Noise, vibration, and harshness

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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 and vibration characteristics of vehicles, particularly cars and trucks. While noise and vibration can be readily measured, harshness 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.

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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, 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.

Sources of NVH

The sources of noise in a vehicle can be classified as:

Many problems are generated as either vibration or noise, transmitted via a variety of paths, and then radiated acoustically into the cabin. [1] 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 harmonics. Here is the noise spectrum of Michael Schumacher'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.

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Instrumentation

Typical instrumentation used to measure NVH include microphones, accelerometers and force gauges, or load cells. Many NVH facilities will have semi-anechoic chambers, and rolling road dynamometers. Typically signals are recorded direct to hard disk via an analog-to-digital converter. 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 lab once per year, and any given setup is calibrated as a whole once per day.

Laser scanning vibrometry 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, rig squeak and rattle tests (complete vehicle or component/system tests), lead cladding, acoustic intensity, transfer path analysis, and partial coherence. Most NVH work is done in the frequency domain, using fast Fourier transforms to convert the time domain signals into the frequency domain. Wavelet analysis, order analysis, statistical energy analysis, and subjective evaluation of signals modified in real time are also used.

Computer-based modeling

NVH 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 cheaper 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 modelling works for structure borne noise and vibration analysis. When the phenomenon being considered occurs below, say, 25–30 Hz, for example 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 (SEA) model may be a better approach.

For the mid-frequency band, various methodologies exist, such as vibro-acoustic finite element analysis, 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:l

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 dampers, subframes, balancing, modifying the stiffness or mass of structures, retuning exhausts and intakes, modifying the characteristics of elastomeric isolators, adding sound deadening or absorbing materials, or using active noise control. 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

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Acoustics Branch of physics involving mechanical waves

Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.

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In music, timbre, also known as tone color or tone quality, is the perceived sound quality of a musical note, sound or tone. Timbre distinguishes different types of sound production, such as choir voices and musical instruments. It also enables listeners to distinguish different instruments in the same category.

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Acoustical engineering

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Spectrum analyzer

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Active sound design is an acoustic technology concept used in automotive vehicles to alter or enhance the sound inside and outside of the vehicle. Active sound design (ASD) often uses active noise control and acoustic enhancement techniques to achieve a synthesized vehicle sound.

Ernst Terhardt is a German engineer and psychoacoustician who made significant contributions in diverse areas of audio communication including pitch perception, music cognition, and Fourier transformation. He was professor in the area of acoustic communication at the Institute of Electroacoustics, Technical University of Munich, Germany.

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Electromagnetically induced acoustic noise , electromagnetically excited acoustic noise, or more commonly known as coil whine, is audible sound directly produced by materials vibrating under the excitation of electromagnetic forces. Some examples of this noise include the mains hum, hum of transformers, the whine of some rotating electric machines, or the buzz of fluorescent lamps. The hissing of high voltage transmission lines is due to corona discharge, not magnetism.

References

  1. Wang, Xu (2010). Vehicle noise and vibration refinement. Cambridge, UK: Woodhead Publishing Ltd. ISBN   978-1-84569-497-5 . Retrieved 5 December 2016.

Bibliography