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Noise, vibration, and harshness (NVH), also known as noise and vibration (N&V), is the study and modification of the
noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...
and
vibration Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
characteristics of
vehicles A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered vehicles (e.g. bicycles, tricycles, velomobiles), animal-powered tr ...
, particularly
cars A car, or an automobile, is a motor vehicle with wheels. Most definitions of cars state that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people rather than cargo. There are around one billio ...
and
trucks A truck or lorry is a motor vehicle designed to transport freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction ...
. While noise and vibration can be readily measured,
harshness Harshness (also called raucousness), in music information retrieval, is a Non-Contextual Low-Level Audio Descriptors (NLDs) that represents one dimension of the multi-dimensional psychoacoustic feature called as musical timbre. Classical timbre� ...
is a subjective quality, and is measured either via jury evaluations, or with analytical tools that can provide results reflecting human subjective impressions. The latter tools belong to the field
psychoacoustics Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...
. Interior NVH deals with noise and vibration experienced by the occupants of the cabin, while exterior NVH is largely concerned with the noise radiated by the vehicle, and includes drive-by noise testing. NVH is mostly engineering, but often objective measurements fail to predict or correlate well with the subjective impression on human observers. For example, although the ear's response at moderate noise levels is approximated by
A-weighting A-weighting is a form of frequency weighting and the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighti ...
, two different noises with the same A-weighted level are not necessarily equally disturbing. The field of psychoacoustics is partly concerned with this correlation. In some cases, the NVH engineer is asked to change the sound quality, by adding or subtracting particular harmonics, rather than making the vehicle quieter. Noise, vibration, and harshness for vehicles can be distinguished easily by quantifying the frequency. Vibration is between 0.5 Hz and 50 Hz, noise is between 20 Hz and 5000 Hz, and harshness takes the coupling of noise and vibration.


Sources of NVH

The sources of noise in a vehicle can be classified as: * Aerodynamic (e.g., wind, cooling fans of
HVAC Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. ...
) * Mechanical (e.g., engine, driveline, tire
contact patch The contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is commonly used in the discussion of pneumatic (i.e. pressurized) tires, where the term is used strictly to describe the portion of the tire' ...
and road surface, brakes) * Electrical (e.g., electromagnetically induced acoustic noise and vibration coming from electrical actuators, alternator, or traction motor in electric cars) * Mainly, noise is either structure-borne noise or airborne noise. Many problems are generated as either
vibration Vibration () is a mechanical phenomenon whereby oscillations occur about an equilibrium point. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...
or noise, transmitted via a variety of paths, and then radiated acoustically into the cabin. These are classified as "structure-borne" noise. Others are generated acoustically and propagated by airborne paths. Structure-borne noise is attenuated by isolation, while airborne noise is reduced by absorption or through the use of barrier materials. Vibrations are sensed at the steering wheel, the seat, armrests, or the floor and pedals. Some problems are sensed visually, such as the vibration of the rear-view mirror or header rail on open-topped cars.


Tonal versus broadband

NVH can be tonal such as engine noise, or broadband, such as road noise or wind noise, normally. Some resonant systems respond at characteristic frequencies, but in response to random excitation. Therefore, although they look like tonal problems on any one spectrum, their amplitude varies considerably. Other problems are self-resonant, such as whistles from antennas. Tonal noises often have
harmonic In physics, acoustics, and telecommunications, a harmonic is a sinusoidal wave with a frequency that is a positive integer multiple of the ''fundamental frequency'' of a periodic signal. The fundamental frequency is also called the ''1st har ...
s. Below is the noise spectrum of
Michael Schumacher Michael Schumacher (; born 3 January 1969) is a German former racing driver, who competed in Formula One from to and from to . Schumacher won a record-setting seven Formula One World Drivers' Championship titles, tied by Lewis Hamilton in ...
's Ferrari at 16680 rpm, showing the various harmonics. The ''x-''axis is given in terms of multiples of engine speed. The ''y-''axis is logarithmic, and uncalibrated.


Instrumentation

Typical instrumentation used to measure NVH include
microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...
s,
accelerometer An accelerometer is a device that measures the proper acceleration of an object. Proper acceleration is the acceleration (the rate of change (mathematics), rate of change of velocity) of the object relative to an observer who is in free fall (tha ...
s, and force gauges or load cells. Many NVH facilities have semi-
anechoic chamber An anechoic chamber (''an-echoic'' meaning "non-reflective" or "without echoes") is a room designed to stop reflection (physics), reflections or Echo (phenomenon), echoes of either sound or electromagnetic waves. They are also often isolate ...
s, and rolling road
dynamometer A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...
s. Typically, signals are recorded directly to the hard drive via an
analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a Digital signal (signal processing), digi ...
. In the past, magnetic or DAT tape recorders were used. The integrity of the signal chain is very important, typically each of the instruments used are fully calibrated in a laboratory once per year, and any given setup is calibrated as a whole once per day.
Laser scanning vibrometry The scanning laser vibrometer or scanning laser Doppler vibrometer, was first developed by the British loudspeaker company, Celestion, around 1979, further developed in the 1980s,Stoffregen, B., Felske, A., “Scanning Laser Doppler Analysis Syst ...
is an essential tool for effective NVH optimization. The vibrational characteristics of a sample is acquired full-field under operational or excited conditions. The results represent the actual vibrations. No added mass is influencing the measurement, as the sensor is light itself.


