Noise, Vibration, and Harshness

Noise, Vibration, and Harshness, also known as Noise and Vibration, abbreviated to NVH and N&V respectively, is the name given to the field of measuring, and modifying, the noise and vibration characteristics of vehicles, particularly cars and trucks. Harshness is a bit of a historical misnomer. Noise, and vibration, can be measured, but harshness is more of a subjective assessment. There is a psychoacoustic measurement called harshness but it does not correlate very well with many harshness issues.

Interior NVH is the noises 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. This is partly because the human body has its own frequency response, e.g. the ear's response is approximated by A-weighting, but this does not mean that two noises with the same A-weighted level are 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, i.e. adding or subtracting particular harmonics, rather than making the car quieter.

Sources of NVH

The sources of noise in a vehicle are many, including the engine, driveline, tire contact patch and road surface, brakes, and wind. Noise from cooling fans, or the HVAC, and alternators, is also fairly common. Many problems are generated as vibrations, transmitted via a variety of paths, and then radiated acoustically into the cabin. Others are generated acoustically, and this is then attenuated via various barriers until it excites the cabin. Vibrations are sensed at the steering wheel, the seat or the floor and pedals. Some problems are sensed visually - such as the vibration of the header rail or rear view mirror on open topped cars.

Tonal vs 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.

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.

Investigative Techniques

Techniques used to help identify NVH include part substitution, modal analysis, rig tests, lead cladding, acoustic intensity, transfer path analysis, and partial coherence. Most NVH work is done in the frequency domain, using Fourier Transforms to convert the time domain signals into the frequency domain. Wavelet analysis, Statistical Energy Analysis, and subjective evaluation of signals modified in real time are also used.

Computer based modelling

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. Sometimes these work. Back-of-envelope calculations are very useful.

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 should be established. In contrast, when the phenomenon being considered occurs at relatively high frequency, for example above 1 kHz, a SEA （Statistical Energy Analysis） model should be established.

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); or
# absorption of the noise or vibration energy, as for example with foam noise absorbers, or tuned vibration dampers.

Deciding which of these to use in solving a particular problem is the challenge 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.

See also

*Acoustical measurements and instrumentation
*Acoustics
*Digital signal processing
*Resonance
*Sound pressure level
*Vibration
*Vibration isolation
*Whole body vibration

References

* 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".
* White and Walker. "Noise and Vibration". ISBN 0-470-27553-7 has many good introductory papers

External links

* [http://www.bksv.com/Library/Primers.aspx Bruel and Kjaer's primers for noise and vibration analysis]
* [http://www.bksv.com/Library/Application%20Notes.aspx Bruel and Kjaer's excellent introductory notes for noise and vibration analysis]
* [http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf Agilent's Fundamentals of Signal Analysis]
* [http://www.diracdelta.co.uk/science/source/n/o/noise%20&%20vibration/source.html The Dirac Delta Science & Engineering Encyclopedia NVH Section]