Metamaterials with resonant inclusions for acoustic stealth under hydrostatic pressure – PANAMA

The objective of this academic proposal is to initiate a technological breakthrough by developing a new class of locally-resonant passive acoustic materials for stealth and discretion in underwater acoustics. These metamaterials are synthesized from polymer engineering and involve strong resonant multiple-scattering phenomena within the medium. The main focus of this project is the engineering of sound/noise control in marine environment for the military and civilian areas. The potentialities of these new materials are: increasing sound absorption levels; the possible reduction of thickness of anechoic or masking coatings; a good compatibility with the industrial constraints of manufacture and use.

This proposal is a strongly multidisciplinary project between three CNRS laboratories from the Bordeaux campus (experts in wave-physics, soft-matter and microfluidics techniques) and a major industrial group specialized in naval defence. The academic partners have more than 7 years of joint research on the topic of metamaterials (design and manufacturing) and DCNS has recently had a CIFRE/DGA action with one of them. This long collaboration coupled with a geographical proximity and a complementarity of skills up to the industrial level, is a key point to meet the materials and acoustics challenges of the project.

The materials challenge. These inclusion-type materials will incorporate sub-millimetric porous micro-resonators (made by emulsions or microfluidics) dispersed in an elastomer matrix adapted to the marine environment. Using "dense" and "resonant" inclusions must make it possible to address two major challenges for better performance of the boat-hull coverings: resistance to hydrostatic pressures during immersion; higher absorption properties due to the resonant multiple scattering.

The wave physics challenge concerns the modeling and the experimental proof of the functions and characteristics sought for the synthesized subwavelength materials/structures. An important phase for ultrasonic characterization under mechanical loading of the laboratory samples will indicate the performance of the latter, in particular in terms of absorption. Contextualized experiments will be conducted to predict the anechoic/masking power of the laboratory materials, as well as acoustic measurements on metric panels placed in a pressurized tank.

The industrial challenge seeks to take into account at the project outset, a number of manufacturing and use constraints that cannot be avoided by the industrial over the medium to long terms. This is why the soft-matter techniques that are easily-to-be-industrialized techniques, and the account for the hydrostatic pressure are two key elements at the heart of this exploratory-research project for naval engineering.

The synoptic operational overview of PANAMA is as follows.
1. Definition of the resonant inclusion media (acoustic design) according to the targeted specifications (absorption level, frequency range, static/dynamic impedance, static loading).
2. Chemistry and synthesis of porous micro-resonators according to certain criteria: size, shape, calibration, controlled polydispersity, mass production. Incorporation of the objects in an elastomer matrix.
3. Acoustic experiments/tests (in laboratory: under loading in open air; in a conventional acoustic water-tank at atmospheric pressure; in a specialized laboratory: in a pressurized tank).