Innovative Acoustical Materials for Aeronautics – MACIA

The new aircraft engine concepts, developed to decrease fuel consumption of airplanes propulsion systems, induce a growth of fan diameter, a reduction of the rotation speed and of the number of blades. These modifications are also associated with shorter and thinner nacelles. Thus, the fan noise becomes the predominant acoustic source of the engine and the frequencies that should be attenuated decrease to reach typical values of the order of 500 Hz. Consequently, the usual acoustic liners cannot be used anymore, because they are quasi-inefficient for such frequencies and cannot accommodate the reduced nacelle thickness.
To solve this problem, other approaches must be investigated and new concepts of acoustic absorbers dedicated to the reduction of turbomachinery noise must be developed. These sound absorbers should fit the specific constraints of the aviation and absorb sound for frequencies up to 500 Hz for a small thickness, while keeping their absorption properties unchanged for higher frequencies such as those emitted by the engine turbines.
To satisfy these new requirements, the SAFRAN group companies: Aircelle, Snecma and Turbomeca have decided to fund an Industrial Research Chair on new concepts of acoustic absorbers. This Chair will be established at LAUM (Laboratoire d’Acoustique de l’Université du Maine) where all the necessary skills are gathered to achieve the conceptual and technological breakthroughs.
Porous materials have particularly efficient properties at high frequencies, because they dissipate the acoustic energy through thermal and viscous effects. They can be used as multi-layered or functionally graded materials in order to increase their efficiency. Another way to improve the efficiency of porous materials is to build them from a complex arrangement of resonators with a subwavelength size (pore size). In order to enlarge the absorption frequency range of these materials, the addition of volume or surface macroscopic heterogeneities to a porous matrix has been recently studied at LAUM. This embedment enables the excitation of different types of mode, which trap the energy inside the structure, therefore leading to an increase of the absorption properties, particularly at low frequencies. In order to increase the absorption at even lower frequencies, these heterogeneities should resonate for a wavelength larger than their characteristic dimensions and a metamaterial is then obtained.
These two types of porous materials will be studied in the frame of this MACIA Industrial Research Chair. The impedance notion usually used to characterize the aeronautical liners fails when porous materials or metamaterials are used. That is why it is important to develop, in the frame of this Chair, a new computer code to optimize the characteristics of these materials when they are located in waveguides containing a large number of propagating modes. This code should account for the effect of large flow especially in the boundary layer just above the porous material.
The last concept studied in this Chair will be the concept of membrane absorbent structures patented by Snecma. The energy of the acoustic wave is then dissipated either by mechanical or by electrical systems. Two realisations will be studied: an absorber using the heat and viscous dissipation that occurs in a cavity under the membrane, an electrostatic device with a dedicated shunt impedance leading to electrical power dissipation.
For all the above concepts, demonstrators in form of large flat panels will be realized and test in a in-flow measurement bench for acoustical absorbent materials developed in parallel to this Chair.