Spectrum**- A spectrum is a definition of the magnitude of the frequency components that constitute a quantity.
**Spectrum Analyser**- An instrument for measuring, and usually recording, the spectrum of a signal.
**Specular Reflections**- Mirrorlike reflections of sound from a flat surface.
**Speech Intelligibility**- A measure of sound clarity that indicates the ease of understanding speech.
**Speech Transmission Index**- A single number that indicates the effect of a transmission system on speech intelligibility.
**Speed of Sound**- Depends on the elastic and inertial properties of the particular medium.
**Spherical Divergence**- The condition of propagation of spherical waves that relates to the regular decrease in intensity of a spherical sound wave at progressively greater distances from the source. Under this condition the sound pressure level decreases 6 decibels with each doubling of distance from the source.
**SPL**- Abbreviation of Sound Pressure Level, ten times the logarithm of the ratio of the time-mean-square pressure of a sound to the square of the reference sound pressure.
**Squeak**- A sound resembling that of an unlubricated hinge.
**Standing Wave**- The interference pattern produced by two waves of equal amplitude and frequency traveling in opposite directions.
**Static Stiffness**- A measurement of the rigidity of a structure to a constant force, defined as the deflection per unit force of input.
**Statistical Energy Analysis**- A technique for analyzing the propagation through, and the sound radiation from, complex structures.
**STC**- Abbreviation of Sound Transmission Class, a single number rating for describing sound transmission loss of a wall or partition.
**Steady State Sounds**- Sounds whose average characteristics remain relatively constant in time.
**Steering Offset**- The distance from the point where the steering axis intersects the ground to the longitudinal line that runs through the centre of the tyre′s contact patch.
**Step Response**- The response of a system to an instantaneous jump in the input signal.
**Stereo**- From the Greek meaning solid.
**STI**- A single number that indicates the effect of a transmission system on speech intelligibility.
**Stiffness**- Stiffness is the ratio of change of force (or torque) to the corresponding change in translation (or rotational) deflection of an elastic element.
**Straight-Through Silencer**- A type of absorption silencer with a single perforated tube surrounded by sound-absorbing fibre. This results in a very low back pressure silencer, but at the same time only attenuates higher frequency noise.
**Stroboscope**- An instrument that allows viewing of rotating or reciprocating objects by producing the optical effect of a slowing down or stopping motion.
**Strouhal Number**- A dimensionless unit.
**Structural Acoustic Coupling**- The physics relating to the vibration of a panel and the sound radiated from it or the acoustic excitation excitation of the panel.
**Structural Modification**- Mathematically determining the effect of changing the mass, stiffness, or damping of a structure and determining its new modal parameters.
**Structure-Borne Sound**- Sound for which a significant portion of the transmission path from source to receiver takes place in a solid structure rather than through a liquid or gas.
**Subharmonic**- Sinusoidal quantity of a frequency that is an integral submultiple of a fundamental frequency.
**Subsynchronous**- Component of a vibration signal that has a frequency less than shaft rotation frequency.
**Sub-Synchronous**- Also called a subharmonic.
**Subwoofer**- A speaker designed exclusively for low-frequency reproduction.
**Synchronous Averaging**- A type of signal averaging where successive records of the time waveform are averaged together.
**Tangential Mode**- A room mode produced by reflections off four of the six surfaces of the room.
**Temporary Threshold Shift**- A temporary impairment of hearing acuity as indicated by a change in the threshold of audibility.
**Third Octave**- Unit of frequency interval describing a band of frequencies such that the highest frequency is 2
^{1/3}times the lowest. **Threshold of Feeling**- The threshold of feeling for a specified signal is the minimum effective sound pressure of that signal which, in a specified fraction of the trials, will stimulate the ear to a point at which there is the sensation of discomfort.
**Threshold Shift**- Temporary threshold shift occurs when a person has been exposed for a few hours to noise levels of about 80 dB and above.
**Timbre**- The quality of a sound that distinguishes it from other sounds of the same pitch and volume.
**Time History**- A continuous record of the variation of a physical quantity (e.g. displacement, acceleration, force, etc.) with time.
