Self-Adaptation in Vibrating Soap Films
is provided by a large loudspeaker located below the film and giving a spatially homogeneous forcing. Using a function generator and a hi-fi amplifier the excitation frequency can be tuned continuously. The whole system is placed in a perspex box to avoid drafts. The film is lit with a large monochromatic sodium lamp located behind a diffusing screen. Observed in reflection the film exhibits equal thickness interference fringes, the thickness varying by 0.22 mm between two neighboring fringes. A video camera placed just above the box is used to record the evolution of the interference patterns. We used the reflexion of a plane laser sheet of light by the film across the middle of its width. The largest deflections at the antinodes give a measure of the amplitude of the film vibration. The transverse profile of the film in the