Investigative techniques

Techniques used to help identify NVH include part substitution,
modal analysis Modal analysis is the study of the dynamic properties of systems in the frequency domain. It consists of mechanically exciting a studied component in such a way to target the Normal mode, modeshapes of the structure, and recording the vibration ...
, rig squeak and rattle tests (complete vehicle or component/system tests), lead cladding,
acoustic intensity Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area, also called the sound power density and the sound energy flux density. The SI unit of inten ...
, transfer path analysis, and partial coherence. Most NVH work is done in the frequency domain, using
fast Fourier transform A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). A Fourier transform converts a signal from its original domain (often time or space) to a representation in ...
s to convert the time domain signals into the frequency domain.
Wavelet A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases or decreases, and then returns to zero one or more times. Wavelets are termed a "brief oscillation". A taxonomy of wavelets has been established, based on the n ...
analysis, order analysis,
statistical energy analysis Statistical energy analysis (SEA) is a method for predicting the transmission of sound and vibration through complex structural acoustic systems. The method is particularly well suited for quick system level response predictions at the early design ...
, and subjective evaluation of signals modified in real time are also used.


Computer-based modeling

NVH analysis needs good representative prototypes of the production vehicle for testing. These are needed early in the design process as the solutions often need substantial modification to the design, forcing in engineering changes which are much less expensive when made early. These early prototypes are very expensive, so there has been great interest in computer aided predictive techniques for NVH. One example is the modeling works for structure borne noise and vibration analysis. When the phenomenon being considered occurs below, for example, 25–30 Hz, the idle shaking of the powertrain, a multi-body model can be used. In contrast, when the phenomenon being considered occurs at relatively high frequency – for example, above 1 kHz – a
statistical energy analysis Statistical energy analysis (SEA) is a method for predicting the transmission of sound and vibration through complex structural acoustic systems. The method is particularly well suited for quick system level response predictions at the early design ...
(SEA) model may be a better approach. For the mid-frequency band, various methodologies exist, such as vibro-acoustic
finite element analysis Finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of structural ...
, and boundary element analysis. The structure can be coupled to the interior cavity and form a fully coupled equation system. Also, other techniques exist that can mix measured data with finite element or boundary element data.


Typical solutions

There are three principal means of improving NVH: * Reducing the source strength, as in making a noise source quieter with a muffler, or improving the balance of a rotating mechanism * Interrupting the noise or vibration path, with barriers (for noise) or isolators (for vibration) * Absorption of the noise or vibration energy, as for example with foam noise absorbers, or tuned vibration dampers Deciding which of these (or what combination) to use in solving a particular problem is one of the challenges facing the NVH engineer. Specific methods for improving NVH include the use of
tuned mass damper A tuned mass damper (TMD), also known as a harmonic absorber or seismic damper, is a device mounted in structures to reduce mechanical vibrations, consisting of a mass mounted on one or more damped springs. Its oscillation frequency is tune ...
s, subframes, balancing, modifying the stiffness or mass of structures, retuning exhausts and
intake An intake (also inlet) is an opening, structure or system through which a fluid is admitted to a space or machine as a consequence of a pressure differential between the outside and the inside. The pressure difference may be generated on the ins ...
s, modifying the characteristics of elastomeric isolators, adding sound deadening or absorbing materials, and using
active noise control Active noise control (ANC), also known as noise cancellation (NC), or active noise reduction (ANR), is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The concept was first deve ...
. In some circumstances, substantial changes in vehicle architecture may be the only way to cure some problems cost-effectively. Not-for-profit organizations such as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) and Vibration Isolation and Seismic Control Manufacturers Association (VISCMA) provide specifications, standards, and requirements that cover a wide array of industries including electrical, mechanical, plumbing, and HVAC.


See also

*
Acoustic camera An acoustic camera (or noise camera) is an imaging device used to locate sound sources and to characterize them. It consists of a group of microphones, also called a ''microphone array'', from which signals are simultaneously collected and processe ...
* Acoustic quieting *
Engine balance Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and Balance shaft#Overview, balance shafts, to prevent unpleasant and potentially dam ...
*
Health effects from noise Noise health effects are the physical and psychological health consequences of regular exposure to consistent elevated sound levels. Noise from traffic, in particular, is considered by the World Health Organization to be one of the worst environ ...
*
Noise control Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation noise control, a ...
*
Noise mitigation Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation noise control, a ...
*
Vibration calibrator Vibration calibrators , sometimes also called reference shakers, are electromechanical instruments which enable calibration of vibration sensors and measuring instruments to traceable standards. They produce sinusoidal mechanical vibration sign ...
*
Vibration isolation ''Vibration isolation'' is the prevention of transmission of vibration from one component of a system to others parts of the same system, as in Building, buildings or mechanical systems. Vibration is undesirable in many domains, primarily engineere ...
* Acoustical measurements and instrumentation


References


Bibliography

* Baxa (1982). ''Noise Control in Internal Combustion Engines''. * Beranek. ''Acoustics''. * Griffin. ''Handbook of Human Vibration''. * Harris. ''Shock and Vibration Handbook''. * Thomson. ''Theory of Vibration with Applications''. * *


External links


Agilent's Fundamentals of Signal AnalysisBasics of NVH Dr. Pawan Pingle
{{Noise, state=uncollapsed Mechanical vibrations Automotive engineering Noise control