**Time of Flight**- A method of locating a source in space based on the arrival times of the signal at a number of receivers.
**Time Weighted Average**- The yardstick used by the Occupational Safety and Health Administration to measure noise levels in the workplace.
**Tonal Colour**- These can be partially quantified using the metrics sharpness, roughness, fluctuation strength, tonality and pitch.
**Tonality**- Tone-to-noise ratio, which is determined from the ratio of the sound pressure level of the tone to the sound pressure level of the noise in the critical band centred at the frequency of the tone but without the tone itself.
**Tone Burst**- A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections.
**Total Harmonic Distortion**- The RMS value of the distortion appearing at multiples of the input frequency to the RMS value of the input sine wave.
**Tracking Filter**- A low-pass or band-pass filter which automatically tracks the input signal.
**Transform**- A transform is a mathematical operation that converts a function from one domain to another domain with no loss of information.
**Tuned Mass Damper**- Another name for a Dynamic Vibration Absorber.
**Tuning Fork**- A small two-pronged steel instrument which when struck produces a fixed tone.
**Tyre Noise**- The noise generated by the interaction of a tyre with the road surface as it rolls over it. This noise may result in vehicle exterior noise or in interior noise. In the case of interior noise it is often referred to as Road Noise.
**Unbiased Annoyance**- A combination of sharpness, fluctuation strength, N10 level, and includes a correction factor for time of day.
**Uniform Window**- The uniform, or rectangular, window does not modify the signal amplitude at all.
**Vehicle Exterior Noise**- Noise produced by a land based vehicle that is radiated to the surrounding environment.
**Vehicle Interior Noise**- The noise inside a vehicle is made up of three main components: Engine Noise, Road Noise and Wind Noise.
**Vehicle Noise**- The noise produced by a land based vehicle that is radiated to the surrounding environment (exterior noise) or that is observed by the occupants of the vehicle (interior noise).
**Vibrating String**- When a string vibrates it radiates as a dipole and is very inefficient and so needs to be attached to a panel that will be excited by the vibrations in the string and act as a far more efficient radiator of sound.
**Vibration Dose Value**- Defined as the 4th root of the time integral of the 4th power of the 1-32 Hz filtered acceleration.
**Vibration Isolation**- Vibration problems are solved by onsidering the system as a number of springs and masses with damping.
**Vibration Isolator**- A resilient support for vibrating equipment designed to reduce the amount of vibration transmitted to the other structures.
**Vibration Severity**- A criteria for predicting the hazard related to specific machine vibration levels.
**Viscous Damping**- Viscous damping is the dissipation of energy that occurs when a particle in a vibrating system is resisted by a force proportional to the velocity of the particle.
**Voice Coil**- Coil attached to the diaphragm of a moving coil loudspeaker.
**Volume Velocity**- Within acoustics the volume velocity U of a source is the pressure p divided by the acoustic impedance Z.
**Waterfall Plot**- A series of spectral maps taken at regular intervals of time or at regularly spaced shaft speeds,
*similar to the flow of a waterfall*. **Waveform**- The shape of a time domain signal as seen on an oscilloscope screen.
**Wavelength**- The shortest repetition length for a periodic wave.
**Wavelet**- The wavelet allows a rapidly changing time history (e.g. door slam noise) to be investigated in the frequency domain.
**Wavenumber**- Wavenumber describes the spatial variation of waves, phase change per unit distance.
**Weber-Fechner Law**- An approximate psychophysical law relating the degree of response or sensation of a sense organ and the intensity of the stimulus.
**Weighted Equivalent Continuous Perceived Noise Level**- A measure proposed by International civil aviation organization to assess the continuous exposure to long-term noise of multiple aircraft.
**Whole Body Vibration**- The vibration experienced by a person whilst operating equipment or vehicle
**Windowing****Woofer**- Large loudspeaker designed primarily to reproduce low frequency audio signals.
**Z Weighting**- Z-weighting is a flat frequency response of 10Hz to 20kHz ±1.5dB. This response replaces the older "Linear" or "Unweighted" responses as these did not define the frequency range over which the meter would be linear.