Search Results

The purpose of the SONVERT project is to create a link between the acoustical sources of a car and the environment in terms of traffic and architecture. Based on well validated approaches, it introduces the notion of a “macro-source” which integrates the major acoustic sources: engine, tires and exhaust, taking into account the low and high frequency aspects, from measurements made on real vehicles. The macro-source is then integrated into an original approach dealing with outdoor propagation. The proposed method can consequently be seen as a first step toward a global approach for the study of traffic noise in real conditions.

The weight reduction challenge has taken a new shape in the past two years due to high pressure on CO2 emissions in the automotive industry. The new question is: what level of acoustic performance can you get with an insulator weighting less than 2500 g/m2? The existing solutions at this weight being mainly dissipative (absorption) concepts give a satisfactory performance only if the pass-throughs are poor and present critical leakages. Respecting the less than 2500 g/m2 weight target, we have developed a wide range of new or optimized concepts switching from extremely absorbing to highly insulating noise treatments playing with multi-layers insulators (typically three to four layers), in combination or not with tunable absorbers on the other side of the metal sheet (in the engine compartment for example).

The lightweighting research on noise treatments since years tends to prove the efficiency of the combination of good insulation with steep insulation slopes with broadband absorption, even in the context of bad passthroughs management implying strong leakages. The real issue lies more in the industrial capacity to adapt the barrier mass per unit area to the acoustic target from low to high segment or from low petrol to high diesel sources, while remaining easy to manipulate. The hybrid stiff insulator family can realize this easily with hard felts barriers backfoamed weighting from 800 g/m2 to 2000 g/m2 typically with compressions below 10 mm. Above these equivalent barrier weights and traditional compressions of 7 mm for example, the high density of the felts begins to destroy the open porosity and thus the absorption properties (insulation works anyway here, whenever vibration modes do not appear due to too high stiffness…).

Due to increasing attention paid to the optimization of leakages and passthroughs in general, measurements on cut out modules in large coupled reverberant rooms are often carried out in the middle and high frequency range, in order to optimize the insulation performance of trims installed in their actual environment (Transmission Loss). Using optimal controlled mounting conditions, we have been able to extend the frequency range to the low frequencies in order to validate trim FEM models of a headliner cut out module with structureborne and airborne excitations.

In order to reach OEMs acoustic treatment targets (improving performance while minimizing the weight and cost impact), we have developed an original hybrid approach called “Vehicle Acoustic synthesis method”[1] to simulate - and therefore to optimize - noise treatments for both insulation and absorption, and to calculate the resulting Sound Pressure Level (SPL) at ear points for the middle and high frequency range. To calculate the SPL, we identify equivalent volume velocity sources from intensity measurements, and combine them to acoustic transfer functions (panel/ear) measured or computed with ray tracing codes using the reciprocity principle. Compared to our first approach [1], this paper shows a new measurement technique using pressure-particle velocity probes [2]. This technique allows to reduce acquisition time by a factor four, and makes therefore possible a synthesis method on a complete car within two weeks.

Whenever the noise source level or the expected acoustic comfort increases for diesel engines for example or for premium petrol vehicles, the required weight per unit area can be specified above 2000 g/m2 for the equivalent barrier of a mixed absorbing-insulating noise treatment. For an ABA foam/heavy layer/felt insulator, this is not a big issue, one has to increase the intermediate heavy layer weight. For hybrid stiff compressed felt backfoamed standard technologies, going above 2000 g/m2 is critical due to absorption properties loss following much too high airflow resistances and progressive porosity loss (above 250 kg/m3) as well as too high bending stiffness presenting resonant modes progressively and assembly manipulation issues. Last but not least, compressed felts begin to present too high costs at these weights against those of the heavy layers of ABA systems.

The Flaxpreg is a green and light very long flax fibers thermoset reinforced sandwich, which can be effectively used as multi-position trunk loadfloor or structural floor in the passenger compartment of a vehicle. The prepreg FlaxTapes of about 120 g/m2 constituting the skins of the sandwich, are unidirectionally aligned flax fibers tapes, with acrylic resin here, easily manipulable without requiring any spinning or weaving step and thus without any negative out of plane crimping of the almost continuous flax fibers. Thanks to their very low 1.45 kg/dm3 density combined with an adaptive 0°/90°/0° orientation of the FlaxTapes (for each skin) depending on the loading boundary conditions, the resulting excellent mechanical properties allow a - 35% weight reduction compared to petro-sourced Glass mat/PUR sandwich solutions (like the Baypreg).

Middle and high frequency vibro-acoustic simulation of complex shape insulators requires using 3D poroelastic finite elements. This can be applied to either the whole part (up to 2500 Hz maximum) or through singly curved pre-computed Insertion Losses (up to 5000 Hz maximum) to be introduced in large SEA or energy-based models. Indeed, a dependence of the Insertion Loss slopes of noise treatments following the curvature is observed both experimentally and numerically. Beyond frequency range limitations, poroelastic finite element simulations following all curvatures and thickness 3D maps typically take too much time of up to a few hours each. A cylindrical Transfer Matrix Method spectral approach significantly reduces the time for the calculation of singly curved Insertion Losses up to 10 kHz to only a few minutes. This simplifies enormously the SEA modeling effort enabling easier, more precise fully trimmed vehicle middle and high frequency vibro-acoustic simulations.

With the increasing importance of electrified powertrains, electric motors and gear boxes become an important NVH source especially regarding whining noises in the high frequency range. Engine encapsulation noise treatments become often necessary and present some implementation, modeling as well as optimization issues due to complex environments with contact uncertainties, pass-throughs and critical uncovered areas. Relying purely on mass spring systems is often a too massive and relatively unefficient solution whenever the uncovered areas are dominant. Coverage is key and often a combination of hybrid backfoamed porous stiff shells with integral foams for highly complex shapes offer an optimized trade-off between acoustic performance, weight and costs.