IDENTIFICATION OF SOME ACOUSTIC NOISE SOURCES IN CARS BY
SPECKLE INTERFEROMETRY
Dan Borza
Institut National des Sciences Appliquées de Rouen, LMR-FIMEN
BP 8, avenue de l'Université, 76800 Saint-Etienne du Rouvray, France
[email protected]
ABSTRACT
Acoustic noise reduction is an important subject of research and development for car and trucks manufacturers. It passes
through the identification of acoustic noise and vibration sources, which are related to the car itself and to the interaction of the
car with its environment, including the road. The testing and study of vibroacoustic behaviour of different car-related structures
uses complex, pointwise and full-field measurement techniques. Among other techniques, acoustical and optical non-contact
techniques are representing today the favourite choice since they are sensitive and do not add any mass to the structure under
the vibroacoustic tests. As long as the noise sources belonging to the car are concerned, speckle interferometry is a very
efficient technique allowing to detect, by full-field and non-contact measurements, the vibrational behaviour of almost any
visible vehicle parts.
The paper presents an experimental study of the some representative sources of vibration and noise. The experimental results
are grouped according to the main interest of the sources. These sources may be of concern because the sound produced
may affect the comfort and the security of the driver and other occupants of the vehicle, may be an environmental nuisance
and affect the persons and buildings situated close to the road where the vehicle is passing by, but also may be a factor of
dissatisfaction for a potential car buyer or hiding a technical disfunctionning. They may also put in evidence some local defects
in car assemblying or repairing.
Introduction. Full-field vibration measurement by speckle interferometry
There are many vibration and noise sources associated to a vehicle. Tires and their interaction with the road and with the
surrounding car structures are probably one the most important of these noise sources. So does the motor and the associated
components. The conditions in which the noise is produced are extremely varied. Some of the parameters involved are vehicle
speed, road properties, tire model and pressure, developed power, environment, different mechanical and acoustical
properties of assemblies. The vehicle components most involved in noise production and most exposed to vibration are tires,
motor, outer panels of the car body, the brakes, electronic components, equipments in the inner part of the car.
Some of the most important sources of noise are investigated by different techniques, mostly acoustical, like acoustical
holography. Speckle interferometry [1] is a fine tool able to help understand and identify some noise sources and thus bring
assistance to acoustical techniques widely used in this field. The most important characteristics of speckle interferometry are
related to its high sensitivity, non-contact and full-field measuring capabilities. A short functional description of the system used
in the Photomechanics Laboratory of INSA Rouen is given in [2]. It uses a CW frequency-doubled YAG laser emitting 100 mW
at 532 nm. In vibration measurement, the simplest operating mode [3] is the real-time, time-averaged interferometry, based on
acquiring groups of 4 successively phase-stepped frames and continuously displaying the time-averaged image given by the
approximate relation:
⎛ 4π
⎞
I TAV = I OBJ J 0 ⎜
d ( x, y ) ⎟
⎝ λ
⎠
where
J0
(1)
is the Bessel function of first kind and zeroeth order, λ is the wavelength and d(x,y) is the vibration amplitude at
object point (x,y).
The normal operating mode is to excite harmonic vibrations of the object under study with the help of an adjustable frequency
generator while simultaneously looking at the currently generated time-averaged hologram given by eq. (1) and displayed on a
monitor. When the excitation is such that the object operational deflection shape is close (or identical) to one of its proper
modes, that hologram shows, like in Figure 1, the image of the object with the vibration amplitude map superimposed on it.
Figure 1. Object under test (vibrating immersed plate) and its real-time speckle interferometry hologram
The tires
Tires and their interaction with the road are the most important noise source related to a vehicle, especially at speeds over 60
km/h. Tire noise has been investigated by many researchers [4], [5], [6], [7]. At lower frequencies, between 150 – 500 Hz, the
noise is mainly produced by the vibration of the tire (Figure 2) as a thorus. Vibration from the tires is transferred to the vehicle’s
structure and to the passenger cabin, where the low frequencies are most disturbing. The biggest amplitudes are those of the
tread in radial direction, originating near the contact with the road. Lower amplitudes are experienced, for the low-frequency
modes, by the sidewalls.
Figure 2. Some of the low-frequency vibration modes of a tire
At higher frequencies, vibrations and noise are mainly due to the impact contact with the road and the "air pumping"; they are
mostly disturbing outside the vehicle.
Identifying the resonant frequencies of a tire allows minimizing the transfer of the vibrations to the cabin and avoids excitation
of resonances of other vehicle components. It also helps understanding how vibrations are transmitted from the region where
they are initiated (near the contact of the tire with the road) to the rest of the tire, wheel and other vehicle structures.
Door assembly
An example of noise source which is related to the comfort of the driver and which might dissuade a potential buyer of a car is
the noise which may be produced by closing a car door. The inner door panel and the outer panel may both contribute to this
noise. Some of the sources of vibration "hidden" between the inner composite thermoformed door panel and the outer door
panel are shown in Figure 3. They are the protection screen of the loudspeaker, the large plane parts of the inner panel and
the parts of the window opening mechanism or pantograph. Between these different parts the producers may also place, as
dampers, small textile patches.
