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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.
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.
Typical solutions
There are three principal means of improving NVH:
1) reducing the source strength, as in making a noise source quieter with a
muffler, or improving the balance of a rotating mechanism;
2) interrupting the noise or vibration path, with barriers (for noise) or
isolators (for vibration); or
3) 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.
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