Soft porous beads vs micro-balloons under pressure

Anechoic coatings for underwater acoustics typically consist of a soft polymer slab loaded with gas-filled micro-balloons whose resonances absorb sound. These coatings must also hold their performance under hydrostatic pressure: at 100 m a submarine already experiences ~10 bar.

This is a three-way coupling:

1. A resonance mechanics problem — the inclusions must resonate in the frequency band of interest;
2. A poroelasticity problem — open-porosity beads collapse under load;
3. A durability problem — repeated load cycles must not degrade the attenuation.

Micro-balloons vs soft porous beads

We compare two resonant inclusions:

• Micro-balloons — a thin polymer shell with a sealed gas core (closed porosity), the reference technology in naval anechoism;
• Soft porous beads — silicone beads obtained by emulsion templating whose open porosity (a silicone alkyl polyether surfactant) forms a cluster of connected air cavities resonating broadly around 200 kHz.¹

Both composites are compressed uniaxially at loads up to 10–20 bar, mimicking hydrostatic pressure at 100–200 m depth, and their attenuation compared.

What we found

• At ambient pressure, soft porous beads attenuate substantially more than micro-balloons over a broad band around 200 kHz;
• Under load, they remain more absorbing at low frequencies even at 10 bar, but their resonance weakens as the porosity collapses;
• Beyond a few bars, the damage is irreversible: after compression/relaxation cycles the resonant attenuation is not recovered, whereas micro-balloons resist better because their shells deform.

The natural next step is a hybrid: reinforce the porous beads with a thin shell — the pressure resistance of micro-balloons with the clusterized air cavities of the beads — or stiffen the surrounding matrix.

• Paper — J. Appl. Phys. 133, 155105 (2023)
• HAL version (open access)