Figure 3. Inner door panel and three of its vibration modes
Another example related to the door, influencing the comfort of the driver, is the acoustical behaviour of the elastic, rubber joint
placed between the door and the car body. One of its essential roles is to achieve a good acoustical isolation between the
vehicle occupants and the world outside the vehicle. At some higher speeds, this acoustical behaviour of the joint is suddenly
worsening. An investigation by speckle interferometry revealed that at some frequencies, the joint may have resonances, as
shown in Fig. 4. The vibration modes at these frequencies are so that the full contact between joint and car body is lost, and a
direct noise propagation from outside the vehicle to the interior becomes possible.
Figure 4. Rubber joint between door and car body and some of its vibration modes
These resonant parts are also interacting with the outer door panel, whose resonant modes are influenced by the inside
structure of the door (Figure 5).
Figure 5. Outer door panel vibrations follow inner door structure
Outer panels; effects of added masses and of assembling imperfections
Outer panels have many resonant modes, depending on their dimensions, boundary conditions and shapes. Exciting and
visualising by speckle interferometry these modes may have as useful results:
the detection of loose, weak assemblies between two panels or between a panel and the car body;
the detection of relatively important masses (of functional assemblies) attached or in contact with the inner part of the
panels, able to excite strong local resonances.
Such less known examples of noise sources, related to all the factors previously mentioned have been investigated and some
results are presented in Figure 6.
a
b
c
Figure 6. (a) strong vibrations induced by the windscreen wiper motor;
(b) loose assembly at the corners of the hinged cover over the engine;
(c) loose assembly at the corner of the wing, of the black plastic wheel protection and resonance induced by the blinker
A characteristic of the vibration-based testing of such panel assemblies is that a loose assembly is detected in an incipient
state, which may simply be described by an insufficient pressure between the panel and the plastic or elastic material by which
it is supported.
These examples show that vibration-based nondestructive inspection or even damage detection may be successfully applied
to vehicles by using the CW speckle interferometry.
Brake disc and brake pads
A typical brake system consists of disk brakes connected by a series of tubes and hoses filled with an incompressible liquid to
a master cylinder into which a plunger is pushed with help of the pedal. External friction pads are forced against the faces of
the disc by the action of a caliper. Brake discs and pads are subjected to high dynamical and thermal stresses. Some discs are
just solid cast grey iron; others are ventilated, which helps dissipating the generated heat. The brake pads are designed for
supporting high friction forces and high temperatures. Sometimes a high-frequency squeal is produced while the pads are in
contact with the disc. Usually it is produced by vibration of the brake pads and discs.
Finding the resonant frequencies and modes of a brake disc is part of the correct design of the system. Figure 7 shows a disc
brake assembly and two of the resonant modes of the disc, at 1079 Hz and 2470 Hz. The disc brake mechanism was mounted
on the vibration-isolated optical table and excited with help of an electro-magnetic shaker.
Figure 7. Assembled brake disc, caliper and pads and first three resonance modes of the disc
At some frequencies the brake pads (partially hidden in the images by the caliper) are also vibrating, independently of the
brake disc, as shown in Figure 8 by the two fringe patterns obtained by time-averaged speckle interferometry.
Figure 8. Vibration of brake pads under the brake caliper, with or without vibration of brake disc
Electronic assemblies and components
There are more and more electronics in a car. All subsystems of a vehicle are controlled or assisted today by analogical or
digital dedicated circuits. They may have various functions in the essential, power related compartments, but also in
communications, security and comfort. These electronic systems must operate in difficult conditions. They are subjected to
vibrations and chocs, accelerations, thermal stresses, and more and more electronic noise.
In these conditions it is essential to insure the reliability of these circuits and detect all possible causes of failure, since many of
the failures may represent direct danger for the vehicle occupants or other persons.
An example is the Airbag Electronics Control Unit(ECU) – an electronic system which has to sense any sudden deceleration
and enable the airbag to protect passengers. The systems are required not only to provide safety for all passengers but also to
be integrated with pedestrian protection electronics. Such a system should always deploy the airbag as soon as necessary,
but only if it's really necessary.
In an ECU there are not only a microprocessor, components, connectors but also piezoelectric sensors for choc detection and
different single-layer externally placed flexible sensors able to detect if the occupant of a seat is a child and thus provide
compatibility with the advanced airbags requirements.
By exciting vibrations of the main electronic circuits and closely observing the holographic results one may detect different
situations which a potentially dangerous for the reliability of the electronics. A few such situations may be seen in the images
presented in Figure 9. Electronic connector blocks which are not correctly fastened on the printed circuit (Figure 9 a) usually
end up in an opened circuit. The ceramic plate, part of a microprocessor casing, vibrating at resonance (Figure 9 b) usually
ends up either by unsoldering one of its terminals or simply by destruction. A heavy passive component vibrating at resonance
(Figure 9 c) produces fatigue by an alternating flexure of its terminals.
a
b
c
Figure 9. (a) loose connectors; (b) resonance of a microprocessor; (c) resonance of a heavy electronic component which
usually leads to destruction of the terminals
Conclusions
Speckle interferometry identification of vibration and noise sources in cars has a great potential in detecting components which
present dangerous resonances. The exact shape of the vibration mode is measured simultaneously for all visible points of the
tested structure, without contact and without use of any transducer. Apart identification of individual noise sources, the speckle
interferometry technique may also be of great help by validating, with a high spatial and measurement resolution, the results
offered by techniques based on a less direct relation between the vibration amplitude (displacement or velocity) of a point and
the estimated result – like, for example, near-field acoustic holography. It is also a source of data useful in damage detection
and non-destructive testing.
References
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