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Spécial « Congrès Acoustics 2012 »
Noise source identi cation techniques : simple to advanced applications
Fig. 2: Left: Measurements on a car window during wind tunnel
tests. A: with mirror, B: without mirror, intensity at 125 Hz
1/3rd octave. Right: Vacuum cleaner, intensity 630 Hz
1/3rd octave.
Gauche : Mesures sur une fênetre de voiture pendant
des tests en soufflerie. A : avec un rétroviseur,
B : sans rétroviseur, intensité au 1/3 d’octave 125 Hz.
Droite : Aspirateur, intensité au 1/3 d’octave 630 Hz.
Panel contribution
Noise source identification techniques, can be used beyond
their traditional use, namely localizing acoustic sources
or estimating the distribution of acoustic quantities on
extended areas. One example is panel contribution based
on SONAH.
The purpose of this technique [4] is to estimate the contri-
butions of some areas to the acoustic pressure at a chosen
point in a cabin. A practical case is for example the acous-
tic contribution of the different inner parts of a car cabin
to the sound pressure level at the ear of the driver.
Fig. 3 : Pressure contribution at the driver’s ear position of a flat
loudspeaker attached on a window of a car cabin. The
measured true contribution corresponds to solid black
curve, the calculated contribution correspond to solid
blue curve. For the red curve only the operational particle
velocity is used.
Contribution de la pression d’un haut-parleur plat
fixé au niveau de l’oreille du conducteur, sur la
fenêtre de l’habitable d’une voiture. La contribution
véritablement mesurée correspond à la courbe
noire, la contribution calculée correspond à la
courbe bleue. Pour la courbe rouge, uniquement la
vitesse des particules opérationnelles est utilisée.
Panel contribution based on SONAH estimates the contri-
bution of a panel
Δ
S to the acoustic pressure at a target
point using pressure and particle velocity [4].
Two steps are required. Firstly, the transfer functions
between a source at a target position and the panel ΔS
using SONAH are measured. Secondly, the operational
pressure and sound velocity on the surface ΔS are obtai-
ned using SONAH again.
To illustrate this technique, figure 3 shows the contribution
of a flat loudspeaker attached to a window of a car cabin,
in presence of background noise. Agreement between the
true and calculated contributions is good, except below
630 Hz, where the signal level provided by the flat louds-
peaker is too low.
Entering intensity
A different example of application using SONAH is the
determination of the entering intensity in a cabin configu-
ration [5]. The purpose is to distinguish between the diffe-
rent intensity components close to walls. In particular, it
is of interest to estimate what is actually entering into the
cabin from outside the vehicle. Another interesting aspect
is the estimation of the panel absorption.
It is assumed that the sound fields due to radiation
(entering intensity) on one hand and due to absorption
on the other hand are mutually incoherent. This inco-
herence assumption can be reasonable in case of very
large number of incoherent excitations, such as, for
example, those created by a turbulent flow around an
aircraft fuselage.
A sound intensity probe cannot distinguish these diffe-
rent components, but will measure instead the total
intensity. Using SONAH with a double layer microphone
array (DLA), it is possible to separate these different
contributions [5].
To evaluate the Entering Intensity technique, tests were
performed in a transmission suite facility, emulating the
noise conditions around an aircraft window.
In the figure 4 (same scale, for a single octave band),
the middle display shows a high level of absorbed power
compared to the radiated power. The Right display shows
that the result is close to the reference case when applying
the Entering Intensity technique.
Fig. 4 : SONAH calculations. Left: Transmission loss noise
without emission from loudspeakers (reference). Middle:
Transmission loss with emission from loudspeakers.
Right: Transmission loss with emission from loudspeakers,
but with Entering Intensity processing applied.
Exemples de calculs avec SONAH. Gauche : perte de
transmission du son d’un haut-parleur sans émission
(référence). Au centre : perte de transmission du son
d’un haut-parleur en fonctionnement. Droite : perte de
transmission d’un haut-parleur en fonctionnement mais
avec l’application d’un traitement d’intensité entrante.
