Self-adaptation in vibrating soap films

The vibration of a soap film set into motion by a sound wave is studied experimentally and theoretically. In contrast with the well separated resonances of a solid membrane, the modes of a liquid film exist in wide ranges of frequencies and the vibration amplitude is large for all forcing frequencies. This is due to the adaptation of the film mass distribution which concentrates at the antinodes as observed by interference fringes in monochromatic light. The theoretical model takes into account the variation of surface tension with thickness and explains the experimental results.