Noise, vibration, and harshness, also known as noise and vibration, 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. These latter tools belong to the field
known as "psychoacoustics." 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 are
many, including the engine, driveline, tire contact patch and road surface,
brakes, and wind. Noise from cooling fans, or the HVAC, alternator, and other
engine accessories is also fairly common. Many problems are generated as
either vibration 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 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. 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,
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. 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 can be
used. In contrast, when the phenomenon being considered occurs at relatively
high frequency, for example above 1 kHz, a Statistical Energy Analysis model may
be a better approach. 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 or isolators; 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 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.
See also Acoustic quieting
Acoustical measurements and instrumentation
Acoustics Engine balance
Noise control Noise health effects
Noise mitigation Resonance
Sound masking Sound pressure level
Soundproofing Vibration
Vibration isolation Whole body vibration
References Baxa. 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
Laser Doppler Vibrometers Structural Dynamics Testing/Modal
Analysis Animating Vibration Test Results
Some NVH application examples from HEAD acoustics
Bruel and Kjaer's introductory notes for noise and vibration analysis
Agilent's Fundamentals of Signal Analysis
The Dirac Delta Science & Engineering Encyclopedia NVH Section
Noise and Vibration Introduction An introduction to Transfer Path
Analysis