Mitigation of sound waves by wet aqueous foams

We present macroscopic experiments that show how the decay phenomenon affects the sound wave mitigation in wet aqueous foams and how both effects depend on the mass concentration of added particles of coal fly ash. To measure the mitigation of a sound wave as a function of the drainage time and particle concentration both a standing-wave and a single-pulse methods were used. To the best of our knowledge, the combination of these two techniques, which complement each other, has not been done before. The tested geometry of the foam samples concerns that the drained liquid and the sound wave are both uni-directed. Generally, this simplifies the phenomenon, which could be treated as one-dimensional but becomes less evident if the particles leaving the foam introduce uncertainties into the analysis. It turns out that the standing-wave method is best suited for testing the dry ready-made or other stable foams, while for wet unstable foams the single-pulse methods is superior. On the limited evidence collected at this stage, it is found that in conventional (without particles) foams, the mitigation of a sound wave decreases in time due to the foam drainage, while in particulate foams, increased particle concentration reduces the drainage rate and increases the sound wave mitigation. From this it follows that a powder of coal fly ash could be safely used as a cheap and an effective solid additive to improve protective capabilities of aqueous-foam based barriers.