An unusual problem in vibration damping: the flat tension mask computer tube

In the flat tension-mask tube recently introduced, the mask is a flat steel membrane only 1 mil (25 mu m) thick, stretched close to its elastic limit. With no air damping, flexural vibrations of this membrane decay slowly and may become noticeable in vehicular applications. Damping such tension-mask vibrations is rendered difficult by three factors: the mask must be kept clear of obstructions, except for a narrow strip along its edges; mask resonance frequencies vary widely during operation, because electron-beam heating of the mask changes with picture content, causing tension to vary correspondingly; and conventional damping materials cannot be used in a vacuum tube. The first two obstacles are overcome by a dynamic vibration damper attached to the mask near its edge in such a way that the resonant frequency of the damper tracks that of the mask. Resistive loading of this damper over a wide frequency range is provided by a flexural transmission line loaded with loose-fitting bushings. The decay time of the fundamental membrane mode is thus reduced from about 10 s to approximately 20 ms.<>