Version HTML de base
Journée SFA / Renault / SNCF
5
Acoustique
&
Techniques n° 44
p-
u
probes [3] in order to drastically speed-up the
measurement protocol while increasing accuracy and
addressing unsteady operating conditions like run-ups.
A first validation of the microflown® p-
u
probes has been
realized in a coupled reverberation room/anechoïc room
testbench where a complete vehicle has been mounted
[4]. In fact, only the dash area has been excited by the
airborne artificial source in order to avoid at first high
reactive fields due to external sources. This work has
proved that it was possible to measure both intensity and
transfer functions without moving the probes by using the
pressure channel of the p-
u
probe alone for the transfer
function measurements.
The results compared to the p-p intensity technique were
very good and promising both in term of quality and of
efficiency of the protocol.
This paper deals with the second industrial validation of
this new technique applied on a Peugeot 1007 but with
all powertrain sources operating this time and without
any absorbing mock-up, proving that it is now feasible
to perform a «Vehicle Acoustic Synthesis Method» on a
complete car within two weeks.
Goals and Basis of the Study
Besides the development of the newmethodology, the goal
of the study has been to design and optimize the acoustic
package of a Peugeot 1007 for the floor and dash area
reaching the challenging target of a weight reduction of
10\% while maintaining the acoustic performance and a
cost reduction of 5\%.
This can be summarized by the lightweight strategy called
«
F
aurecia
A
coustic
C
oncept
T
riangle» (FACT) [1, 5]
This lightweight strategy is represented by two triangles
(cf. figure 1), the first one represents the target imposed by
the carmaker summarized in terms of acoustics / weight
/ price, the second one represents the related acoustic
package summarized in terms of a barycenter between
the acoustic concepts: classical insulation, broadband
absorption and mixed absorption/insulation. The link
between the triangles is performed by the «Synthesis
Methods: Vehicle Acoustic Synthesis Method», showing
the necessity of such hybrid simulation tools.
Synthesis Methods with Microflown
The first task of the «Vehicle Acoustic Synthesis Method»
consists in determining and digitizing the 3D measurement
mesh of the interior skin of the vehicle. This has been
carried out with an ultrasonic digitizer leading to 654
elements (radiating facets measuring 23 cm2 in average)
defined by 1331 geometric nodes.
The center points of the elements needed for the channel
tables of Ideas-Test are computed with MATLAB to ensure
that the measurements points are «inside» the car and
that the numbering follows the elements numbers.
The originality of this way of working is that this mesh
will live throughout the study being used to realize the
measurements, to represent the 3D power, transfer
functions and projected SPL maps, but also to build the
Ray-Tracing model...
As requested by the carmaker, the measurement
conditions are :
- 2nd gear, 4000 rpm
- 4th gear, 100 km/h
- 3rd gear, WOT run-up (1200 rpm - 5000 rpm)
The measurement protocol is the following:
a)
the p-
u
probes are positioned on the center points of the
elements;
b)
vehicle running / engine «on», the intensity
measurements are done for the two constant speed road
conditions;
c)
vehicle stopped / engine «off» but volume
velocity source «on», the transfer functions are measured;
then
d)
vehicle running / engine «on», the intensity run-ups
measurements are realized.
The calibration procedure of the p-
u
probes, which is
rather delicate, can be found in reference [4].
Sound Power Measurements
The VASM is based on the hypothesis
that each sub-area S(j), having a
global radiated power W(j), can be
modeled by n(j) fictitious uncorrelated
substitution monopoles, the total
mean
squared volume velocity Q
2
eq
(j) being
determined with ka<<1 by [6,7] :
€
W(j) =
ρω
2
4
π
c Q
eq
2
j
( )
C j
( )
(1)
WithC(j)weightingfactor representing
the influence of the surroundings on the power radiated by
the equivalent monopoles (compared to that in free field):
- C(j) = 2 for a hard reflecting wall
-
€
C
(
j
)
≈
2
-
α
for an absorbing surface where
α
is a
diffuse field absorption coefficient (plane wave
approximation.
Figure 2 shows the measurement conditions and how the
p-
u
probes are positioned.
Fig. 1 : Faurecia Acoustic Concept Truangle (FACT)
Vehicle Acoustic Synthesis Method 2nd Generation: an effective hybrid simulation tool to implement acoustic lightweight strategies